How To Detect & Measure Radiation

$3.95 Cat. No. ADR-2

HOW TO DETECT & MEASURE

RADIATION by *

Harold S. Renne

#*■

HOWARD W. SAMS & CO., INC. THE BOBBS-MERRILL Indianapolis

•

COMPANY, New York

INC.

T hoeniic

FIRST EDITION FIRST PRINTING— JANUARY,

1963

HOW TO DETECT & MEASURE RADIATION Copyright © 1963 by Howard W. Sams & Co., Inc., Indianapolis 6, Indiana. Printed in the United States of

America.

Reproduction or use, without express permission, of editorial or pictorial content, in any manner, is pro hibited. No patent liability is assumed with respect to the use of the information contained herein.

Library of Congress Catalog

Card

Number:

63-11935

?7X

v//x

PREFACE

Many changes have taken place in the field of nuclear science in just a few short years. Not too many years ago, atomic power plants and radioactive isotopes had very limited applications in technology and medicine. Today we have a fleet of nuclear-powered submarines on active duty, and nuclear-powered ships are joining the fleet. A number of atomic power plants are providing quantities of electric ity on a regular basis. Radioactive isotopes are readily available and are used in a multitude of routine applica tions. Many new uses are being found almost daily. New devices and techniques for detecting nuclear radia tion, such as the solid-state silicon P-N junction, have re cently been exploited. In the field of analytical chemistry, a powerful new tool—called neutron activation analysis — is available. This tool permits the detection and measurement of extremely small quantities of chemicals. It was recently used to measure the amount of arsenic in Napoleon's hair and, when the percentage was found to be far above normal, led to the theory that he was poisoned. Before the methods of detecting and measuring radia tion, and before the various means of employing radiation for useful purposes can be understood, some background knowledge is necessary. Therefore, an introduction to the field of nuclear science is given in the first portion of this book. The structure of the atom, the principles of nuclear fission and fusion, what radiation is and how it can be detected, and the various terms used in the field of nuclear science are included in this portion of the book.

Next, the various types of instruments used to detect and measure radiation are discussed. Individual chapters are included for a discussion of each of the three types of detection and measuring devices — Geiger counters, scintilla tion counters, and dosimeters. Another chapter includes circuits and parts lists for building your own Geiger counter. The final chapter describes the various applications for radiation in such diversified fields as medicine, biology, agriculture, industry, insect control, food preservation, power sources, and research. It is hoped that the content will excite the interest of high school science students in the field of nuclear science. Businessmen and laymen in all walks of life will also find much of the information valuable. Many technicians em ployed in the field of nuclear science will also find material which will be of interest and value to them in their every day work. As with any book, the material for this book has stemmed from many sources. The Atomic Energy Commission has been particularly helpful, as has the Brookhaven National Laboratories. To the many manufacturers who have pro vided information about their products and activities I extend my thanks. Without their wholehearted cooperation, this book could not have been written.

Harold December, 1962

S.

Renne

Table of Contents CHAPTER

1

Nuclear Radiation and Its Effects

Blocks — Atomic Structure — Isotopes — Energy and the Electron Volt — Nuclear Reactions — Atomic Radiation — Radioactivity— Standards — Radiation Effects on Humans Atomic

7

Building

CHAPTER

Detecting Nuclear Radiation

2

Cloud Chambers — Ionization Chambers — Geiger Tubes — Electroscopes and Electrometers — Scintillation Crystals — Conducting Crystals— Chemical Indicators — Photographic Emulsions — Solid-State Detectors — Miscellaneous

CHAPTER

24

3

Ionization Counters

Geiger-Tube Instruments — Ionization Counter Instruments — Proportional Counter

CHAPTER 4

Scintillation and Solid-State Counters

....

Solid Scintillometers — Scintillation Probes — Commercial Instruments — Liquid Scintillometers — Solid-State Detectors

41

69

CHAPTER 5

Dosimeters

Film — Chemical — Radiophotoluminescence — Ionization — Solid-State Devices — Summary

92

CHAPTER 6

Home-Built Counters

Simple Counters — Counter With Amplifier — Novel Power Supply — Counter With Transistor Amplifier — Counter With Meter Indicator — Counter With Interrupter-Type Power Supply — Counter With Transistorized Power Sup ply—Counter Operated from Power Lines — Conclusion

104

CHAPTER 7

Nuclear Radiation Applications

— Medical Applications — Agriculture — In dustrial Applications— Power Generation — Miscellaneous

126

Instrumentation

APPENDIX

I

Abbreviations

151

APPENDIX II

Definitions

152

Index

157

Chapter

1

Nuclear Radiation and Its Effects

Since time began, man has been delving into the secrets of nature in an attempt to determine the exact structure of matter. Progress has been particularly rapid since the dis covery of atomic fission before World War II. Many secrets have been uncovered; however, a great deal remains to be explained. Some knowledge of atomic theory is essential to an under standing of nuclear reactions and atomic radiation. There fore, in this chapter those essentials that are considered necessary for understanding this book will be reviewed. ATOMIC BUILDING

BLOCKS

The atom has long been considered to be the smallest par ticle of matter that retains the properties of the original ele ment. However, it is not the smallest particle existing in the universe. It is convenient to consider the atom as made up of three basic building blocks—electrons, protons, and neu trons — each of which is smaller than the atom itself. This, of course, ignores the many additional particles, such as the meson, positron, neutrino, and the like, which have been discovered by physicists. For our purposes, however, the simplified picture is adequate. Electrons are very small and very light. Each one carries with it a unit charge of negative electricity. When electrical current flows through a wire, vacuum tube, or any other de vice, the current is made up primarily of electrons. The proton is much heavier than the electron. In fact, it is about 1840 times as heavy, and carries with it a positive charge equal in value but opposite in sign to that carried by

Protons are somewhat elusive, and, except when nuclear reactions are taking place, are seldom found outside the nucleus of an atom. The neutron is the third item in the series of basic build ing blocks. It has the same mass as the proton but has no electrical charge. That is, it is electrically neutral. It cannot be deflected by an electric or a magnetic field, making it dif ficult to study. The neutron was not positively identified until 1932, but it is now extensively used in nuclear reactions and for bombarding atomic nuclei. In general, the nuclei of atoms can be considered to be made up primarily of neu trons and protons. the electron.

ATOMIC STRUCTURE

As an approximation sufficient for our purposes, the atom can be considered to be made up of a compact core, or nucleus, around which a number of electrons travel in or bits. Normally, the number of electrons is equal to the num ber of protons in the nucleus. Thus, in its normal state, the atom is electrically neutral. Normal hydrogen and helium atoms are shown schematically in Figs. 1-1A and B. If one or more of the orbital electrons is removed by some means, the atom is left with a net positive charge and is said to be

(A) Hydrogen

Jf.

(B)

©

PROTONS

0

ELECTRONS

O

NEUTRONS

(C) Deuterium iH'. Fig. 1-1.

Helium ,He'.

(D) Tritium

S'.

of a hydrogen atom, helium atom, and two isotopes of hydrogen.

Schematic representation

The ionization process plays an important role in our study of nuclear radiation. The chemical properties of a normal atom are determined by the number of electrons surrounding the nucleus, which, in turn, is the same as the number of protons in the nucleus. This quantity is called the atomic number and is very useful in describing the various kinds of atoms. Normal hydrogen (Fig. 1-1A), the lightest element, has one orbital electron and its nucleus has one proton. Therefore it has an atomic number of one. Uranium, one of the largest and heaviest atoms occurring in nature, has 92 orbital electrons and 92 protons in its nucleus ; therefore its atomic number is 92. Neutrons form an essential building block for atomic nu clei, but they have no effect on the chemical properties of the atom and thus do not affect the atomic number. They do affect the mass, however, and since the total mass of an atom is important, a mass number which is equal to the sum of the protons and neutrons in the nucleus is assigned to each atom. Normal boron, whose nucleus contains five protons and five neutrons, has a mass number of ten. Hydrogen, with a single proton and no neutrons, has a mass number of one. To clearly indicate the mass number and atomic number of an element, a shorthand notation is used. In this notation, the atomic number is written as a subscript at the left of the symbol for the element, and the mass number as a super script at the right. Thus, boron, with a symbol B, an atomic number of five, and a mass number of ten, would be writ ten as : SB10. Occasionally the atomic number may appear at the right, or it may be omitted completely. Thus, BB10, BB10, and B10 all have the same meaning. The mass number of an element is very nearly equal to its atomic weight, which is the weight in grams of a specified number of atoms. However, there may be slight but impor tant differences in these two numbers. ionized.

ISOTOPES

As mentioned before, the chemical properties of an atom are determined by the number of orbital electrons, and are by the number of neutrons in the nucleus. Therefore, you might expect to find atoms of an element with the same atomic number as the normal atom, but with a different mass number. Such atoms are present, and they are called isotopes. An isotope has the same number of pro not affected

tons in its nucleus as a normal atom, but it has either more or fewer neutrons. They play a very important role in the field of nuclear science. Chemically, an isotope behaves the same as a normal atom so that the only way of telling them apart is by the difference in atomic weight. There are over 280 known stable isotopes, and many which are unstable, or radioactive. The number of isotopes per element is distributed very unevenly and is determined by rules that govern the stability of nuclei. Boron with an atomic weight of eleven, written symbolically as „BU, is an example of an isotope. It has five protons and six neutrons in its nucleus, compared with five protons and five neutrons for the normal boron atom (5B10). There are two isotopes of hydrogen. One, called deuteri um (,H2), is shown schematically in Fig. 1-1C, and the other, tritium GH8), is shown in Fig. 1-1D. In nature hydro gen contains only a small proportion of deuterium and an extremely small amount of tritium. However, ways have been found for increasing the concentrations of these iso topes, and nearly pure deuterium is now available. Water made with deuterium is called heavy water, because the deu terium atom is nearly twice as heavy as the normal hydro gen atom. This difference in weight makes it possible to sep arate isotopes from normal atoms. ENERGY

AND THE ELECTRON VOLT

In atomic physics you are continually dealing with energy in one form or another. To reduce energy to a common de nominator, the term electron volt (ev) is commonly used. An electron volt is the amount of energy acquired by a unit charge of electricity (such as an electron) when it moves

through a difference in potential of one volt. For example, if an electron should start at a potential of zero volts and travel to a point having a potential of 300 volts, it would ac quire an energy of 300 electron volts. An electron volt is a very tiny amount of energy ; for ex ample, a 10-watt lamp burning for one second consumes the equivalent of about 62 quintillion (62 X 10") electron volts. A more common term is a million electron volts (mev). Even this unit is small, but since you will be dealing mostly with individual atoms, it is a very convenient unit to use. Many years ago Albert Einstein postulated that mass and energy are equivalent and gave us a famous equation : 10

E =

mc2

where, E is the energy in ergs, m is the mass of the matter in grams, c is the velocity of light in centimeters (c2

=

9-1020)

per

second

.

Early experiments with nuclear reactions verified this equa tion. Where the end products of a reaction weighed less than the initial products, it was found that energy was released in the reaction, and the energy was exactly equal to the loss of mass as given by Einstein's equation. An extremely small amount of mass is equivalent to a large amount of energy. For example, if one pound of matter could be completely transformed into energy in accordance with Einstein's equation, the energy would be equivalent to that produced by burning 1Vk million tons of coal. Thus, in nuclear reactions the loss of a small amount of mass can re sult in the release of a tremendous amount of energy. The energy equivalent of an electron is about 0.51 mev, and that of a proton or neutron is about 931 mev. NUCLEAR

REACTIONS

Work in nuclear physics is concerned largely with the dis ruption and reorganization of atomic nuclei. Disruptions may be the result of natural radioactivity, or they may be caused by firing particles at the nuclei with sufficient energy to cause a reaction of some kind. Working with projectiles can be difficult, however, since the atom is practically all empty space and the nucleus represents an extremely small target. The diameter of an atom is roughly 10,000 times the diam of the nucleus. Another factor which makes it difficult to strike a nucleus with a positively charged particle is the potential barrier sur rounding it due to its concentrated positive charge. This po tential barrier strongly repels a positive charge. For this rea eter

son, neutrons, which do not have a charge, are frequently used as projectiles. If neutrons are traveling very rapidly,

however, they may pass right through a material without ever striking a nucleus — this is due to the fact that atoms are mostly empty space. Therefore in many reactions it is necessary to slow down, or moderate, the neutrons. This can be done by such moderators as heavy water, graphite, and paraffin. The Bevatron, a giant atom smasher which acceler 11

ates protrons to 6.2 billion electron volts, is pictured in Fig. 1-2. To get an idea of the types of nuclear reactions involved

in nuclear physics and the energy obtained, it is instructive

(Courtesy Lawrence

Radiation Laboratory.)

Fig. 1-2. The Bevatron atom smasher, capable of accelerating protrons 6.2 billion electron volts.

to study a specific reaction in which a neutron dn0) is fired into a boron atom. The results are a lithium atom (8LiT), an alpha particle (helium nucleus, 2He4) , and some energy. The complete equation can then be .B1

+

1n0-

written as

3LiT + 2He4 +

:

energy.

This is an example of a reaction where the weights of the end products are less than the weights with which you started, giving a chance to verify Einstein's equation. The products on the right of the equation have an atomic weight less than those on the left by 0.00385 units which, according to Einstein's equation, is equivalent to 2.85 mev of energy. This agrees precisely with experimentally observed results. A rather startling fact is revealed by the reactions just de scribed. By striking a boron nucleus with a neutron, the boron completely disappears and two new elements — lithium and helium — are formed. Thus, transmutation of elements, a dream of the alchemists of old, has been accomplished. 12

Although neutrons are useful as projectiles in nuclear re actions, they are somewhat difficult to handle. They can be formed only as a result of a nuclear reaction, and once formed, cannot be accelerated or deflected by electric or magnetic fields. One method of producing neutrons is to fire high-speed alpha particles (helium nuclei) at beryllium or boron. The resulting second-hand neutrons can then be used to bombard other nuclei. In 1934, Enrico Fermi, while bombarding various heavy elements with neutrons, found that when uranium was the target, a new element, called plutonium, was produced. The process is now well understood, and is used in making plu tonium for nuclear reactions. The steps are as follows: First the uranium nucleus captures the neutron to form a uranium isotope : ,2U"8 + .n0 -> MUm

This isotope is unstable, and the nucleus immediately gives off an electron (^e0), forming a new element, neptunium, with an atomic number of 93 : 92 US3»

-+ 03NpM»

+

.^

Neptunium is also unstable, and immediately gives off an electron to yield the desired element, plutonium : 93 Np2se

-»

94Pu288

+

.,60

Plutonium is relatively stable until bombarded with slow neutrons. It then literally blows itself to pieces, giving a whole series of atoms of smaller atomic weight and releas ing vast quantities of energy. This process is known as nu clear fission, and is the basis for the atomic bomb and for nuclear reactors. Emission of an electron from the nucleus of an atom re quires some explanations which are not as yet completely satisfactory. So far nothing has been said about electrons in the nucleus. The best explanation at present is that one of the neutrons gives off an electron and becomes a proton. Just how this takes place is one of the atomic mysteries still defying solution. Another process which should be discussed briefly at this time is that of nuclear fusion. In this process, atoms of a 13

light element (such as hydrogen) are caused to combine to form a heavier element (such as helium). However, some of the mass is converted to energy so that the net weight of the resulting matter is less than that of the original components. This is the basis for the operation of the hydrogen, or thermonuclear, bomb. Extremely high temperatures are required to produce nu clear fusion — on the order of one hundred million degrees or more. In the thermonuclear bomb this temperature may be produced by the explosion of a conventional atom bomb, thus triggering the thermonuclear reaction. Much research is being conducted in various laboratories throughout the country in an attempt to produce controlled thermonuclear reactions. When this feat is accomplished and the energy is made available for useful purposes, our exist ing raw materials will produce all the energy we can ever possibly use. The fear of eventually exhausting our supplies of organic fuels, such as coal and oil, or fissionable materials, such as uranium and thorium, will no longer exist. ATOMIC RADIATION When a nucleus disrupts spontaneously, it is said that the material is radioactive. Several elements, such as radium, are naturally radioactive; radioactive isotopes of many other elements can be formed artificially by neutron bom bardment. Various kinds of radiation are frequently given off when atomic nuclei break up in the radioactive process. The three main types of radiation are alpha rays (a), beta rays (ft), and gamma rays (y) . Alpha rays are made up of particles having the same mass as the helium nucleus ; in other words, they contain two pro tons and two neutrons and have a net positive charge of two. They are deflected only slightly by strong electric and mag netic fields because of their weight. The range of alpha particles is limited because they ionize the material through which they are passing very heavily, and thus lose their energy rapidly. Alpha particles are un able to penetrate the unbroken skin, but if an element lib erating them is deposited in the body, severe damage may result. In any specific nuclear reaction all the alpha particles given off have essentially the same energy and thus, the same range. 14

Beta rays are made up of electrons and so are easily de flected by rather weak electric and magnetic fields. Because electrons are so light, they are easily bounced around in any material through which they pass. Therefore their actual range is variable. At most, they will pass through about a third of an inch of body tissue. They do not ionize as heavily as alpha particles but can cause a moderate amount of dam age both externally and internally. Damage to tissues is a direct result of ionization. Individual electrons (beta particles) given off in a nuclear reaction can differ widely in energy content. This con tributes to the variable range. Gamma rays cannot be deflected by either a magnetic or an electric field. They consist of extremely short electro magnetic waves and act somewhat like X rays, although their wavelength in general is much shorter. They are less damaging, quantity for quantity, than alpha or beta rays, but they have much greater penetrating power; therefore, they are a major problem. Very dense materials are required for effective shielding against gamma rays. For this reason lead is widely used. It is about 11 times as effective as the same thickness of water, and about half as effective as gold. Six inches of lead will effectively shield the most penetrating gamma rays. Other particles may be encountered under certain circum stances, such as in cosmic ray showers and in the output of high-energy accelerators. Included are neutrons, posi trons, mesons, and the like. Neutrons are by far the most common, since they are plentiful in nuclear reactors and in the explosion of atom or hydrogen bombs. A beam of neutrons has a very great penetrating power which depends more on the composition of the shielding material than on its density. Any material containing hydro gen, such as water or the human body, absorbs neutrons much easier than lead. Thus, a beam of 1-million volt neu trons is slowed down only slightly by a foot of lead, while the same thickness of water will form an effective shield. Neutrons do not ionize directly but instead produce ioniza tion by transferring their energy to the nuclei which they strike. These moving nuclei then produce the ionization which is detected by suitable instruments. A positron is a particle having the same mass as an elec tron and carrying a positive charge. If a positron and an electron combine, they are annihilated and two oppositely 15

directed quanta, or gamma rays, are formed, each with an energy of about one-half a million electron volts. Here again, Einstein's equation is verified and can be used to calculate the energy produced from the mass of the electron and proton. Mesons are particles of varying mass and charge that have a tremendous penetrating power. They do not exist in ordinary nuclear reactions but may be present in cosmic-ray showers or in the output of large nuclear accelerators. Their lifetime, in general, is extremely short. RADIOACTIVITY

Probably the best-known naturally radioactive elements are radium and uranium. Radium has been used for many years because of its therapeutic value in cancer and other illnesses. Uranium has been widely publicized because of its use in nuclear reactors and atomic weapons; plutonium has been publicized for similar reasons. Uranium is plentiful throughout the world ; radium is much more scarce ; and plu tonium, for the most part, is obtained only from certain nu clear reactions and does not exist in appreciable quantities in nature. The rate of disintegration of a radioactive material is fixed by nature and cannot be altered by physical or chemi cal changes. Each material has a characteristic rate of decay which remains constant in spite of variations in tempera ture, pressure, chemical reactions, or any other known pro cess. This characteristic is usually designated by a factor known as a half-life. If you start with a certain quantity of radioactive material the quantity will decrease by half in a certain length of time. During the next equivalent period of time the remaining quantity will decrease by half. This length of time is known as the half life of the material ; it gives a precise yardstick for comparing the lives of various radioactive elements. The concept of half life is illustrated in Fig. 1-3. This diagram assumes a radioactive isotope with a half life of 3 hours. The abbreviation mc at the right of the diagram is for millicurie — a measurement of the radioactivity given off by a radio isotope. The term is explained later in this chapter. Half lives of the various radioactive elements vary over an extremely wide range — from a small fraction of a second to millions of years. Radium has a half life of about 1,590 16

1HALF- LIFE

©

© 3PM

2 HALF-LIFE

0

3 HALF-LIFE

4 HALF-LIFE

©

O

is a measurement of the •Miilicurie(mc) radioactivity given off by a radioisotope

Fig. 1-3.

The concept of half IIfe.

years, and ordinary uranium about 4.6 billion years. Radon, a radioactive gas resulting from the disintegration of rad ium, has a half life of 3.825 days. Artificial radioactivity may be induced in many different elements by bombarding them with nuclear particles, such as neutrons, alpha or beta rays, or other particles. In gen eral, such a bombardment may cause a transmutation of the nucleus with the formation of a different element, or it may produce an isotope which is radioactive. The large majority of artificial radioactive isotopes in use today are formed from bombardment with neutrons. A versatile mobile neu

(Courtesy Nuclear-Chicago Corp and Shell Fig. 1-4. The TNC mobile

Oil Co.)

neutron generator.

17

tron generator for the advanced

researcher is pictured in

Fig. 1-4. As a typical example, examine what may happen when the nucleus of an aluminum atom (13A127) is struck by a neutron. First, the nucleus absorbs the neutron (13A128) ; then a beta particle is given off, leaving a new element which is an isotope of sodium („NaM). This isotope is radioactive, and has a half life of 14.8 hours. The complete reaction may be indicated as follows: 13

Al"

+

..n1

-»

13A128

-*

^Na24

+

2He4

Not all isotopes

formed by neutron bombardment are radioactive. For example, the reaction mentioned previously where a neutron strikes a boron nucleus results in a stable lithium nucleus plus an alpha particle. This reaction is com monly used as a test for neutrons. Nuclear fission, which has been mentioned briefly, pro vides the basis for the operation of a nuclear reactor and for the explosion of an atom bomb. When a neutron strikes the nucleus of a fissionable material, the nucleus disintegrates, giving off two or three neutrons and a tremendous amount of energy. These neutrons in turn strike other atoms and produce further disintegrations, etc., setting up a chain re action. In the atom bomb this chain reaction is encouraged and allowed to proceed uncontrolled ; in nuclear reactors, it is controlled at all times so that no explosion takes place. The energy (heat) which is released can then be harnessed for useful purposes. Fissionable materials include an isotope of uranium and plutonium. When a fissionable atom disinte (o2U235) grates, many different elements may be formed. STANDARDS

With the foregoing background, you can now consider various methods of measuring nuclear radiation and its ef fect on the human body. In order to do this use of the stand ards which have been set up will be made. The curie is a basic unit of measurement which describes an amount of radioactive material. The technical definition is not of much use for our purposes, but the commonly ac cepted definition is that the curie is that quantity of radio active material which will produce 37.1 billion (3.7 X 1010) 18

disintegrations per second. For convenience, the terms millicurie (mc) and microcurie (/*c) are frequently used. These are, respectively, one-thousandth of a curie and one-mil lionth of a ourie. Another measurement of the quantity of radioactive ma terial which is sometimes used is the rutherford (rd). This unit is equivalent to one million disintegrations per second. As with the curie, prefixes can be added for convenience in order to give both larger and smaller units. A kilorutherford (Krd) is 1,000 rd, and a megarutherford (Mrd) is 1,000,000 rd. The relationships between the curie and the rutherford are as follows : 1

curie = 3.71 X

104

rutherfords

= 37.1 Krd

1

rutherford = 2.7 X 10° curie = 27/*c.

Radioactive isotopes are normally purchased by curies or millicuries. Thus, one millicurie of a radioactive iodine (or any other radioactive material) would be that amount which produces 37.1 million disintegrations per second. The by products of the disintegrations do not enter into the definition.

Fig. 1-5.

A

selection of radioactive standards National Bureau of Standards.

issued by the

19

The National Bureau of Standards is custodian of our standards of radioactivity. A special laboratory has been constructed for radioactive cobalt standardization, because of the comparatively low cost and availability of this isotope. The Bureau distributes various radioisotope standards at nominal prices. A group of standards is shown in Fig. 1-5. The measurement of the total quantity (or dose) received by an object such as the human body when it is exposed to radiation is another important measurement in the field of nuclear science. For this measurement, a unit called the roentgen (r) has been established. It is based on the ionizing capabilty of the radiation, which, in turn, is a direct meas ure of the effect of the radiation on the human body. The technical definition of a roentgen is of little value for our purposes ; however, it might be of interest. A roentgen is the amount of radiation which will produce one electrostatic unit of electricity of either sign in a cubic centimeter of air at standard temperature and pressure. In the average tis sue, one roentgen will produce an ionization equivalent to an energy concentration of 93 ergs per gram of tissue. This quantity of radiation is frequently called a rep, which is the abbreviation for roentgen equivalent physical. The impor tant point to remember is that the greater the dosage in roentgens, the greater will be the number of ion pairs formed in human tissue, and the greater will be the damage to the tissue. For small doses, the term milliroentgen (mr) , equal to one thousandth of a roentgen, is frequently used. The rem (roentgen equivalent wan) is another term that is occasion ally employed. It is based on the equivalent biological effects in man. The unit for specifying an absorbed dose of radia tion is called the rod, and is equal to 100 ergs per gram. This is very close to the 93 ergs per gram specified for the rep, so adoption of the rad to replace the rep does not require a change in numerical values of dosages given in rads. Some types of radiation are more damaging to the human body than others. In order to reduce all measurements to a common denominator, the term relative biological effective ness (RBE) is employed. Beta and gamma rays are used as the basis for damage and are considered to have an RBE of 1. Neutrons are much more destructive; slow neutrons have an RBE of 10. Fast neutrons are less destructive and have an RBE of 5. Alpha rays, the most destructive of all to body tissues, have an RBE of 20. 20

Many instruments in common use measure the intensity of radiation, usually in roentgens per unit time. Calibration may therefore be in terms of roentgens or milliroentgens per hour. To obtain the total dose, the intensity is multiplied by the time of exposure. For example, if a person is exposed to a radiation intensity of 2 milliroentgens per hour (mr/ hr) , for an 8-hour period, the total dose would be 16 milli roentgens. Instruments -for measuring the total dose are called dosimeters. RADIATION EFFECTS

ON HUMANS

It has been known for many years that excessive exposure to X rays or to radium products many harmful effects in the human body, such as damage to the bloodforming organs (leukemia), and inflammation of the irradiated area. It is also known that the results of such radiation can be cumu lative, i.e., the effects of many small doses over a long period Table 1-1. Maximum Radiation Dosages. TYPE

OF EXPOSURE

CONDITION

RADIATION WORKER: Whole body, head and trunk, active blood-forming organs, gonadf, or lem of eye.

1

feet

Bone

dose

year

13 weeks 1

year

13 weeks Body burden

HEM)

of years 5,000 mrem

5 times the number

beyond a year. 13 weeks

Skin of whole body and thyroid. Hands and forearms, and ankles.

Accumulated

DOSE

1 8 at

(3,000 mrem) 30 rem (30,000 mrem)

3 rem

10 rem (10,000 mrem) 75 rem (75,000 mrem) 25 rem (25,000 mrem) 0.1 microgram of radium 226 or its biological

equivalent Other organs

POPULATION: Individual, whole body Average, gonads

1

year

15 rem (15,000 mrem) (5,000 mrem)

13 weeks

5 rem

1 year 30-year

0.5 rem (500 mrem) 5 rem (5,000 mrem)

of time can add up to the same effect as a large dose in a short period. A tremendous amount of research has been expended in attempting to determine just how much radiation the human body can stand with neither short-range nor long-range harmful effects. Data has been accumulated from the time X rays and radioactivity were first discovered ; innumerable 21

(Courtesy Bendix Fig. 1-6. Typical results of shorMerm exposure

22

on humans.

Corp.)

experiments have been conducted on animals of various kinds. On the basis of this evidence, maximum dosages have been prescribed which indicate safe limits of exposure. De tailed information on this subject is contained in a report made by the Federal Radiation Council in May, 1960. The essence of this report is contained in Table 1-1. Effects of radiation can be both immediate and delayed. The late effects can be due to the fact that ionizing radia tions are capable of producing changes in individual genes and chromosomes. These changes are generally deleterious to the individual in his lifetime and to the future generations when they occur in the germ cells. A basic working guide has been set up by the Civil Defense Administration and others which indicates results that can be expected after ex posure to various doses. This guide, presented in Fig. 1-6, should be taken as a guide only and not a positive indication of exact results. Individual tolerances vary widely, making it impossible to predict exact effects in individual cases. Radiation affects many things besides the human body. For example, radiation will speed up the mutations in both plant and animal life because of its effect on the genes con trolling various hereditary factors. It can affect the poly merization of plastics and alter the properties of fabricated plastic materials. Some metals are affected by continuous exposure to high-intensity radiation. Sterilization of certain foods and drugs can be facilitated by proper use of radiation. Many of these effects will be discussed in greater detail in Chapter 7.

23

Chapter 2

Detecting Nuclear Radiation

There are many characteristics of atomic radiation which may be exploited in developing detection and measuring instruments. Such devices as cloud chambers, ionization chambers, Geiger tubes, and electroscopes depend on the ionizing properties of the rays for an indication of intensity. Some crystalline substances (for example, sodium iodide) will give off flashes of light or scintillate, when struck by atomic radiation. Other crystals, including diamonds, will change resistance when irradiated. Certain chemical indi cators change color to a degree determined by the amount of radiation received. Photographic film is sensitive to radia tion, and, more recently, solid-state detectors have been de veloped which are sensitive to radiation. These and other phenomena have been -applied in experi mental and commercial detectors. The discussions in this chapter include the basic devices and techniques most com monly used. CLOUD CHAMBERS

The Wilson cloud chamber, named after the British physi cist, C. T. R. Wilson, is probably the first and most widely used instrument for studying various kinds of rays and par ticles. It operates on the principle that when air saturated with water or other vapor is expanded suddenly, the air be comes supersaturated and tiny droplets condense on dust or any other particles which are present. These droplets persist long enough to be photographed. Ions are ideal as nuclei for condensing droplets. The air in the chamber is made dust-free and the ionizing rays under study are introduced. 24

When expansion takes place, a droplet is formed around each ion along the path of the ionizing rays, thus indicating the exact paths of the rays. An alpha particle ionizes very heavily; therefore, it pro duces a dense path of droplets. Beta particles ionize much less heavily, resulting in a much sparser number of droplets. Gamma rays, which ionize only slightly, form only a few droplets as they pass through the chamber. By applying an external magnetic or electric field to the chamber, the rays may be deflected, and the velocity of the particles in the rays can be calculated when field strengths are known. This technique has been applied in determining the energy of particles resulting from collisions, and for de tecting new particles which may result from nuclear reactions.

Fig. 2-1.

The Cfoudmaster

continuously

sensitive

cloud chamber.

A

commercial cloud chamber is shown in Fig. 2-1. This unit displays ionizing radiation tracks by maintaining a con tinuously supersaturated region of isopropyl-alcohol vapor near the bottom of the chamber. Droplets form around any ions produced in this region. A strong light assists in observ ing the tracks. Electrons are swept out of the chamber by a 1,200-volt potential so that droplets are formed on the posi tive ions only. A radiation source is provided with the unit; but even when this source is removed, occasional tracks caused by high-energy cosmic rays can be distinguished. 25

IONIZATION CHAMBERS

As the name indicates, ionization chambers depend on the ionizing properties of radiation for their activation. In gen eral, an ionization chamber consists of a cylindrical enclosure of metal (or glass coated with a conducting material) with a coaxial rod or wire centrally located within the cylinder and insulated from it. The total enclosure is sealed. It may con tain air, argon, or other gases and may be below, at, or above the atmospheric pressure, depending on the specific use for which it is constructed.

w

Fig. 2-2. Schematic of a Geiger tube which may be operated the proportional or the Geiger region.

in either

The schematic of an ionization chamber is given in Fig. 2-2. The central conductor (w) is operated at a positive po tential with respect to the outer enclosure. When ions are formed as a result of exposure to ionizing radiation, the pos itive ions are attracted to the negative outer enclosure, and the negative ions (usually electrons) are attracted to the center conductor. Thus, a current can be made to flow in an external circuit. The magnitude of this current depends on the amount of ionization. Fig. 2-3 shows the relative amplitudes pf current pulses which can be expected at various voltages for an ionization chamber connected in a circuit such as that shown in Fig. 2-2. For this analysis assume that there is a constant value of incident radiation which is affecting the tube. There are four general voltage ranges where radiation can be detected ; the tube operates differently in each of the four ranges. No exact voltages have been assigned to this dia gram, but there is a set of voltages that will apply for each style and size of tube which may be used. 26

ION COLLECTING RANGE Fig. 2-3.

CONTINUOUS DISCHARGE Relationship berween for a circuit »uch

puise output and applied voltage that of Fig. 2-2.

at

In the first section, from 0 to V,, the only current flowing through load resistor R (Fig. 2-2) is that due to the ions, which are produced directly from incident radiation. When the voltage across the tube is 0, there is no force that will draw the ions into the external circuit; consequently, no

current pulses are formed. If a small voltage is applied, some of the ions will pass through the external circuit; pulse am plitude depends on how many of the ions reach the tube elec trodes. Some will reach the electrodes ; the remainder will recombine without passing through the external circuit. As voltage across the tube is increased, more and more of the available ions will reach the electrodes and flow in the ex ternal circuit. Soon the voltage is high enough to sweep all the ions produced by the incident radiation to the electrodes before they recombine. This is the beginning of the hori zontal line in the diagram, known as the saturated region. As the voltage is increased through the saturated region, the ions will travel through the tube at greater speeds, but the total number of ions, and therefore the pulse size, will re main the same. When the voltage is increased above value V„ the ions acquire sufficient speed to ionize other atoms by collision, thus increasing the current and the size of the output pulse. Because of the geometry of the tube chamber, the strongest field is close to the center conductor, or wire, so that most 27

of the additional ionization takes place in the immediate vi cinity of the wire. Such ionization, caused by a series of suc cessive electron collisions, is called an electron avalanche. At any particular voltage in the proportional range the ava lanches produced by individual electrons are similar to each other. The net effect, then, is to amplify the original pulse size.

This is called gas amplification. An ionization chamber

operated in this region is called a proportional counter, be cause the resulting pulse size at any particular voltage is di rectly proportional to the number of ions formed due to radiation. Amplification factors as high as 1,000 to 10,000 are possible. The curve in Fig. 2-3 shows that the resulting pulse size, and therefore the amplification factor, in creases gradually through the range of applied voltages

from Y1 to V3. Beginning at the point identified as V3, each ionizing event, no matter how small, will initiate an electron ava lanche which spreads quickly through the entire tube. This point is called the Geiger threshold, and the section of the curve from V3 to V4 is known as the Geiger range. Charac teristic operation in this range calls for one large pulse for each ionizing event to which the tube is subjected. The pulse size does not depend on the amount of radiation from the outside source, but it does increase as greater voltages are applied. When voltages applied to the chamber are great enough to cause this condition, the chamber is operating in the Geiger region and a tube operating under these condi tions is called a Geiger tube. If a voltage greater than V« is applied to the tube, a single ionizing event, no matter how small, will start a discharge which will not stop until the external voltage is reduced. This is called the continuous discharge region. Little is to be gained by using this region of operation — there is no relation between the current through its external circuit and either the quantity or the amplitude of ionizing events. When the voltage is less than V3, the pulse amplitude is proportional to the amount of incident radiation. In the ioncollecting range the amplitude is very small ; it is difficult to measure the pulse with conventional equipment. Insulation resistance must be extremely high, and the measurement method must not take any power from the chamber circuit. Sometimes an electroscope can be used to measure the pulses, or the current may be amplified to a readable value with an electrometer tube. 28

When dealing with radiation, you are frequently con cerned with a single particle, or quantum, and the resulting ionization. Such ionization produces a single pulse of current in the external circuit, because all of the ions are formed al most simultaneously. Therefore it is necessary to consider the pulse size or magnitude resulting from a single ionizing event. Pulse sizes below the saturated region are small and variable, but in the horizontal portion of the curve they are constant for a given quantity of ionization. Incoming rays lose approximately 32 ev of energy for every ion pair formed. Assuming that all the energy is used to form ions within the chamber, rays with equal energy will form equal pulses. Conversely, if the rays have unequal energies, vari ous pulse sizes will be produced. The individual pulse size then indicates the number of ions formed by each ioniz ing event. In the proportional range, pulse size is still directly re lated to the number of ions in the ionizing event. But the am plitude of each pulse is greater because of gas amplification. This makes it easier to measure the relative pulse sizes with conventional equipment. Usually two stages of pulse ampli fication are sufficient to operate headphones or a meter. When the tube is operating in the Geiger region, there is no relationship between the amplitude of incident radiation and the size of an output pulse. There is, however, one out put pulse for each ionizing event. The pulse amplitude is large enough to operate headphones or a meter without any amplification.

1

1

U

f

1

i

(-

1

1 1

1

I

z o O

o

'1

o 900

l 1000

I

1 1100

1200

1 1300

1

1 WOO

VOLTS APPLIED TO COUNTER Fig. 2-4. The operating

plateau

of a typical Geiger tube.

29

The curve in Fig. 2-4 shows the performance of a typical Geiger tube over a wide range of voltages. As can be seen, there is quite an extensive region of operation where rela tively large changes in voltage produce only slight changes in pulse size. This is known as the Geiger plateau, and the slope and length of this plateau give an indication of the condition of the tube. Ordinarily, the plateau should be at least 100 volts long, and the slope should not be greater than about 0.01% per volt. GEIGER TUBES

Although, strictly speaking, Geiger tubes are ionization chambers, they will be discussed separately because they are so widely used and have so many characteristics which do not apply to other types of counters. A schematic diagram of a Geiger tube circuit is given in Fig. 2-2. When a tube is operated in the Geiger region, a discharge, once started, will continue unless something is done to quench or extinguish it. One technique is to make load re sistor R (Fig. 2-2) so large that the voltage drop across it re duces the tube voltage below the point where continued ionization can take place. Other types of external circuitry can also be applied to produce quenching, but self -quench ing counters are used almost exclusively. If an organic gas, such as alcohol vapor, is introduced into a Geiger tube, it has the ability to stop the ionizing process once the original discharge has taken place. Alcohol ions, when they reach the center conductor, are neutralized and then dissociated rather than forming new electrons for con tinuing the discharge. Thus, organic quenching takes place, and external resistor R in Fig. 2-2 can be low in value. One disadvantage of this type of quenching is that some of the organic material is used up every time a discharge takes place, placing an upper limit on the life of the tube. Counts of the order of 108 to 1010 (one hundred million to ten bil lion) are typical for organically quenched tubes. Another technique, called halogen quenching, is now in widespread use. This technique has one tremendous advan tage over organic quenching —the life of the tube is not lim ited by the quenching material. The quenching mechanism involves only a change in the halogen gas from the molecular to the atomic state and back again* none of the halogen is consumed in the process. One company manufactures tubes 30

which have operated for as many as 10" counts with no substantal change in characteristics. Originally, halogen quenching was subject to disadvantages, but these have been overcome in current designs. For example, the sensitive vol ume of the counter is reduced by about 80 to 95%, depend ing on the type of tube. This reduction is now considered in the original calibration. Also, the slope of the counting plateau is slightly greater than that attainable during the early life of organically quenched tubes. This latter disad vantage is readily overcome by the use of regulated power supplies. The quantity of halogen in a halogen counter is limited to a few micrograms. Halogen gases (chlorine, bromine, etc.) are very active chemically ; therefore tubes must be en gineered and built to prevent even these infinitesimally small quantities from reacting with the envelope and other elements. For example, one manufacturer uses stainless steel construction with ceramic insulators. When properly made, halogen-quenched Geiger tubes will give satisfactory performance over a temperature range of -50 to-f75°C, because the physical state of the gases utilized is not affected through this range. The tubes can neither be damaged by sustained over-current nor limited in life by operation. A group of typical halogen-quenched Geigercounter tubes is shown in Fig. 2-5.

rt

(Courtesy N. Wood Counter Lobs., Fig. 2-5. An assortment

Inc.)

of counter tubes.

Stainless steel, aluminum, and glass are used for Geigertube envelopes. When glass is used, the inside wall is coated with a conducting material, such as aquadag. The inner wall of the envelope is the electrical negative terminal for the tube. 31

Wall thickness for the envelope is determined by the type, or types, of radiation to be measured. The thickness is speci fied in terms of weight per unit area of envelope material, such as milligrams per square centimeter (mg/cm2). This method of specifying wall thickness is used because the depth of penetration of nuclear radiation is determined by the mass of material being penetrated.

All commonly encountered alpha and

held back by a wall thickness

beta radiations are

of about 300 mg/cm2, but

gamma rays will pass through it. So this is a common wall thickness for Geiger tubes that are designed to measure gamma radiations. This thickness provides adequate me chanical strength for any of the commonly used envelope materials. Stainless steel tubes and some small glass tubes are strong enough mechanically when they have wall thicknesses as thin as 30 mg/cm2. Glass wall tubes like this admit all gamma radiations and most beta radiations but exclude the weaker beta rays and all alpha radiations. Stainless steel tubes with walls this thin are common and have similar ray separating characteristics. The beta particles from strontium 90 have an energy of 0.65 mev ; when they are directed at a wall thickness of 30 mg/cm2, 31% of them will pass through the wall. At an energy of 1.712 mev, 72.4% of the beta particles will penetrate the wall. Mechanical considerations prevent the utilization of walls much thinner than 30 mg/cm2. If a thinner wall is desired, it may be applied in the form of a window at the end of the tube. In such a location the thickness can be reduced considerably while still maintaining adequate mechanical strength. Windows are commonly made of mica, glass, stain less steel, and pliofilm. The diameter of the window varies, but it may be as great as 1^ inches. For measuring alpha radiation the window thickness is generally less than 5 mg/cm2. Since the range of alpha par ticles is very limited in air, the source of radiation must be Table 2-1. Alpha Energy Requirements and Range WINDOW THICKNESS

MG/CM' 1.4

32

ALPHA INITIAL KINETIC ENERGY (MEV) greater

than

1.9

MEAN ALPHA IN AIR ICMI

RANGE

greater

than

1.0

2.0

2.6

1.5

3.0

3.6

2.2

4.0

4.5

2.9

placed close to the window in order to obtain a true indica tion. Table 2-1 indicates the alpha-particle energy required to penetrate various window thicknesses. The ranges in air for these alpha-particle energies are included for compari son. As an example, if the window thickness is 2 mg/cm2,

at least 2.6 mev of alpha-particle energy is required for the alpha particle to pass through the window and into the ioni zation chamber. An alpha particle with 2.6 mev of energy could pass through 1.5 centimeters of air. Many times it is desirable to use a single tube for different applications. For example, it may be desirable to measure beta radiation in the presence of gamma radiation, and vice versa. Such measurements can be made with a tube de signed for beta-gamma counting (thin wall, approximately 30 mg/cm2) by providing a sliding cover for cutting out the beta rays without eliminating the gamma rays. A measure ment with the cover removed then indicates both beta and gamma intensity, and with the cover in place, gamma inten sity alone. By subtracting gamma from the total, beta in tensity is obtained. The same principle can be applied to end-window tubes — shields of different thicknesses placed over the window provide selective absorption. With the large scale use of high-energy particle acceler ators and reactors the measurement of neutrons has become very important. Since neutrons produce very little ionization in a gas, some other method of detection must be used. A common technique is to fill a conventional tube with boron trifluoride gas (BF3) under pressure, particularly when thermal (slow) neutrons are to be measured. The boron contains a high percentage of the isotope B10 and has a high neutron capture cross section, i.e., it captures a high percent age of neutrons traveling through the chamber. When a neu tron strikes a B10 nucleus, a nuclear reaction takes place, as explained in Chapter 1. The resulting products are a lithium atom and an alpha particle having an energy of nearly 3 mev. This alpha particle ionizes the gas and produces a pulse in the output circuit. Another technique is to line the walls of the tube with metallic boron. The reaction is the same as when BF3 gas is used. A group of tubes filled with boron trifluoride for detecting neutrons is shown in Fig. 2-6. Gamma rays are very inefficient ionizers of gas in a counter tube; therefore, practically all of the ionization is due to secondary electrons emitted from the walls of the tube. To increase the number of secondary electrons pro33

(Courtesy Nuclear-Chicago Corp.) Fig. 2-6.

A

group of Boron Trlfluorlde detecting neutrons.

filled tubes for

duced by the gamma rays, the walls of the tube are made as thick as possible without introducing too much loss. Also, the tubes are sometimes lined with bismuth or other ma terial to increase efficiency still further. Operating potentials for Geiger tubes vary from 300 volts to 1,500 volts or more, depending on individual design. Cur rent requirements vary with the intensity of the radiation — the higher the counting rate, the higher the current drain. In general, the current through the Geiger tube and load re

sistor will be a few microamperes at most. The load resist ance in series with the tube varies from one to ten megohms. Individual pulses will produce a peak output of approxi mately one volt, sufficient to give an audible signal in head phones without any amplification. ELECTROSCOPES AND ELECTROMETERS

An electroscope charges repel each called the gold-leaf insulated from the

takes advantage of the fact that like other. In the simplest unit of this type, electroscope, the leaf and its support are container and all other materials, and charged to a potential of several hundred volts. The leaf is repelled from the support and remains in a repelled position until the charge disappears. If the insulation is very good 34

will leak off very slowly. However, if ionizing radiation is present, the charge will leak off more rapidly. The rate at which the gold leaf falls back towards the support then gives an indication of the intensity of radiation. and there are no ions present, the charge

MICROSCOPE

3

^SCALE

I

INSULATOR METAL SUPPORT

FIBER

'-•—CHARGING

KEY

(Courtesy National Fig. 2-7.

Lauritsen

electroscope

WINDOW

Bureau

of Standards.)

diagram.

The Lauritsen electroscope, diagrammed in Fig. 2-7, re presents a big improvement over the gold-leaf electroscope. The sensitive element consists of a fine metallized quartz fiber mounted on an insulated parallel metal support which may be charged from a battery or other source of potential. A small piece of quartz fiber mounted across the end of the metallized fiber serves as an index that is viewed through a microscope with an eyepiece scale. On being charged, the metallized fiber is deflected from the support and returns toward the position of zero charge when the gas in the cham ber is ionized. About 200 volts is required to produce fullscale deflection. Sensitivity is about two divisions per minute for one milligram of radium at one meter. Electroscopes require steady auxiliary potentials to pro duce electrical fields when measurements are started. Radia tion produces ionization, which discharges the electric field, causing the indicating fiber to change its position. The fiber is sometimes called a "needle." Electrometers also indicate the presence of radiation by means of a moving needle. However, this needle is the pointer of a voltmeter. When connected to an ionization chamber, the electrometer indicates the chamber potential. The rate at which the needle moves is determined by the amount of ionization produced per unit of time in the cham ber, which, in turn, is determined by the radiation intensity. 35

Electrometer tubes are widely used for amplifying ex tremely small DC currents. Tubes in which the control grid is very carefully insulated have been developed and operate with very low grid currents. The Type FP-54 tube used in the DuBridge-Brown circuit shown in Fig. 2-8 is such a tube. It operates with a very low value of filament-to-plate voltage (4 to 5 volts) ; yet it will amplify currents as small as 1014 ampere (0.01 micromicroampere) to readable values. This limit can be extended so that it is possible to measure very small amounts of ionization with the equipment under special conditions. 10,000 .n.

VM soil

12V

(Courtesy National Fig. 2-8.

Bureau

of Standards.)

DuBridge-Brown electrometer circuit.

Fig. 2-8 shows that a 12-volt DC source furnishes both filament and plate voltages. Thus, the electrometer can be made compact, a completely portable instrument. The G in the diagram is a sensitive galvanometer that will register plate current for the tube. The electrometer requires a separate ionization chamber for measurement of ionizing radiation. Current pulses from the ionization chamber are connected to the electrometer cir cuit across the high value of grid resistance. The terminals for chamber connections are shown at the left side of Fig. 2-8. SCINTILLATION

CRYSTALS

Certain materials give off small flashes of visible light when struck by alpha or beta particles, gamma-ray quanta, or neutrons. These flashes, which are called scintillations, may be detected and counted by a photomultiplier-type 36

photoelectric cell, thus giving an indication of the intensity of radiation in the vicinity. Sodium iodide (NaI) is widely used for the detection of gamma rays because of its high light output and efficient gamma-ray absorption. A small amount of thallium (Tl) is usually added to NaI crystals as an activator to shift the fluorescence into the spectral region most easily detected by photomultiplier tubes. The symbol for sodium iodide acti vated in this way is usually written NaI (Tl) . Since this ma terial is highly hygroscopic (absorbs moisture from the air) , it is normally supplied in a hermetically sealed container. Counters using scintillating crystals, called scintillation counters, have many advantages and are given a detailed treatment in Chapter 4. CONDUCTING

CRYSTALS

crystalline materials become slightly conducting when subjected to atomic radiation. Only crystals which are normally nonconducting can be used as radiation indicators, because the conduction current is very small. The first ex periments for investigating this property were made on crystals of silver chloride at the temperature of liquid air. Since then, it has been found that diamonds exhibit these characteristics at room temperature. In practice, the crystal is clamped lightly between two electrodes and a polarizing voltage of several hundred volts is applied. A pulse is then produced whenever the crystal is struck by an alpha or beta particle or a gamma-ray quan tum. The pulses developed are all small but have a consider able range of magnitude. Therefore a relatively high gain amplifier is required to produce a usable output. This method of measurement has not seen widespread use outside the laboratory, although it appears to have certain advantages in the measurement of gamma radiation. Be cause of the high density of the crystal, gamma sensitivity is much higher than with ionization chambers. The crystal has a tendency to polarize so that the magnitude of the output pulses decreases with continued use. Some

CHEMICAL INDICATORS Some solids and liquids change color when exposed to atomic radiation, the amount of color change giving an indi 37

cation of the quantity of radiation absorbed. Alkali halides, such as lithium fluoride, potassium bromide, and sodium chloride, have been used as chemical integrating indicators for both high energy photons (gamma rays) and nuclear par ticles. The so-called Fricke, or ferrous sulphate, chemical dosimeter is employed rather widely ; it is discussed in more detail in Chapter 5. Chemical indicators are employed primarily in the meas urement of extremely high levels of radiation intensity, and in dosage indications. The Fricke dosimeter, for example, is capable of measuring doses in the range of 1,000 to 44,000 rads. PHOTOGRAPHIC EMULSIONS Photographic film is one of the most widely used sensing elements for radiation. One example is its application in X-ray photographs of all kinds. It is used extensively in badges for monitoring the dosage of radiation received by personnel. Special types of films have been developed for various purposes. Sensitive films are employed for monitoring lowenergy beta and gamma rays and insensitive films for highenergy rays. Other films are better suited for indicating neutrons. Thick films can be used for detailed studies of the tracks followed by nuclear radiation. Film has the advan tage of providing a fairly permanent record of a nuclear event.

Film must

before it can be read, and the emulsions must be very carefully controlled if consistency of readings is desired. A densitometer of some type is neces sary to compare the film density with a standard. However, in spite of these disadvantages, improvements are continu ally being made and film is being more and more widely applied to radiation measurements. be developed

SOLID-STATE

DETECTORS

Recently it has been found that a reverse-biased silicon P-N junction makes a good particle detector. Its operation is closely analogous to the gaseous ionization chamber, in that an ionizing particle striking the sensitive portion of the detector will create hole-electron pairs (ions) which result in a pulse of current in the external circuit. 38

Solid-state radiation detectors have a number of basic ad vantages over other types. Solids in general have lower ioni zation energy, resulting in a larger number of ions per inci dent particle. Also, solids have greater stopping power for penetrating radiation, and can provide windowless opera tion. These detectors will be described more fully in Chap ter 4. Recently, it has been discovered that a silicon solar bat tery can be used to measure nuclear radiation. A specially prepared cell of the N-on-P type is employed, and the cell connected to a microammeter or sensitive current-indicat ing device. Intensities of as low as 100 rads per hour can be detected, and up to a billion rads per hour can be measured if the radiation is not too energetic. MISCELLANEOUS

There are several other techniques available for the de tection and measurement of atomic radiation, but most of them are either in the very early stages of development or are suitable for use only in laboratories where skilled per sonnel and highly specialized equipment are available. Since all of the energy is eventually converted to heat, the total amount of energy given off by a radioactive material can be measured by means of a sensitive calorimeter. How ever, since the total energy given off per unit time is small, except for large quantities of radioactive materials, only ex tremely sensitive equipment can determine this energy with any degree of accuracy. A microcalorimeter, which can measure the generation of heat at the rate of 0.005 calories per hour with an overall accuracy of 2 or 3%, has been de veloped for this purpose. Barium titanate appears to be affected slightly by nuclear radiation. If a barium titanate crystal is employed to control the frequency of an oscillator circuit, the frequency will shift slightly when the crystal is irradiated. A frequency meter will indicate the amount of frequency change, and if other conditions are carefully controlled, will thus give an indica tion of radiation intensity. Under suitable conditions, the intensity of alpha and beta rays may be measured by determining the rate at which an insulated receiver accumulates a charge. No gas can be pres ent when such measurements are being made, or ionization may upset the measurements. Relatively strong sources of 39

radiation are necessary ; for example, 100,000 beta particles per second is equivalent to a current of only 1.6 x 1014 am pere.

Many of the techniques described in this chapter have been perfected and applied to instruments of various kinds. Some of these instruments will be described in detail in the following chapters.

40

Chapter 3

Ionization

Counters

A

large proportion of the commercially available gas-tube radiation detectors use Geiger tubes for the detection ele ment. However, an appreciable share employ detectors which utilize other types of ionization phenomena for their operation. These include ionization chambers, proportional counters, and the like. This chapter is concerned with all such ionization detectors but will be primarily concerned with Gegier tubes. A basic requirement for all ionization-type counters is a source of high voltage for the operation of the ionization tube. High-voltage batteries can be used for this source; however, most manufacturers prefer to use low-voltage bat teries and various circuit techniques to boost the voltage to the desired value. The high voltage sweeps the ions out of the gas when an ionizing event occurs and thus produces a pulse for every such event. Once the pulses are obtained, it is necessary to use them in some manner. An indication of pulse rate is necessary to determine the intensity of radiation ; this indication can be given by counting the clicks per unit time in headphones, or a speaker, or by flashes of a neon light. As the circuitry becomes more complex, a meter and integrating circuit may be employed. The integrating circuit serves to smooth out the pulses, which usually occur at a very irregular rate, and steadies the meter reading so that it indicates the average rate at which pulses are being received. Such a device is called a ratemeter. Where very accurate indications of radioactivity are de sired, an instrument called a scaler may be employed. This instrument counts all the pulses which it receives and can be 41

used with any radiation detecting device that produces an individual pulse for each ionizing event. The selection of instruments for discussion in this chapter is by no means comprehensive, and should be taken only as illustrative of the many types available. Three general classes will be covered: (1) Geiger tubes, (2) ionization chamber, and (3) proportional counter instruments. In one or two cases an instrument may be equipped to handle two or more different types of tubes. GEIGER-TUBE

INSTRUMENTS

The various types of counter tubes which depend on ioni zation of a gas for their operation were described in some detail in the previous chapter, so those descriptions will not be repeated here. Rather, in this chapter we will de scribe a number of commercial instruments. In some cases, schematics are included. Portable Transistorized Units

A

portable, transistorized Geiger-counter survey meter designed for low- and medium-intensity radiation measure ments is pictured in Fig. 3-1. It has three ranges — 0.2, 2, and 20 milliroentgens per hour (mr/hr) full scale, equiva lent to 500, 5,000, and 50,000 cpm (counts per minute).

Model Fig. 3-1. Th« Atomaitere-Buntaine GSM5 traniittorized iurvey m»t»r.

(Courtesy Atomasters-Buntaine Corp.)

This instrument is available with two different probes. One incorporates a beta-gamma probe with a rugged metal Amperex 90 NB Geiger tube having a window density of 30 milligrams per square centimeter (mg/cm2). This thick ness effectively screens out alpha particles. 42

The other model uses an alpha-beta-gamma probe incor porating an Amperex 200 NB counter tube with a mica end window having a density of about 1.4 mg/cm2. This par ticular probe is designed for highly sensitive surveying and is provided with a protective plastic |grille over the window. Operating principles of this instrument can be determined by referring to the circuit diagram in Fig. 3-2. The highvoltage power supply uses a single PNP transistor (Q1), which operates as a blocking oscillator, producing pulses at a frequency of about 2 kilocycles. These pulses are stepped

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Fig. 3-2. Schematic of the Aiomasten-Buntalne

Model GSM5 survey meter.

43

up in voltage by transformer T1 and rectified by a conven tional voltage-doubler circuit. The count-rate circuit consists of a transistorized one-shot multivibrator (Q2 and Q3) driving a count-rate meter. Each time an incoming pulse of greater than 2 volts arrives, the multivibrator is triggered and produces an output. Potentio meters P2, P3, and P4 are range-calibration resistors. Transistor Q4 serves as an audio amplifier and produces a click in the headphones for each incoming pulse. A speaker could be driven in this manner, if desired, or the output could be fed to an external amplifier-speaker combination. This instrument is designed exclusively for battery opera tion and requires three standard size "D" flashlight batter ies. Such batteries will provide about 200 hours of operation. For longer life, alkaline cells (350 hours) , or mercury cells (in excess of 450 hours) may be employed.

(Courtesy Nuclear-Chicago Corp.) Fig. 3-3. The Nuclear-Chicago Series 2660 multirange, transistorized survey instrument.

The transistorized survey instrument pictured in Fig. 3-3 has a multitude of ranges. It has full scale ranges of 0.1, 0.3, 1, 3, 10, 30, and 100 mr/hr, and counts-per-minute ranges of 150, 1500, 15,000, and 150,000. It is available with two different probes —one, a side-window Geiger tube with a stainless-steel window of 30 mg/cm2 thickness, and the other an end-window Geiger tube with a window thickness of 1.5 to 2 mg/cm2. This instrument is suited for general laboratory survey work, such as checking suspected contamination areas, con tamination prevention, monitoring of isotope shipments and packing material, and a wide variety of other radiological survey and health-physics applications. It is also ideal for general prospecting and civil defense work.

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Schematic of the Nuclear-Chicago Series 2660 survey meter.

45

A circuit diagram of the unit pictured in Fig. 3-3 is given in Fig. 3-4. The power supply consists of transistor oscillator Q5, step-up transformer T1, and the conventional half -wave rectifier with a corona voltage regulator. This system pro

vides the necessary 600 volts for operating the Geiger tubes. The monitoring circuit consists of an emitter-coupled monostable multivibrator (Q2, Q3) triggered by an emitterfollower amplifier (Ql). Output pulses from this multi vibrator are fed to the integrating circuit and to the meter. For headphone operation, buffer amplifier Q4 is included. Four conventional size D flashlight cells provide about 300 hours of operation at about 8 hours per day. Recording-Rate Meter

The recording-rate meter shown in Fig. 3-5 is quite dif ferent from conventional survey meters. It indicates the ra-

(Courtesy Gelman Instrument Fig. 3-5. The Gelman

46

Model 31100 recording-rate

meter.

Co.)

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fiflflflflfifi

recording-rate

meter.

47

diation intensity in any given area and makes a permanent record on a chart. A single chart will provide a continuous record of radiation intensity for up to 31 days. The unit can Geiger-tube be equipped with a Mullard Type MX-115 gamma probe, or a Mullard Type MX-108 end-window beta probe. Both tubes are the halogen-quenched type. The ranges are 0.1, 1, 10, and 100 mr/hr full scale. Al though readily portable, it is not battery operated and must be used where 11 7- volt, 60-cycle current is available to power the unit. A circuit diagram of the recording-rate meter is given in Fig. 3-6. High voltage for the Geiger tube is obtained from a secondary winding on the power transformer and rectified by a half -wave rectifier. The series of neon lamps form a voltage regulator. Plate supply for the vacuum tube is pro vided by another secondary winding on the power trans former and a bridge-type rectifier. Pulses from the Geiger tube are amplified by the E80CC dual triode tube and fed to the integrating circuit and the miniature recorder. The meter of the recorder has a fullscale range of one hundred microamperes. The stylus is pressed against pressure-sensitive chart paper to record the deflection of the microampere. A small motor and gear wheel, driven at 2 rpm, causes the stylus to strike the paper once every two seconds. Because of the com paratively rapid succession of dots thus impressed, a contin uous trace is obtained without the use of ink, electric cur rent, or heat. The standard chart speed is 1 inch per hour, although other speeds can be obtained by changing the gear train. At 1 inch per hour, the standard chart will provide 31 days of continuous operation. Pocket Meters

Two other interesting meters are shown in Fig. 3-7. The radiation alarm (Fig. 3-7 A) produces an audible sound con sisting of a violent crackle at 1 mr/hr, a violent buzz at 10 mr/hr, and a whistle at 100 mr/hr. This sound is emitted by the tiny speaker which is visible in the photograph. A high range model is also available in which the output sound is a violent buzz at 100 mr/hr. This unit is small enough to be slipped into a shirt pocket, and weighs only 5 ounces. It is powered by two standard penlight batteries. 48

(A) Fig. 3-7.

Radiation meter.

(B) Sparrow

radiation alarm.

Pocket radiation m«i«r and Sparrow radiation alarm instrument.

The pocket radiation meter (Fig. 3-7B) has a two-range compressed scale. The standard instrument has ranges of 0.1 to 50 mr/hr, and 0.01 to 1 r/hr full-scale. For the highrange unit the scales are 1 to 500 mr/hr and 0.2 to 50 r/hr. The instrument weighs only 8 ounces, and is powered by two standard penlight batteries. Radiometer

A small, readily portable, transistorized

survey meter called a radiameter is shown in Fig. 3-8. It has a variety of ranges, depending on the type of Geiger tube and window employed. For example, with a low-dosage Geiger tube and a window thickness of 650 mg/cm2, gamma measurements of 0.5 and 25 mr/hr and 1 r/hr full scale are available. With the same Geiger tube and a window thickness of 40 mg/cm2 the unit can be used for both beta and gamma measurements in the ranges 0-320 and 0-10,000 counts per minute. With a high dosage counter tube and window thick ness of 650 mg/cm2, the range is 0-50 r/hr. To avoid any possibility of error in reading this instru ment, the control knob simultaneously changes the circuitry, measuring scale range, and tube shielding. A basic block diagram showing the operating principles of this instrument is given in Fig. 3-9. A transistorized highvoltage generator and the regulator tube supply the opera ting voltage for the Geiger tube. 49

(Courtesy Kahl Scientific Instrument

Corp.)

Fig. 3-8. The Kahl Model FH40TV radiometer.

Pulses from the Geiger tube are fed to the transistorized matching stage and then to a pulse-former stage. This stage

£Z3

HIGHVOLTAGE GENERATOR

_\

GEIGER TUBE

REGULATOR

T

PULSE-

4~l

FORMER STAGE

INTEGRATING AND INDICATING SECTION

AMPLIFIER

Fig. 3-9. Block diagram of the Kahl Model FH40TV

50

radiometer.

EARPHONE CONNECTION

transforms the counter-tube pulses of varying energy into square-wave pulses of a definite size and feeds them to the integrating and indicating section. For headphone opera tion, pulses are fed to the earphone amplifier.

(Courtesy Kahl Scientific Instrument Corp.) Fig. 3-10.

The Kahl Model FH40K miniaturized, radiometer. transistorized

A unit

which has been miniaturized still further is shown in Fig. 3-10. This unit has a single logarithmic scale of 0-50 mr/hr, and weighs only 7 ounces. Overall dimensions are 4 by 2% by 1% inches. Power is supplied by two penlight cells. Radiation Monitor

A self-contained radiation monitor which measures gam ma radiation, detects beta radiation, and is provided with a visual and audible alarm is shown in Fig. 3-11. It is operated by power from the ordinary 117-volt, 60-cycle house cur rent. Nominal range is 0-10 mr/hr, but it can be provided 51

with a variety of other ranges, such as 0-20, 0-30, 0-40, or 0-50 mr/hr.

n/4

X

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(Courtesy Victoreen Fig. 3-1 1 . The Victoreen

Instrument

Co.)

Vamp Model 808

radiation monitor.

This instrument contains a number of unique features.

For

example, a minute quantity of

or U238 is mounted next to the Geiger tube. This provides a continuous, small meter reading when the instrument is operating properly and represents the operating zero point on the scale. If the meter reading drops to true zero, it indicates that the instrument is not operating properly. This is a "fail-safe" indication. Also, adjustments can be made so that a light will flash and a loud, warbling tone will be emitted by the speaker if the radiation level exceeds a certain prede termined value. The circuit diagram for this unit is given in Fig. 3-12. As can be seen, the circuit is transistorized, and the power sup ply is more or less conventional. High voltage for the Geiger tube (780 volts) is regulated by V3, a corona regulator, and the low-voltage supply of 10 volts for the transistor cir cuitry is regulated by zener diode CR7. When an ionizing event occurs, a pulse is fed through C2 in order to trigger the monostable multivibrator circuit (Q1 and Q2). The average collector current through Ql is a function of the number of pulses received per second; this is the current indicated by meter M1. An alarm circuit consisting of Q5 and relay K1 is incorpo rated. If the radiation level exceeds a certain predetermined value, Q5 starts oscillating and produces a loud tone in the 52

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Fig. 3-1 2. Schematic of the Vamp radiation monitor.

53

speaker. Also, relay

K1 operates, switching on light

13

and

discharging integrating capacitor C5. Q5 continues to con duct by virtue of the discharge of capacitor C8 through the Q5 base circuit. When C8 has discharged sufficiently, Q5 ceases to conduct and relay K1 opens. Light 13 goes out and the tone from the speaker ceases. If the ambient field is still above the trip level, the cycle will repeat itself at a rate de termined by the field. If it is below, the alarm remains quiet. The "fail-safe" relay (K2) is actuated whenever Q4 con ducts. When everything is operating properly, the internal radioactive source will provide an average voltage across R16 which will keep Q4 conducting. If the voltage should drop below this value for any reason, Q4 will cease to con duct, causing relay K2 to drop out and I1 to light up, indi cating that the device is not functioning properly. This "fail-safe" operation can produce a signal at a remote point, if desired. Low-Level Counter

A

Geiger counter for measuring low-energy beta rays at extremely low levels is shown in Fig. 3-13. It novel

Fig. 3-13.

The Atomasters-Buntaine

Model G-301 for measuring low-energy beta rays.

(Courtesy Atomasters-Buntaine Corp.)

ability by means of a double counter and an anticoincidence circuit that is arranged to provide a back ground count of well under two per minute. This unit incorporates a dual detector assembly that con achieves this

sists of a sample and a guard detector. The purpose of the guard detector, which practically surrounds the sample de 54

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tector, is to provide for cancellation of external background radiation which passes through both detectors simultane ously. This is accomplished by the anticoincidence circuit (Fig. 3-14), which is arranged to register only those counts in the sample channel not appearing in the guard channel. The instrument is heavily shielded with both iron and mercury to keep the background count as low as possible (approximately 8 counts per minute) . Reduction of the effec tive background to under 2 cpm is accomplished in the anti coincidence circuit, which can be arranged to read out both channels, or to read only the output of the sample channel. The sample detector has a mica window with a thickness of 1.4 mg/cm2. In general, the sample will emit only beta radiation, such as carbon-14. Because of the anticoincidence characteristics and careful shielding, accurate measure ments can be made on samples having very low activity. IONIZATION COUNTER INSTRUMENTS

A

gun-type survey meter which uses an ionization cham ber as the radiation detector is shown in Fig. 3-15. It has a three decade logarithmic scale for indicating radiation in tensities from 3 mr/hr to 3 r/hr full scale. The ionization

Fig. 3-15. The Baird-Atomlc Model 414 gun-type survey meter.

(Courtesy Baird-Atomic, Inc.)

chamber is 3 inches in diameter and 6 inches long, and has an active volume of 600 cubic centimeters. The beta window is Mylar plastic coated with aluminum and has a thickness of less than 0.9 mg/cm2. The unit consists essentially of an ionization chamber and a triode-connected subminiature pentode vacuum-tube pulse amplifier (Fig. 3-16). The indicating meter (M1) is in the plate circuit of the tube. 56

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57

Power is provided by batteries. The 67.5 volts used on the ionization chamber is obtained by connecting three 22.5volt batteries in series. Switch positions are provided for checking the voltages of the various batteries. Electrometer Units

The survey meter shown in Fig. 3-17 has found wide spread acceptance wherever radioisotopes are handled. It is an ionization-chamber instrument and is designed for meas uring beta particles, and gamma and X rays. Two versions

(Courtesy Technical Fig. 3-1 7. The Technical

Associates

Associates.)

Cutis Pie Model CP-3 survey meter.

are available : one version has ranges of 50, 500, and 5,000 mr/hr full scale; and the other has 25, 250, and 2,500

mr/hr. The unit is battery operated and completely portable. The ionization chamber has a volume of 36 cubic inches and a rubber hydrochloride window 0.45 mg/cm2 thick. It is filled with air at atmospheric pressure as the ionization medium. The schematic diagram of Fig. 3-18 shows that the indi cating portion of this instrument is essentially an elec 58

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Cutie Pie Model CP-3.

59

trometer consisting of VI and associated circuitry. The electrometer tube is a specially constructed pentode operated as a triode. It requires a very low-power signal on its grid, permitting the use of an extremely high-value grid resistor. The plate current flows through microammeter M1, which provides a visual indication of the level of radiation. Because the electrometer tube will always draw enough current to cause a slight deflection of the meter, a battery (64) is used to buck out this current and allow a true-zero meter reading. A photograph of another ionization-chamber survey meter is shown in Fig. 3-19. The circuitry of this instrument is quite similar to that of the unit in Fig. 3-18 ; therefore, it is not reproduced here. This instrument is primarily intended for the inspection of flat surfaces and is widely used wher ever a high degree of accuracy is desired.

(Courtesy Technical Associates.) Fig. 3-1 9. The Technical

Associates

Juno Model 7

survey meter.

The unit in Fig. 3-19 is designed to measure the intensity of, and discriminate between, alpha, beta, and gamma radia tion. Discrimination is accomplished by two absorbers in order to reject either alpha or beta radiation. Standard ranges for the instrument are 50, 500, and 5,000 mr/hr full 60

scale, and 250, 2,500, and 25,000 model.

mr/hr for the high-range

Gamma-Radiation Detector

A

portable survey meter of the ionization-chamber type, which is designed to measure gamma radiation over a broad energy range and at extremely low intensities, is shown in Fig. 3-20. It is especially useful where stray leakage radia tion is likely to be encountered, such as that resulting from X-ray machines, high-powered RF transmitter installations, and wherever high-voltage equipment is likely to produce X rays.

(Courtesy Victoreen Instrument Fig. 3-20.

The Victoreen

Co.)

Model 440 survey meter.

The lowest range of this instrument is 0-3 mr/hr, mak ing it easy to detect radiation intensities as low as 0.3 mr/hr. Thus, it is well suited to the measurement of gam ma radiation encountered in health-physics problems. Four other ranges are provided: 0-10, 0-30, 0-100, and 0-300

mr/hr.

The ionization chamber of this instrument is filled with air at atmospheric pressure. An end window of Mylar 1 mg/cm2 thick permits the entry of alpha, beta, and gamma radiation. An aluminum end cap is provided for betaparticle discrimination. 61

a> _ m- to- «

-

(Courtesy Victoreen Instrument Fig. 3-21.

62

Schematic of the Victoreen

Model 440 survey meter.

Co.)

This instrument uses a vibrating reed electrometer to pro vide high sensitivity and stability. Operation of this electro meter can be studied with reference to the circuit diagram in Fig. 3-21. The vibrating reed is driven at its natural resonant fre quency of 300 cps by the transistorized oscillator (Q6 and Q7). This reed causes the capacity of input capacitor C2 to vary at a 300 cps rate, and in turn converts the input volt age across R2 to a 300 cps signal. This signal is then ampli fied by V1, Q1, Q2, and Q3 and is fed to a discriminator circuit where its phase is compared to the phase of the os cillator voltage. The resultant voltage is detected and used as a feedback voltage—the amount of feedback is deter mined by the resistance attenuator and hence determines

the range of the instrument. A saturation circuit is included in this instrument in order to prevent the input voltage from becoming excessive when the ionization chamber is subjected to extremely high fields. Its operation can be studied by referring to the circuit .. ;.

(Courtesy Victoreen Instrument Fig. 3-22.

The Victoreen

Rodgun

Co.)

survey Instrument.

63

diagram. The chamber collection voltage is supplied through resistor R30 which also functions as the load resistor for Q8. Normally Q8 is cut off so that the chamber voltage is de termined by the supply voltage. When the chamber is sub jected to an intense field, the output voltage of the dis criminator causes Q8 to conduct through CR4. Conduction of Q8 drops the chamber voltage to a point where the meter remains just off scale. The circuit will keep the meter off scale at intensities up to and exceeding 1,000

r/hr.

The power supply regulator consists of Q4, Q5, and CR1 connected in a conventional series type regulator with CR1 being used as the reference voltage. A portion of the input voltage is coupled to the base of Q4 through R17 to com pensate for variations in the input battery voltage. Still another type of ionization-chamber survey meter is shown in Fig. 3-22. It has a tremendously wide range, being capable of reading intensities from 0.01 mr/hr to 10,000 r/hr in three ranges. It is equipped to discriminate between gamma and beta-gamma levels with a simple movable end window. The ionization chamber is filled with argon gas under pressure. Isotope Analysis Kit

The isotope analysis unit shown in Fig. 3-23 is an inter esting variation from the more conventional survey meters. Essentially, it consists of an ionization chamber with an electroscope for indicating the voltage on the chamber. This instrument is somewhat similar to the ionizationchamber type of pocket dosimeter which will be discussed at some length in Chapter 5. The electroscope and ionization chamber are charged up to about 145 volts with an external voltage source. This produces a reading of about zero on the electroscope. As ionizing radiation enters the chamber, ions are formed which are swept to the chamber electrodes, discharging the chamber and therefore reducing the voltage across it. This reduction in voltage causes the electroscope indicator to change its position. The change in electroscope reading per unit time then gives an indication of the inten sity of radiation, and the actual deflection indicates the total quantity of radiation received by the ionization chamber. The change in chamber voltage for full-scale movement of the quartz fiber of the electroscope, the electrostatic ca64

(Courtesy Landsverk Electrometer Fig. 3-23. The Landsverk

Co.)

Model L-75D isotope analysis unit

pacity of the system, and the volume of the ionization chamber are all factors in determining the sensitivity of a unit of this type. In this instrument these constants are 52 volts, 3 micromicrofarads, and 200 cubic centimeters, re spectively. Actual sensitivity is hard to pin down precisely in a unit of this type, but a rough idea can be obtained from the following. Assume a 150-cc sample of a liquid solution of the phosphorus 32 isotope; if the measured drift of the quartz fiber of the electroscope is six divisions per hour on the 100-division scale (0.1 division per minute) , the quantity of radioactive material in the solution is indicated at about per cubic centimeter — a very small 15 micromicrocuries quantity of radioactivity. This unit is useful for measurements of very small amounts of radioactivity. For example, it can be used for measuring the radioactive contamination in food and drink ing water, thus determining if these materials are fit for human consumption. PROPORTIONAL COUNTER The final instrument to be described in this chapter (Fig. 3-24) uses a proportional counter for the active element. 65

(Courtesy Nuclear-Chicago Corp.) Fig. 3-24.

The Nuclear-Chicago Model 2112 survey meter.

Two different probes can be employed— the one in the cen ter in Fig. 3-24 is for measuring alpha radiation and uses an air proportional-counter tube; the probe at the right is a neutron counter and employs an enriched BFS propor tional detector. A transistorized high-voltage supply (Fig. 3-25) mounted in the case provides the voltage for either probe. It consists 13) RE 042

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66

Schematic of the high-voltage supply for the Model 2112 proportional counter.

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of a transistor oscillator, a step-up transformer, and solidstate rectifier. This supply can provide 1,500 to 2,300 volts, depending on the counter tube used. Each probe is equipped with its own built-in transistor ized preamplifier which gives an output of about 0.25 volt. The preamplifiers are practically identical; therefore, only one circuit diagram is given here (Fig. 3-26). It is a straightforward three-stage preamplifier operating from an 8-volt battery power supply. The alpha probe uses an unsealed, air proportional counter for measuring surface alpha radiation. It will detect from 2 to 2,000 alpha particles per square cm per minute and operates at a voltage of 2,000 to 2,150 volts. Surface area is 75 cm2, and the window is rubber hydrochloride with a conductive coating. The neutron-probe proportional-counter tube is filled with boron trifluoride gas to a pressure of about 20 cm of mer cury and operates at about 1,400 volts. It will detect thermal neutrons, or fast neutrons when equipped with a removable shield of 1-inch thick paraffin. The operation of this type of counter tube was described in Chapter 2. Each type of counter has advantages for certain applica tions ; selection of the proper instrument for a given applica tion is greatly simplified, if some basic knowledge of the operating principles has been acquired.

68

Chapter 4

Scintillation and Solid-State Counters

Solid-state counters have been included in the same chap ter with scintillation counters because most scintillation counters are solid state devices. Solid-state counters could have just as easily been included in the chapter on Geiger counters ; in fact, one of the effects of radiation on a solidstate counter is essentially a form of ionization, as in gas counter tubes. SOLID SCINTILLOMETERS

In Chapter

1

it was mentioned that certain materials will

give off flashes of light (scintillate) if struck by nuclear radiation. These flashes of light can be detected and counted, giving the basis for the scintillation counter or detector. A scintillation detector can be a very simple, inexpensive device. At one time a unit selling for as little as a dollar was on the market. It consisted primarily of a small tube with some scintillating material mounted inside one end and a small hole and magnifying glass at the other end. By look ing through the glass, flashes of light could be seen in the scintillating material when radiation was present. Counting the number of flashes per minute would give a rough idea of. the intensity of the radiation. This device has many limitations. The operator's eye must be adapted to the dark before he can see the flashes, and the source of radiation must be close to the end of the tube which holds the scintillating material. Such a device is not very useful for general survey work or for prospecting, but it has some value as a demonstration, or instructional, de 69

vice and can be used to check clothing, rocks, etc. for the presence of radioactivity. In commercial scintillation counters the quantity of active material is, in general, much greater than in the previously mentioned simple device. The flashes of light (scintillations) are detected by a photoelectric tube which creates an elec trical impulse for each flash. These impulses are then amp lified and counted. Often an indication of their amplitude is also given. The reason for this is that the intensity of each flash is proportional to the energy of the particle or gammaray photon striking the material. Thus, measuring the am plitude of the pulse and determining the pulse repetition rate offers a means for measuring both the intensity of radiation and its energy. ANODE

CATHODE

OUT

Fig. 4-1 . Operation of a photomultlplier

tube.

Since the flashes of light given out by the scintillation ma terial are in general quite faint, a very sensitive photoelec tric tube called a photomultiplier tube is used to detect them. Fig. 4-1 shows the operation of a photomultiplier tube. It consists of a photocathode, a series of dynodes, and an anode. The photocathode serves to detect the flashes of light — it gives off electrons when light strikes it. These elec trons are attracted to the first dynode, which is at a higher positive potential than the cathode. The dynode is made of a material which gives high secondary emission — that is, for every electron that strikes it, several electrons are given off. These electrons are then focused to strike a second dynode which is operated at a higher positive potential than the first dynode. Here again, several electrons are given off for each electron that strikes it, leading to further multiplication. This process is carried on for several stages — usually 10 —so that there may be as many as two million electrons striking the final anode for every electron emitted by the cathode. Thus, the tube may be said to have a gain of upwards of two million; it provides a useful output signal for extremely weak flashes of light. 70

In practice

the voltage difference between two adjacent dynodes is around 100 volts so that the photomultiplier tube requires approximately 1,000 volts DC for proper opera

tion. This voltage can be provided by a conventional highvoltage transformer and rectifier combination with proper filtering. For portable units, a battery and interrupter may be used in the primary circuit of a properly designed trans former in order to produce the desired high voltage, or a pulse generator and a "flyback" system can be used.

Fig. 4-2. Throo photomultiplier tubes mounted on a thallium-activated sodium-iodide crystal.

(Courtesy Harshaw Chemical Co.)

Since streams of electrons are extremely sensitive to both magnetic and electrostatic fields, photomultiplier tubes must be carefully shielded. An assembly of three photomultiplier tubes mounted on a single large scintillation crystal is shown in Fig. 4-2. As can be seen, the photomultiplier tubes are Table 4-1. Characteristics of scintillation materials.

SCINTILLATOR

WAVE LIGHT

GM/CM'

OF EMISSION

YIELD

DECAY TIME

1.25 1.16

1.00 0.73 0.55 0.15

.025 .007 .012 .075

DENSITY

LENGTH

REMARKS

Napthalene

1.23 1.15

4,450 4,100 4,150 3,450

ZnS (Agl

4.1

4,500

2.0

Nal (Tl)

3.67

4,100

2.0

Ccl (Tl)

4.51

white

1.5

p-Terphenyl in Xylene

0.87

3.700

0.48

.007

Liquid

solution

Terphenyl In polystyrene

1.06

4,000

0.30

.005

Plastic

solution

Anthracene Stilbene Terphenyl

Large crystal,

not clear

Good crystals Good crystals Good crystals Small crystal, poor transparency

1

.25

Excellent

Excellent

1

crystals,

but

hygroscopic crystals

(Courtesy of Baird-Atomic, Inc.)

71

carefully shielded. If it is desired to keep the background count extremely low, the base is removed from the photomultiplier tube and the string of voltage divider resistors, which provide the proper voltage for the individual dynodes, is mounted in the stainless steel cup shown in the photo graph. This technique minimizes leakage currents. The characteristics of some of the most commonly used scintillators are given in Table 4-1. As can be seen, the ma terial called NaI (Tl) can be formed into excellent crystals and gives a high light output. However, it is hygroscopic; that is, it readily absorbs water from the air and dis integrates. Therefore it must be protected from the air. In spite of this disadvantage, it is probably the most widely used scintillator. The symbol NaI (Tl) stands for thallium-activated so dium iodide. Addition of a small amount of thallium serves to shift the frequency of the light output into a region more easily detected by the photomultiplier tube. A material not mentioned in the table is Europium-acti vated lithium iodide [LiI (Eu)]. This material is effective as a thermal neutron detector because there is a good prob ability that a thermal neutron will interact with a lithium atom to give an alpha particle and triton, each with a con siderable amount of energy. Inorganic crystals, such as those indicated in Table 4-1, have many good points when used as scintillators. However, organic crystals or phosphors have certain advantages which make them extremely useful in some applications. For ex

( Courtesy

Fig. 4-3.

72

Nuclear Enterprises,

An assortment of plastic phosphor

NE102.

Ltd.)

ample, the decay time is much more rapid than with inor ganic crystals ; this means that they can separate very closely spaced pulses and therefore can detect pulses that are gen erated at a much higher rate than can the inorganic type.

For

example, organic phosphors are suitable for counting in the millimicrosecond range. Also, these materials are nonhygroscopic and are much less susceptible to shock. Organic phosphor NE102 is an example of a com mercially available organic material. It is essentially a mix ture of scintillation crystals in a plastic material known as polyvinyltoluene. It can be provided in a wide variety of shapes and sizes and is easily machined. An assortment of shapes and sizes of this material is shown in Fig. 4-3.

(Courtesy Nuclear Enterprises, Fig. 4-4.

A

Ltd.)

flow counter formed of NE102 plastic-phosphor capillary tubing.

plastic-phosphor

To demonstrate the versatility of this plastic scintillation material, Fig. 4-4 is included. This picture shows a length of capillary tubing made of NE102 coiled into a flow counter. The overall diameter of this flow counter is 2 inches so that it may be viewed by a single 2-inch photomultiplier tube, or a tube may be mounted on each side. SCINTILLATION

PROBES

To better understand how these various solid scintillation materials are employed, a few commercial applications will 73

be described. These descriptions are, of necessity, brief, but they give a general idea as to how commercial scintillation counters are designed. Universal Scintillation Detectors

A universal scintillation detector and photomultiplier as sembly is shown in Fig. 4-5. The different heads are for measuring alpha, beta, gamma, and neutron rays. Sealed NaI (Tl1) crystals are used with this unit.

(Courtesy N. Wood Counter Lab.) Fig. 4-5.

N. Wood Counter Laboratories scintillation

Model SC-1U universal

counter.

The object in the top of the picture is a probe-type counter for clinical and laboratory use. It contains a 1-inch length of 1-inch diameter sealed NaI (Tl ) crystal and has a built-in lead shield to reduce stray counts. It also has a de tachable collimator to permit checking the direction from which radiation is coming. Mu-metal magnetic shields com pletely surround the photomultiplier tube in order to prevent stray magnetic fields from affecting its operation.

Model 81 SB Fig. 4-6. The Boird-Atomie scintillation probe.

(Courtesy Baird-Atomic, Inc.)

Another versatile scintillation probe of many uses is shown in Fig. 4-6. This probe features a large, 3-inch crystal 74

m±°s

h

it

B

-HtX

Ur

Fig. 4-7.

|°s

Schematic of the Model 81 8B scintillation

prebe.

75

that may either be solid or may be provided with a well for special applications. Again the crystal and phototube sec tion is shielded by Mu-metal. This probe comes equipped with a transistorized preampli fier with a gain of 10 so that it provides about 10 times as large an output pulse as could be obtained from the photo tube alone. A DuMont 6363 photomultiplier tube is used. The schematic of the probe shown in Fig. 4-6 is given in Fig. 4-7. This diagram is not included to encourage anyone to build his own but rather is intended to show the basic connections for a photomultiplier tube and also to show how transistors are used in instruments of this type. As can be seen, an external source of high voltage (500 to 1,400 volts) is necessary to power the photomultiplier tube. The transistors are powered by an external source of 220 to 280 volts at 10 ma. With a 200-volt power supply, output pulses of up to 4 volts can be obtained; with 150 volts, the range is up to 3 volts. The pulse is taken off the last transistor (Q105) through capacitor C1 11. Output impedance at this point is 100 ohms. The rise time for this unit is 0.2 microsecond, and the decay time 1.6 microseconds. Alpha-Ray Probe The probe pictured in Fig. 4-8 is designed primarily for detecting and measuring alpha rays. It is about 6 inches in diameter; therefore a large active area is provided. The stainless steel cover has 19 holes, each one an inch in di-

Fig. 4-8. The Balrd-Atomic Model 870 alpha scintillation probe.

"■V

J (Courtesy Baird-Atomic, Inc.)

ameter. Thus, a sensitive area

of approximately

100 square

centimeters is provided. The sensitive material in this probe is zinc sulfide activated with silver [ZnS ( Ag) ] . It is covered with a very thin layer of aluminized Mylar. To direct the flashes in the scintillation material to the photomultiplier 76

tube a silicone fluid called DC-200 is employed as an optical coupling medium.

Medical Probes

A

series of miniature scintillation probes that are very useful in medical applications is made by the Nuclear-Chi cago Corp. These are called the DS8 series; each unit con sists of a very small housing (1" x 10") which contains the photomultiplier tube and preamplifier. This basic unit can be used with a number of different probes. The DS8-1 set with several different probes is shown in Fig. 4-9. With the longest needle probe in place the complete unit weighs less than 8 ounces, making it very easy to manipulate in body tissues.

(Courtesy Nuclear-Chicago Corp.) Fig. 4-9. The Nuclear-Chicago

Model DS8-1 surgical scintillation-probe tet.

Each probe contains a beta-gamma

sensitive crystal at its

tip [usually NaI (Tl)]. The smallest diameter probe is de signed primarily for neurosurgical use in determining the depth and limits of brain tumors. The other needle probes find important use in the location and identification of radioactive concentrations in tissue, such as the thyroid, dur ing surgery and for estimating radiation levels delivered to various body organs from therapeutic doses. The probes are sensitive only at the ends with an equal distribution of sen sitivity around the tip. 77

This basic unit can also be equipped with a very long light pipe and probe tipped with a thermal neutron sensitive phosphor. A pipe up to 42 inches long can be used, thus permitting the probe to be inserted in a nuclear reactor. The phosphor has a very short decay time, allowing neutron fluxes as high as a million neutrons per square centimeter per second to be measured without appreciable loss. Probe Construction

The unit pictured in Fig. 4-10 is used for measuring gamma radiation. It has a self-contained preamplifier uti lizing a 6U8 vacuum tube. A DuMont Type 6292 photo-

. (Courtesy Technical

Fig. 4-10.

The Technical

Associates scintillation probe.

Associates.)

Model DS-2

multiplier tube is used to detect the flashes from the NaI(Tl) scintillation crystal. This crystal can either be solid or can be provided with a well, depending on the desired application. The basic construction of the alpha and beta units man ufactured by Technical Associates is similar, but different heads, or active portions, are employed. For detecting alpha particles, a head, such as that shown in Fig. 4-11, is useful. A tightly packed layer of alpha sensitive phosphor crystals (1) is mounted on a lucite disc (2) and covered by a holefree aluminum foil (3). This assembly is mounted in a sealed aluminum container (4) which has an ultraviolet transmitting glass window (5) . The end of the container is perforated (6) , providing access for alpha particles and pro tection to the thin end window. Beta measurements may be made by means of the betaray crystal detector shown in Fig. 4-12. A ^-millimeter 78

(A) Photo.

(B)

Cross-sectional drawing. (Courtesy National

Fig. 4-11. An Alpha scintillation

Radiac, Inc.)

detector.

thick slice of a single crystal of stilbene (1) that is used as a scintillating material is mounted on an ultraviolet trans mitting glass window (2) and covered by thin aluminum foil (3). The outer case (4) is aluminum. This brief section on scintillation probes is intended merely to illustrate the wide variety that is available and is by no means comprehensive. Many other manufacturers have similar probes ; scintillation crystals and phosphors are available from a number of suppliers. COMMERCIAL INSTRUMENTS

It

might be instructive to look at one or two complete instruments using scintillation probes. Such instruments can be used for a variety of purposes, including surveying, checking for radioactive contamination, and the like. Again, the coverage is only representative and by no means complete. 79

oy-tt

00

0

■.Wf.'.'.XsB rgjne^^s

(A) Photo.

\WW

(B) Cross-sectional

drawing.

(Courtesy National Fig. 4-12. A Beta scintillation

Radiac, Inc.)

detector.

(Courtesy Victoreen Instrument Fig. 4-13.

80

The Victoreen

Model 589 Thyac II portable

survey meter.

Co.)

Survey Meter

A

transistorized portable survey meter is shown in Fig. 4-13. An interesting aspect of this instrument is that it can be provided with either a scintillation probe or Geiger tube probe. Here, we will be concerned primarily with the scin

tillation probe. The high voltage provided by this unit can be used to power either the Geiger probe or the scintillation probe. The schematic is given in Fig. 4-14. As can be seen, except for the voltage regulators, solid-state devices are used throughout the circuit.

1Z

*****

CI

(Courtesy Victoreen Fig. 4-14.

Instrument

Co.)

Schematic of the high voltage power supply for the Victoreen Model 489 Thyac II.

Basically the circuit consists of a blocking-oscillator driven flyback-type of circuit. The blocking-oscillator por tion is made up of Q2, R7B, transformer T2 windings 3-4 and 5-6, and batteries BT1 and BT2. When the instrument 81

is turned on, Q2 conducts and an increasing current flows through winding 3-4. This current induces a voltage in winding 5-6 which maintains conduction of Q2. The collector current increases until Q2 saturation cur rent is reached. Then the circuit rapidly turns off, due to the regenerative action of the transformer. During this turn-off action, large flyback voltages appear across all transformer windings. A peak voltage of up to 1,400 volts appears across winding 1-2. This is rectified by CR5, fil tered by C5, R12, and C4, and regulated by either V2 or V3, depending on the output voltage desired.

HI! < io M-«. >

.01.( 600V

V^

(Courtesy Victoreen Instrument

Co.)

Fig. 4-15. Schematic of the scintillation probe used with Victoreen Models 489-50, 489-55, and 489-60.

When a scintillation probe (Fig. 4-15) is employed, the output voltage of 1,200 volts is used to operate the photomultiplier tube in the probe. This voltage is fed to the probe jack via resistor R11 (Fig. 4-14) which serves as the load resistor. When an ionizing event occurs, a pulse appears across R11. The pulse-shaping circuit is another blocking oscillator somewhat similar to the power supply. It produces a rec tangular voltage pulse of about a 110-microsecond duration 82

across winding 3-4 of T1 for each detector pulse. This shaped pulse charges integrating capacitors C2 and C6 via rectifier CR2. The amount of charge is determined by range switch S1A which connects resistors Rl, R2, or R3 and R4 in the circuit. The charge placed on these capaciors is discharged through meter M1. The meter current then is dependent on the charge per pulse and the pulse repe tition rate. Several different scintillation probes can be used with this instrument. For gamma-ray detection, an NaI (Tl) crystal is mounted in optical contact with a photomultiplier tube. Different size crystals may be employed, depending

(Courtesy Victoreen Fig. 4-16. Schematic of Hie Victoreen "Poppy" probe.

Instrument

Co.)

Model 702

desired. For alpha rays a Model 702 probe (Fig. 4-16) is very useful. This is a large-area probe (about 168.5 square cm active area) and the active ma terial is ZnS(Ag), which is covered by a strong, opaque aluminized mylar window. This particular probe is some times called the "Poppy" probe. on the sensitivity

83

Underwater Detector

An underwater scintillation detector is shown in Fig. 4-17. This unit uses a plastic scintillation sphere which is 7 inches in diameter and is covered with a waterproof coating.

It detects

both beta and gamma radiation.

(Courtesy Franklyn Systems, Fig. 4-17.

The Franklyn Systems Model 60-4 scintillation a submersible detector probe.

Inc.)

detector using

The scintillator is mounted on a stainless steel water proof submersible probe which contains the photomultiplier tube and the transistorized preamplifier and high voltage supply. A cable provides mechanical and electrical connec tions to the probe. The instrument accompanying the probe in the photo graph is the readout package. The pulse from the cable is amplified and selected by the pulse-height discriminator. A differential or integral pulse-height discriminator selects the pulse amplitude that drives the scaler. Thus, a measure ment of the total count is provided. A counting rate meter, which gives an indication of the instantaneous intensity of the radiation being measured, is also included. 84

LIQUID SCINTILLOMETERS

In Table 4-1, a liquid solution, p-Terphenyl in Xylene is listed. This is known as a liquid scintillometer. One such liquid is available under the trade name Liquifluor; it is a mixture of fluorescent chemicals in toluene. Liquid scintillators are useful for detecting radioactivity from liquids in solution with the scintillator. Alpha, beta, and gamma radiation can be detected in this manner. How ever,

because

of the

lower

mass,

the

efficiency

of

gamma-radiation detection is much lower than that of a sodium-iodide crystal. Therefore the main usefulness of liquid scintillation in small-volume detectors is in its appli cation to alpha- and beta-emitting isotopes.

(Courtesy Baird-Atomic, Inc.) Fig. 4-1 S. The Baird-Atomic Model 845 liquid scintillation

spectrometer.

A liquid scintillation spectrometer is shown in Fig. 4-18. It provides an unusually simple method for counting liquid The detector unit, at the left, contains a single photomultiplier tube detection system. Liquid scintillation counters have been employed in radioactive carbon dating by the Texas Bio-Nuclear Corp., using a liquid spectrometer called the Tri-Carb that is man ufactured by the Packard Instruments Co. This instrument is designed for dual-channel operation — that is, it has two photomultiplier tubes and two outputs which go to two dif ferent analyzers. This permits what is called double-label analysis. samples.

85

Fig. 4-19. One of the Nuclear-Chicago 720 series liquid scintillation systems.

(Courtesy Nuclear-Chicago Corp.)

Another type of liquid scintillation

system

is shown in

Fig. 4-19. This unit utilizes a technique known as channels ratio counting. This technique is based on the fact that when quenching occurs, the height of the pulse produced

by a beta particle is, on the average, decreased. If the spectrum is then properly divided into two counting chan nels, the ratio of the count rate in the two channels will change relative to the amount of quenching in the samples. By careful adjustment, the ratio of the two count rates can be calibrated to represent the degree of quenching. The instrument shown in Fig. 4-19 can accommodate up to 150 samples. A completely transistorized, three-channel analyzer that is specifically designed for liquid scintillation counting is included. Fast amplifier recovery time is in corporated in the amplifier to eliminate data losses due to circuit overloading. This is necessary in an instrument such as this because the scintillation produced by a beta particle in a liquid fluor has a decay time only about 1/50 of the produced in a decay time of a gamma-ray scintillation sodium-iodide crystal. 86

SOLID-STATE

DETECTORS

The fact that solid-state devices, such as silicon PN junc tions, can be used to detect and measure nuclear radiation was mentioned in Chapter 2. These devices are still in the very early stages of development, but some commercial units are available. In general, three different phenomena can be employed in detecting and measuring radiation in solid-state semicon ductor devices. The first involves the direct conversion of the incident radiation into electrical current, in much the same manner that a silicon solar cell converts light energy into electricity. The second covers the cumulative damage to a semiconductor that can be measured; by proper cali bration processes, this damage can be converted to an in dication of the total amount of radiation received by the device. The third technique utilizes the depletion layer in a reverse-biased diode. Electrons and holes are formed in this region much as gas is ionized in a Geiger tube. Silicon Solar Cells

Recent experiments have revealed that the silicon solar cell can be one of the simplest and most useful devices available for measuring high-intensity nuclear radiation since the radiation is converted directly to measurable amounts of electricity. The cells are made by forming a thick layer of N-type semiconductor on a base of P-type material — the reverse of the original solar cell. This type of construction makes the cells much more resistant to radiation damage. A silicon solar-cell radiation detector is extremely simple. It is only necessary to connect the solar cell leads to a microammeter — the output under high-level radiation can be easily read on an inexpensive, moderately sensitive meter. As an example, a radiation intensity of a million rads per hour of X or gamma radiation can produce a cur rent of 37 microamperes. Intensities as low as 100 rads per hour can be measured with more elaborate instrumen tation, and radiation as intense as a billion rads per hour can be measured, provided that it is not too energetic. One disadvantage is that cells of this type cannot be employed to measure very low levels of radiation but, in many ap^ lications, they are very useful. 87

The cells can be permanently damaged by high-energy radiation, but tests have shown that 300,000 volt electrons or X rays do not produce appreciable damage. The cells can also be used to measure alpha particles and protons, but such radiation damages the cell more quickly. However, this damaging effect can be useful in measuring cumulative exposures, such as in dosimeters.

(Courtesy Bell Telephone

Labs.)

Fig. 4-20. A silicon solar cell being placed In the test chamber of a Van de Graff generator.

A

silicon solar cell is shown being placed in the test chamber of a Van de Graff generator in Fig. 4-20. Such a generator permits measurements to be made at certain very precise energy levels. Radiation Damage Effects

As mentioned previously, the damaging effect of radia tion on a semiconductor device can be used to determine the total exposure to which the device has been subjected. This technique appears to be particularly useful in design ing dosimeters to determine total exposure to nuclear radi ation. Damage in the case of a silicon PN-j unction diode can result in a change in the forward current for a certain applied voltage. This voltage need not be applied contin uously, but the diode can be removed occasionally from its 88

environment and the forward current measured. The read ing obtained gives an indication of the total dosage of radi ation to which it has been exposed. Readings can be made as often as desired, as they do not affect the diode. This phenomenon, then, can be utilized in making a dosimeter. Ionization In Depletion Layer

The third phenomenon is the fact that radiation striking a semiconductor material produces holes and electrons in much the same manner that radiation ionizes a gas. Tech niques have been developed for sweeping these charges out of the semiconductor material as soon as they are formed. Thus, an indication is obtained of the fact that radiation has been received. Also, by determining the number of charges swept out, or the amplitude of the output pulse, the energy of the incident particle or photon can be determined. A diode formed of P-type silicon with a very thin heavily doped N-type layer on top takes advantage of this phe nomenon. If the diode is biased in the reverse direction, very little current will flow, and all of the charges will be swept out of a region of the diode, leaving what is called a depletion layer. Such a device is sketched in Fig. 4-21. When an incident particle enters the depletion layer, it gives up its energy to form holes and electrons. The elecBIAS VOLTAGE

P-TYPE

SILICON

DEPLETION

LAYERS-

"yr-

A reverse-biased diode, showing the formation of a

Fig. 4-21.

depletion

layer.

METAL OR THIN p+ LAYER THIN n+--» REGION

HIGH ELECTRIC . HELD

(Courtesy Solid State

h Radiations, Inc.)

89

trie field produced by the reverse bias sweeps these charges to the terminals, producing a pulse of current in the ex ternal circuit. This pulse can then be counted, analyzed, or treated in any desired manner. Commercial Solid-State Counters

This third phenomenon just discussed has been utilized to form a semiconductor neutron detector. A develop mental model is shown in Fig. 4-22. The device consists

1

2

J.0THS lOOTHS 1

2

I

*

3 ? 4

5

■

UUI

e 9

5 _ •

7

0 Q

1

XT^^:*

iJflrtililil:*

(Courtesy Molechem, Fig. 4-22. A developmental

model of a neutron

semiconductor

Inc.)

detector.

of a reverse-biased N-P silicon diode to form the chargesensitive depletion layer, and a coating of boron, lithium, or uranium to convert the incoming neutrons to charged particles. These charged particles then produce the electronhole pairs in the depletion layer. By proper choice of coat ing material, a detector can be made for either fast or slow neutrons. Without the coating, a semiconductor particle detector is available which will detect and measure electrons, pro tons, alpha particles, heavy ions, and fission fragments. Such a device has an extremely short response, in the millimicrosecond range, and has exceptional energy resolu 90

tion. The N-type region is made extremely thin— less than one micron — so that the particles do not lose any appre ciable energy in traveling through this region.

(Courtesy Molechem, Inc.) Fig. 4-23.

A

series of silicon radiation detectors using surface

barrier construction.

Another series of silicon radiation detectors is shown in Fig. 4-23. These units can be used to detect high energy charged particles and, by proper conversion films, will de tect either slow or fast neutrons. The depth of the depletion region is an important factor in a device of this nature, because it determines the active volume of the device. The greater the thickness of the de pletion region, the more efficient the device will be in ab sorbing incident particles. Because this field is so new, it can be expected that man ufacturers will soon be on the market with many commer cial devices. Techniques for purifying silicon and forming very thin junctions by diffusion and surface-barrier tech niques are now highly developed. Thus, it is possible to manufacture semiconductor radiation detectors in quantity and at relatively inexpensive prices.

91

Chapter 5

Dosimeters

Dosimeters have been denned in various ways; a con cise, workable definition which partially suits our purposes is as follows : "A dosimeter is a device worn or carried by an individual to measure his accumulated exposure to nu

clear radiation or X rays." However, this definition should be broadened slightly, because in many instances, it is de sirable to measure the total dose of radiation received by objects other than human beings. For example, in irradi ating food to preserve it, it is desirable to know the total dose, but personnel dosimeters are inadequate because of their limited range. In general, a dosimeter depends for its operation on the effects of ionizing radiation on its sensitive elements. Con struction varies, depending on the methods used for gaug ing the magnitude of these interactions. Some dosimeters are completely self -indicating ; that is, they require no aux iliary apparatus for giving an indication of exposure. Others are self-indicating after they have been prepared for use by auxiliary equipment. Still others require indicating ac cessories.

Dosimeters can be arbitrarily divided into five general classifications. Instruments in each classification depend for their operation on a specific physical or chemical effect of radiation on the sensitive elements. Nuclear radiation can (1) expose photographic film, (2) produce chemical reac tions, (3) change the properties of certain kinds of glass, (4) produce ionization in gases, and (5) affect solid-state devices such as silicon diodes. Each general class will be discussed separately. 92

FILM

Ordinary or specially-prepared photographic film will be exposed when subjected to nuclear radiation. The amount of exposure will depend on the total amount, or dosage, of radiation to which the film has been subjected. The amount of exposure determines the amount of blackening of the developed film, and the degree of blackening can be used to determine the total amount of radiation received by the

film. This characteristic has led to the development of the film dosimeter, one of the most widely used and popular types in existence today. Essentially, the film dosimeter consists of a holder in which a strip of photographic film is placed. This film "badge," as it is called, is worn by the individual during the time that he is subject to possible radiation exposure. After a period of time, depending on the degree of potential ex posure, the film is removed and developed. Then, the amount of blackening of the exposed film is compared to previously established standards. Thus, the total amount of radiation to which the wearer was subjected during the given period can be determined with a high degree of ac curacy. By using a predetermined amount of shielding over parts of the film, an extremely wide range of dosages can be measured.

/*»'»X

(Courtesy Nuclear-Chicago Fig. 5-1. Film badges

manufactured Nuclear-Chicago Corp.

Corp.)

by the

93

Film

have been designed to be pinned onto a garment, worn on the wrist as a watch, or on the finger as a ring, depending on the circumstances of the wearer. In most designs the film itself can be readily removed and replaced with fresh film without removing the entire badge. badges

Typical film badges are shown in Figs. 5-1 and 5-2.

Fig. 5-2. A Dim badge manufactured by Hie Atomic Film Barge Corp.

ATOMIC FILM BADGE CORP.

(Courtesy Atomic Film Badge Corp.)

A

number of companies provide a film-badge service. This service consists of developing and reading the exposed film, reporting to the user, or company, the amount and kind of radiation received during the given period, and pro viding fresh film for the badge. Services for monitoring X, gamma, and beta rays, and neutrons are available. The pe riod of exposure can be a day, a week, two weeks, or more. The film badge has many advantages where the user is subjected to very low levels of radiation extending over long periods of time. It is light in weight and fairly easy to wear. Typical accuracy is within ±10% ; extremely low levels of total dosage can be determined. One company, for example, claims to report dosages as low as a milliroentgen, or one thousandth of a roentgen. The upper limit depends on the shielding used in the badge, and runs as high as 1000 roentgens. Usually the film badge company keeps a very close check on individual exposures, and routinely re ports the total exposure to which the wearer has been subjected.

This service suffers from a couple of disadvantages, neither of which is serious for most applications. First, it 94

is not possible to determine the exposure without removing the film and developing it under carefully controlled con ditions ; second, there is a certain amount of time lag before the wearer knows what his exposure has been. CHEMICAL

Certain chemicals suffer changes in composition when subjected to nuclear radiation. For example, some hydro carbons, such as chloroform, break down to form an acid — the amount of acid being proportional to the total quan tity, or dosage, of radiation received. This effect has been successfully exploited in the development of chemical dosimeters. One technique to determine the amount of acid which has been formed is to use a dye whose color depends on the acidity of the solution. Aqueous Phenol red is one such dye; its color changes from red to yellow with only very slight changes in acidity.

A

typical dosimeter of this type contains chloroform in with an indicating dye. When the ampoule is irradiated, some of the chloroform is converted to acid, changing the color of the dye. By means of a suit able color chart, total dosage can be determined with a fair degree of accuracy. A different type of reaction is involved in a chemical dosimeter that is employed in the study of radiation dam ages to electronic components situated in nuclear environ ments. The chemical solution consists of ferrous sulfate and sodium chloride dissolved in sulfuric acid. It responds pri marily to gamma radiation. When this solution is irradiated, the ferrous ion is oxidized to the ferric ion, with a corre sponding change in the color transparency. The dosage received by the solution is then measured spectrophotometrically by comparing the optical density with that of an unirradiated solution. This dosimeter is capable of measuring doses from 1,000 to 44,000 rads. Dose rate has no effect on the yield up to about 37-million rads per hour. Chemical dosimeters are not normally worn by person nel, but they are useful in determining total dosages in areas where the radiation level is extremely high. They were employed quite extensively in early atom bomb tests to determine radiation dosages close to the source of the a small ampoule, together

95

explosion. Their indication is essentially independent of the rate at which radiation is absorbed; that is, they indicate total dosage accurately, regardless of radiation intensity. RADIOPHOTOLUMINESCENCE

Certain types of glass will fluoresce, or give off visible light, when subjected to ultraviolet radiation. It has been found that the amount of the fluorescence is directly re lated to the amount of nuclear radiation to which the glass called radiophotohas been exposed. This phenomenon, luminescence, can be employed in the design Of a dosi meter.

Silver-activated phosphate glass is particularly well suited for this type of dosimeter. Its fluoresence is not affected by heat, humidity, or aging. Changes in fluorescence due to nuclear radiation are permanent and additive; that is, regardless of when or under what conditions the glass is subject to nuclear radiation, its changes in fluorescent char acteristics are always the same. Thus, the glass, together with a suitable reader, makes an ideal long-term dosimeter.

(Courtesy Bausch Fig. 5-3. Radiophotoluminescent

& Lomb

Inc.)

glass rod.

The glass can be formed into any shape or size desired. One manufacturer forms it into tiny rods only one milli meter (one twenty-fifth of an inch) in diameter and six millimeters long. Fig. 5-3 shows such a rod compared to a penny and a quarter-inch lead from a mechanical pencil. These rods can be located in areas where it would be diffi 96

cult to place any other type of dosimeter. For example, in isotope therapy, these rods can be inserted into a diseased organ of the human body to accurately measure the total dosage received by a specifically irradiated area, without discomfort to the patient.

Fig. 5-4. Reader for determining the radiation dosage to which a glass rod has been exposed.

(Courtesy Bausch & Lomb

Inc.)

The usefulness of this dosimeter depends

on the ease

with which it can be read ; that is, how an indication of radiation exposure is obtained. An effective reader is shown in Fig. 5-4. Here, the glass rod is irradiated with ultraviolet light and the amount of visible light produced is indicated on a suitable meter. By careful attention to details and by proper calibration, this combination of rod and reader can accurately indicate total radiation exposures from 10 to 10,000 roentgens with an accuracy of better than 4%. Since the change in fluorescence of the glass rod is es sentially independent of temperature and humidity effects and the rod can be read as often as desired without altering it in any way, it is conceivable that a rod of this kind could be carried around with a person from the cradle to the grave and could be checked whenever it was desired to de termine his total lifetime exposure to radiation. A casualty-range dosimeter of the glass type for use in the event of an atomic attack has been developed by the Industrial Electronic Hardware Corp. Known as the Gammax, it is inexpensive, rugged, always ready for use, has a long shelf life, and will work under severe conditions. The glass element of this dosimeter is sensitive to the energy of the impinging of radiation. For example, it is 18 times more sensitive at 70 kev than at 1.3 mev. For this reason, lead shields of carefully determined thickness and with suitably chosen perforations are employed to smooth out the response. 97

IONIZATION Nuclear radiation is capable of ionizing air and other This effect is utilized in a class of instruments referred to as ionization dosimeters. Such instruments bas ically contain a sealed chamber with a central conductor insulated from a conducting coating on the outer wall of the chamber. If a voltage is applied between the center conductor and outer wall, any ions formed in the air (or other gas) in the chamber will be swept to one of the elec trodes, depending on polarity. If the source of voltage is removed, the voltage across the chamber will drop as ions are formed. This drop in voltage can be used to determine the total amount of ionizing radiation, or dosage, received by the chamber. In actual practice a number of refinements are incorpo rated; for example, the chamber is made light and small, the voltage is indicated by a quartz fiber electrometer, and a scale and magnifying glass are built into the instrument so that when it is pointed at a source of light, a direct read ing of total dosage is obtained. A number of manufacturers have built dosimeters of this type in the shape and form similar to a fountain pen, making them easy to carry. gases.

Fig. 5-5. Dosimeter being its charger.

(Courtesy Bendix

inserted

into

Corp.)

An auxiliary unit, known as a charger, must be em with these dosimeters to charge them up to the de sired voltage (typically 150-200 volts). A dosimeter being inserted into its changer is pictured in Fig. 5-5. The scale

ployed

on the dosimeter is arranged

so that

it reads

zero

when

fully charged, and the reading increases as the voltage 98

(Courtesy Bendix Fig. 5-6. A Dosimeter,

ratemeter,

and a charger designed

Corp.)

for civil defense.

drops off due to exposure to ionizing radiation. A typical charger and dosimeter are shown in Fig. 5-6. A ratemeter, a device for determining the rate at which radiation is

Fig. 5-7.

(Courtesy Landsverk Electrometer

A

group of dosimeters different ranges.

having

Co.)

being received, is also included in Fig. 5-6. A group of dosimeters having different ranges and a charger which can be used with all of them is shown in Fig. 5-7. The schema tic diagram of a charger is presented in Fig. 5-8. Operation of the charger is fairly simple. When switch S1 is closed, oscillations are set up by the transistor and transformer T1. This oscillating voltage is stepped up to the desired value by Tl and rectified by diode D1, giving the desired DC output voltage of approximately 200 volts. 99

JM

.tL^—oWEP.

RED

^T-i

-e-

CD V-7S6 (Courtesy Bendix

Corp.)

Fig. 5-8. Schematic of a typical transistorized dosimeter charger.

Potentiometer R2 is then used to select the desired voltage to fully charge the dosimeter. In the preceding dosimeters, the quartz fiber electro meter is an integral part of the unit worn by the user. Con siderable simplification is possible if only the ionization chamber is incorporated in the dosimeter. The dosage can then be read by inserting this chamber in a suitable reader. Such a unit is usually called a roentgen meter (Fig. 5-9).

Fig- 5-9. A roentgen meter with assortment of chambers.

(Courtesy Landsverk

Electrometer

Co.)

The basic operating principle is the same as before. An ionization chamber is charged to the desired voltage and then is worn by the user. Whenever he wants to determine his accumulated radiation dose, he plugs the chamber into the reader. The same reader can be used for recharging the chamber.

An interesting variation of the ionization chamber dosi meter is provided by the personal radiation monitor called 100

Chirpee. This device warns the user when he encounters an unexpected radiation field. The warning consists of both a flashing neon lamp and a chirping subminiature speaker. Both are activated simultaneously, at a rate proportional to the intensity of the radiation. Experimental pocket-sized radiation dosimeters which in clude variations of the conventional units have been de to either give an indicated veloped. One is designed measurement of accumulated dose or to provide an alarm when a preselected dose level is reached.

TEST

(Courtesy General Electric Fig. 5-10.

Co.)

Experimental alarm-type of radiation dosimeter.

(Courtesy General Electric

Co.)

Fig. 5-11. Experimental automatic radiation dosimeter. recharging-type

In this unit (Fig. 5-10) a pencil ionization chamber is modified to include an illuminated quartz fiber, an optical system, and a cadmium sulfide light detector. By proper charging, the system can be arranged so that the light de tector is activated when a predetermined dose has been reached, thus activating an alarm or indicator of some kind. Another unit (Fig. 5-11) provides a digital indication of the received dose on a miniature register. It is essentially an automatic recharging-type dosimeter. Again an optical arrangement and light source are incorporated in such a manner that the charger is operated when the dosimeter is discharged. This charging pulse also operates a counter so that the number of times the charger has operated is in dicated directly on the counter. Ionization-chamber type pocket dosimeters are widely used, and a number of manufacturers produce such instru a full scale ments. For laboratory or clinical purposes, range of 200 milliroentgens is typical, while for civil de fense applications, ranges as high as 600 roentgens are provided. 101

These devices are small and rugged but do suffer from one or two disadvantages. Although most of them can be read at any time by merely pointing them towards the light, an auxiliary unit is necessary to charge them up before they can be used. Also, there is some slight leakage after the dosimeter is charged up. This leakage is small, perhaps on the order of 1% per day, or less, but this means that such an instrument cannot produce a reliable indication of total dosage over a long period of time without recharging it periodically and keeping track of the dosage each time it is recharged. SOLID-STATE

DEVICES

Recently much research has been carried out on the ef fects of nuclear radiation on solid-state devices, such as semiconductor diodes. It has been found that the forward resistance of a silicon diode fabricated in a certain manner changes in accordance with the amount of neutron radia tion to which it has been subjected. For example, diodes have been formed as shown in Fig. 5-12. In this unit, Base n+ Region

-

p Region

X

400 ohm-cm

Silicon

.

+ p Re9'on

Nickel Connector

Silver Paste (Courtesy General Electric

Fig. 5-12. Typical silicon-diode neutron

Co.)

dosimeter.

the N+ region is 0.001-inch thick, and the P+ region is 0.002-inch thick. The base region varies from 0.03 to 0.1 inch. It was found that with diodes having a base-region thickness of 0.075 inch, the change in forward resistance due to fast neutron damage was approximately linear and could be readily measured. Thus, although much further work remains to be done, it appears that diodes of this type can be developed into effective neutron dosimeters. Devices of this nature appear to have limited use for doses below 0.1 rad. However, the resistance changes ap proximately 28% after a dose of 40 rads — a change which 102

is easily measured. An interesting and useful characteristic of these devices is that they are apparently completely in sensitive to gamma radiation, at least with doses as high as 500 rad. These devices show definite promise for use as personnel neutron dosimeters. Their sensitivity, dose range, and sta bility indicate applications as annually-read units. Future studies will be directed towards better techniques of meas uring the forward resistance and using different doping ma terials for the diodes. SUMMARY Many different types of dosimeters are available. A few have been described in this chapter; these descriptions are not intended to be all-inclusive but are merely representa tive of the major types available. For a specific application, the various characteristics should be studied carefully, and the unit which best serves the purpose should be selected. Civil defense authorities appear to favor the ionization type of unit for determining dosage due to radioactive fall out in case of a nuclear bomb attack. Such a unit can be very useful over short periods of one or two weeks. It is fairly cheap and rugged, easy to read, and easy to re charge. Other types may be more suitable for other appli cations, but one point should be emphasized: Nuclear radiation of all kinds can be dangerous and great care should be exercised to avoid unnecessary exposure.

103

Chapter 6

Home-Built Counters

A

simple Geiger counter is neither difficult to build nor excessively costly. Several designs will be discussed in this chapter, the simplest costing less than $20.00 at current prices. All components are readily available from most of the larger supply houses. The required tools consist pri marily of a soldering iron, drill, and pliers — very little knowledge of electronic circuitry is necessary. Before attempting to build a Geiger counter, you should carefully consider what the instrument will be used for. If only a very rough indication of radioactivity is desired, such as in prospecting, a home-built instrument can be entirely satisfactory ; however, if very precise measurements are re quired, it would perhaps be better to invest in a commer cial unit. Scintillation counters are in general more sensitive than Geiger counters, but at the same time are much more complicated and more difficult to build. Whenever you work with electronic circuits, there are several basic construction points to keep in mind. For in stance, always keep hook-up wire leads as short as practi cal. Avoid unnecessarily long leads which will tangle in the equipment — this makes circuit tracing and troubleshooting easier. If too long, they often increase the capacity between leads enough to interfere with proper circuit operation. Make each connection mechanically secure before you solder it, so the soldered joint will not break due to strain. Always use rosin-core solder (rather than acid-core) to in sure that connections will not corrode. Heat the material which is to be soldered before applying the solder, and 104

flow

just enough solder into the joint to provide

a

good

connection. SIMPLE COUNTERS

As mentioned in Chapter 3, a Geiger counter can be ex tremely simple, or it can be more complex —depending on the results desired. In some cases, clicks in a pair of head phones provide sufficient indication of radiation intensity; at other times the flashing of a neon light makes a desirable indicator; and in perhaps the majority of applications, a meter reading provides the most satisfactory indication.

®1B86


QUANTITY

(A) Schematic.

R3Rlmeq

HEADPHONES =t|lf«-

ITEM

DESCRIPTION

Vj-watt resistor or equlv.

1

13

1 m»g,

1

VI

Geiger

1

Bl

300-volt battery.

1

tube (Victoreen

Headphones (sensitive impedance magnetic).

1 B86).

high-

(B) Parts list. Fig. 6-1 . A simple Geiger

counter.

An extremely simple Geiger counter is diagrammed in Fig. 6-1A. It is a straight series circuit with the battery, Geiger tube, protective resistor, and headphones connected in series. Although the parts list in Fig. 6-1B specifies a 300-volt Geiger tube, other tubes operating at different voltages could be substituted. However, the tube and bat tery must match — that is, a 300-volt battery must be used with a 300-volt tube, and a 900-volt battery with a 900volt tube. Fortunately, very small, lightweight 300-volt bat teries have been developed so that the power supply can be kept small and light. Certain precautions are necessary in building even this simple unit. In the first place, the 300 volts from the bat tery can give a "wicked bite," so watch out ! Secondly, highimpedance magnetic-type headphones must be used rather 105

than the crystal type, since a DC path must be available for the battery current. Actual battery drain is very small, and battery life approaches shelf life, even with extended use.

Mechanical construction will depend on the type of Geiger tube used — some have wire leads, and some fit into a special socket. Since most of the tubes are glass, a metal enclosure should be provided for protection. This enclo sure should be perforated at appropriate points below the tube in order to avoid blocking the rays which are to be detected. One of the penalties which must be paid for an extremely simple circuit of this nature is lack of headphone volume. Even with sensitive phones that have an impedance of 2,000 ohms or more, volume will leave a lot to be desired

— particularly in noisy locations. Crystal

©

T

headphones

can

c

.01 MFD ^HEADPHONES

h|,|,|2_Ime5g

X

Fig. 6-2. Circuit for «liminating DC from the headphones.

for increased sensitivity by taking advantage of the circuit shown in Fig. 6-2. Here, the DC path is pro

be utilized

vided by a load resistor, and the impulses from the Gegier tube are coupled to the headphones by means of a capaci tor. The capacitor is a 0.01-mfd ceramic unit, and the re sistor may be from 1 to 5 megohms. COUNTER

A

WITH AMPLIFIER

simple, one-stage amplifier may be added to the circuit

of Fig. 6-2 in order to give greatly increased volume. A complete circuit and parts list are shown in Fig. 6-3. Views of the complete unit are presented in Figs. 6-4 and 6-5. The amplifier consists of a triode-connected 1U5 tube whose filament power (50 ma at 1.5 volts) is supplied by an ordinary penlite cell. This particular unit uses a 300volt Geiger tube and a 300-volt battery. To avoid an extra B battery for the 1U5 plate circuit, the 300-volt battery and a large plate-load resistance (1 megohm) are employed. 106

(A) Circuit. QUANTITY

ITEM

2

Rl,

1

R3

2

CI,

DESCRIPTION

R2

4.7 meg, Vi-watt resistors.

C2

0.01 -mfd ceramic capacitors.

1 meg,

Vj -watt resistor.

SI

DPST toggle

VI

Gelger tube (Vlctoreen

V2

1U5 tube.

■1

300-volt battery.

B2

switch. 1 B86

or equlv.)

114 -volt penlight cell. Headphones

(sensitive

high-impedance).

3" x 4" x 5" aluminum Kitchen cabinet Fahnestock

box.

handle.

clip.

Battery clip. 2

Phone-tip jacks. Tube socket (7-pin miniature). Terminal

strip (S-lug).

Screws. Nuts.

Wire. Solder. (B) Parts list. Fig. 6-3. A simple Geiger counter with an amplifier stage.

If

you use a 900-volt Geiger tube for this circuit, place two additional 300-volt batteries in series between the plus terminal of B1 and the bottom of R1 as shown in Fig. 6-3A. Connect the 1U5 plate circuit across only one bat tery so that its total plate voltage is only 300 volts. Be sure the voltage rating of the capacitors is high enough to match the circuit voltage, whether 300 volts or 900 volts. Ceramic capacitors take up less space and usu ally have a higher voltage rating than tubular types, but 107

Fig. 6-4. Overall view of completed

either

critical;

kind the

may 20%

unit.

Resistance values are not commercial tolerances are entirely

be

used.

satisfactory.

Fig. 6-5. Interior

view of the counter in Fig. 6-3.

Either crystal or high-impedance magnetic phones will

work in the output circuit. Use blocking capacitor C2 for protection from shock. The original model of this particular counter was built in a commercial 3-inch by 4-inch by 5-inch aluminum cab inet with an ordinary kitchen-cabinet door pull for a handle. A series of holes were drilled underneath the Geiger tube in order to permit easy entry for the rays being detected. The Geiger tube may be fastened in place by any suitable clamp ; a clamp bent from a Fahnestock clip has been found to be highly satisfactory. It is a good idea to glue two or three small pieces of felt or sponge rubber between the tube and the clamp, providing protection from shock and excessive clamping pressure. 108

Use solid hook-up wire for the tube socket connections. It will support the socket and tube as well as eliminating the necessity of mounting the socket rigidly to the case. This method of assembly provides a type of shock mount and helps protect the tube in case of a severe shock. Use a terminal strip to simplify the wiring; locate it near the lower end of the Geiger tube (Fig. 6-5) . No adjustments are necessary after the unit is built. If all wiring is correct and all components are good, you will hear clicks in the headphones as soon as you turn on the switch. These clicks are the background count and will be heard at the rate of about 40 or 50 a minute. Any appreciable increase in this rate indicates the presence of radiation of some kind, probably due to radioactive material in the vicinity. This instrument is particularly useful for prospecting, since it is light, rugged, and reliable; however, it can also be used for checking objects for the presence of radioactive material, such as clothing, laboratory benches, and the like. It is difficult to determine exact radiation levels with this unit, but comparative measurements may be made by counting the clicks in a certain period of time, say 1 minute.

Fig. 6-6. Simple Geiger counter with a novel high-voltage power supply.

109

NOVEL

POWER

SUPPLY

Another simple counter is pictured in Fig. 6-6. The circuit diagram and parts list are given in Fig. 6-7. It uses a high ratio step-up transformer

and a spark-gap rectifier

to obtain about 900 volts for the 1B85 Geiger tube. Transformer T1 has to be connected "backwards" in or der to obtain the high step-up ratio. Connect the 8-ohm winding as the primary in Fig. 6-7A, and the 8,000-ohm winding as the secondary. Use a normally open microswitch for S1. When you press down on the switch, bat tery B1 sends a current through the 8-ohm winding. Due to the inductance in the winding, current builds up slowly to its maximum. When the switch is released, current stops suddenly, and a high voltage pulse is induced in the sec ondary. Voltage is also induced in the secondary when the switch is closed but it is smaller. Set the finely filed and stoned points of the two spark-gap set screws close enough together so that they will ionize with the larger voltage ap-

~ _QP

GAP

Q

© II

1S4 -

3/

(A) Schematic. QUANTITY

DESCRIPTION

ITEM

RI

CI Tl

'/4-wott resistor. .05-mfd, 600-volt capacitor.

10-meg,

Universal-type output transformer (pri. 5000 to 8000 ohm, sec. 4 to 8 ohm).

SI

Microswitch

(normally

S2

SPST toggle

switch.

open).

VI

1S4 tube.

V2

Geiger tube (Victoreen

Bl

1.5-volt flashlight cell.

B2

22.5-volt hearing-aid battery.

1 B85

or equiv.)

Spark gap. (see text and Fig. 6-8). Headphone. (B) Parts list. Fig. 6-7. Schematic and parts list for counter shown

110

in Fig. 6-6.

plied — but not with the smaller voltage. The specially con structed spark gap (Fig. 6-8) thus acts as a rectifier and allows a current to flow and charge up capacitor C1. When an ionizing event occurs in the 1B85 tube, the ca pacitor discharges slightly. This produces a voltage pulse which is applied to the grid of the 1S4 amplifier tube. The amplified pulse will then cause a click in the headphones. 6-32 MACHINE SCREW «

1 -

s 8-32 TAPPED HOLE ~^_ FOR MOUNTING

_L

UK

T

'*°-

1

,■»,

I MAT'L.-LUCITE

Fig. 6-8. Spark gap construction

details.

The voltage, which is applied to the Geiger tube, needs to be positive at the center wire and negative at ground. If the polarity is wrong when you connect the circuit as de scribed, reverse the connections of either the primary or the secondary of T1. To start circuit operation press and release switch S1 sev eral times — until sufficient charge has accumulated on ca pacitor C1 to operate the Geiger tube. Stop when you hear normal clicks in the headphones, or you will increase the voltage enough to damage the Geiger tube. When the charge on C1 has been reduced so that you can no longer hear the clicks, renew it by pressing switch SI several times. When C1 has been charged, the counter should oper ate for a period of from 5 to 30 minutes. The total time will depend on the quality of the components and, to a certain extent, on the relative humidity. High humidity will allow a more rapid discharge of the capacitor by reducing the external leakage resistance. The 1.5-volt cell B1 furnishes power for the high volt age circuit, and also for the filament of the 1S4 amplifier. B2 furnishes plate and screen voltage for the 1S4. Because current drain through them is very small, both batteries have a fairly long useful life.

Ill

Be careful when you build and operate this device — use high quality components throughout. Capacitor C1 must have a good insulation resistance so as to prevent it from losing its charge too rapidly. Be sure to discharge the ca pacitor before you reach into the unit after it has been used; the voltage which is stored up in the capacitor can give a severe shock. Clicks in a pair of headphones serve as the normal indi cation of radiation intensity. If desired, a neon flasher ( Fig. 6-9) can be added to provide a visual indication.

1S4 NE51 Fig. 6-9. Adding a neon light flasher to the circuit of Fig. 6-7.

Use the primary winding of a push-pull audio output transformer for the neon lamp circuit. To increase the amplification of the 1S4 tube change battery B2 to 45 volts. Clip the transformer leads off short, and connect the center tap to B-(- through the high impedance headphones. Con nect one side of the transformer winding to the tube plate. Connect the NE51 neon lamp across the entire transformer winding. Do not connect anything to the transformer secondary, or short its leads together. COUNTER WITH TRANSISTOR

AMPLIFIER

can be made in the circuitry and battery requirements of a Geiger counter by using a tran Some simplification

sistor amplifier instead of a vacuum tube. An extremely

A Geiger counter with a transistor amplifier.

112

simple grounded-emitter amplifier circuit is given in Fig. 6-10. It provides a gain of about 7, depending on the tran sistor characteristics. Although this specific circuit employs a 900-volt battery and 900-volt Geiger tube, operation would be satisfactory with a 300-volt tube and battery. Remember, 900 volts is dangerous! Be careful when you are working around such a high voltage. Do not touch any part of the circuit where this high voltage is present. COUNTER

A somewhat

WITH METER INDICATOR

complicated Geiger counter circuit, which includes a meter for indicating radiation intensity, is shown in Fig. 6-11. Power is obtained from two seriesconnected 67.5-volt batteries and three parallel-connected standard 1.5-volt flashlight cells. Notice that voltage is ap plied to only one-half of the-filaments of V1 and V4. This connection cuts the battery drain in half with no decrease in performance. Tube V1 and its associated components form a relaxa tion oscillator. The B battery voltage is applied to capacitor C1 through resistor R1. Neon lamp NE1 will ionize when the capacitor has stored up enough voltage to fire it. Then more

®

® GM TUM

CK1013/ 5517

Fig. 6-1 1A. Geiger counter with meter (Parts

liit

on Page 114.)

113

ITEM

QUANTITY

DESCRIPTION

RI

6.8 meg, Vi-watt

R2

22 meg, Vi-watt resistors. 47K, Vi-watt resistor. 50K potentiometer.

R3

M

1 00K,
R5 R6,

resistor.

R9,

RIO

'/j-watt

10 meg,

resistors.

470K, Vi-watt resistor. 30 meg, Vj -watt resistor (two

R7 R8

1 5 meg

in series). 1 meg, Vi-watt resistor. 400-mmf mica capacitor.

R11

CI C2

.001 -mfd mica capacitor.

C3

.002-mfd mica capacitor.

C4

.002-mfd, 2000-volt capacitor. (Glassmike LSG202 or equiv.)

C5

.05-mfd, 2000-volt capacitor ASG17 or equiv.).

C6,

C7

CI

500-mfd,

1 2-volt

500-mmf,

2000-volt capacitor

LSG501

CIO

3

.005-mfd mica capacitor. CH2

(UTC type 0-7

SI

DPST toggle

switch.

S2

DPDT toggle

switch.

B1

67 Vi -volt

B2

Vi-volt flashlight cells. NE-2 neon lamp.

B batteries.

1

NE1

2

Choke coils (UTC type 0-1 3 or equiv.). Interstage transformer or equiv.).

Red.

1

1 N34 crystal

diode.

0-100 microam meter. V4

3S4 tube.

V2

CK1 01

V3

Geiger tube (Victoreen

Jl

Fig. 6-1

(Glassmike

or equiv.).

Tl

Ml VI,

capacitors.

.01 -mfd, 400-volt capacitor.

C9

CHI,

electrolytic

(Glassmike

3/3517

tube. 1B85 or equiv.).

Midget open circuit phone jack.

IB.

Parts list for Geiger counter in Fig 6-11 A.

the capacitor will discharge through the neon lamp until the voltage is low enough for the lamp to be extinguished. This cycle repeats as long as the voltage is applied. In this cir cuit the oscillator waveshape is a sawtooth. The operating frequency of the oscillator is approximately 800 cycles per second.

The 3S4 tube (V1) amplifies the sawtooth voltage. The plate load is the primary winding of transformer T1; the output voltage is transformer-coupled to the input of the cold cathode-rectifier circuit. A very high voltage results 114

from the connections (Fig. 6-11). After rectification by V2, the DC voltage is applied to a filter consisting of resistor R7 and capacitors C4 and C5 which removes most of the ripple. Variable resistor R4 controls the high-voltage output of the power supply over a range of about 500 to 1,250 volts with new batteries in use. As the batteries age, the control is adjusted to permit operation at 900 volts output until the batteries are expended. This point will have been reached when the battery voltage (under load) drops from its orig inal 135-volt value to about 95 volts. The output voltage changes only slightly with aging of the filament batteries, until they have dropped from the original 1.5 volts to about 0.8 volt. The output voltage regulation, as a function of the external load, is quite adequate for a Geiger tube having a normal flat-plateau characteristic. If the tube does not have a flat characteristic, at unusually high counting rates the voltage is readjusted to the proper operating potential by a slight variation of output control R4. A switch in the output circuit of amplifier stage V4 per mits either the headphones or meter M1 to be selected to monitor the count. The signal pulses from the Geiger tube are coupled into the signal grid of V4 via C8. The ampli fied pulses appear across the plate circuit inductance CH1, where they are capacitively coupled to the indicating cir cuit via C10. The "Phones" switch S2 is connected so that in its On position headphone jack J1 is connected across the output of V4, allowing individual pulses to be counted. Stray circuit capacity from the neon oscillator and associ ated circuits provides a weak audio tone in the V4 ampli fier output, indicating that the instrument is operating, and that the batteries are not expended. In the same switch position the negative terminal of M1 is returned directly to ground. The meter, therefore, is ef fectively connected in series from the bottom end of the high-voltage bleeder resistor R8 to ground and provides an accurate indication of the power supply output voltage. This assumes that the bleeder resistance value is accurately known. For example, if the bleeder resistance is 30 meg ohms, a current of 30 microamperes indicates the presence of 900 volts across the bleeder, according to Ohm's law. Other current values are interpreted accordingly, as the voltage output control is varied. 115

When S2 is thrown to its Off position, the headphones are disconnected and the amplifier output is connected into the rectifier circuit. The rectified pulses are fed to inte grating capacitors C6 and C7, which allow the meter to indi cate the average counting rate. The other side of S2 connects the positive terminal of the meter and the bottom of the bleeder resistance directly to ground, thereby re moving the meter from the bleeder circuit. It is unnecessary to provide a scale on the meter other than its original calibration, because the meter is used mainly in a relative sense when it is indicating a counting rate. The background count will show a small reading,

Fig. 6-12.

116

Overall view of the Geiger counter shown in Fig. 6-11.

which is used as the reference. When the meter reading in it indicates the presence of radiation. Do not adjust voltage control R4 to a position where the voltage is great enough to cause the Geiger tube to "spill" or discharge continuously — this would reduce its life. If the voltage will not reach 900 volts, as indicated by a meter reading of 30 microamperes when switch S2 is set at On, resistor R6 may have too small a value. Increase its resistance in 100,000-ohm steps to try to get the 900-volt output. If necessary, remove the resistor completely and leave pin 4 of tube V2 floating. Fig. 6-12 shows the completed counter; interior views showing some of the construction details are given in Figs. 6-13 and 6-14. Follow this parts layout as closely as possible to prevent too much feedthrough of the oscillator tone into the amplifier. Excessively long leads, resulting in a large stray capacity, can cause such trouble. creases,

Fig. 6-13.

Interior

view of the counter.

A probe housing inside the case is partially visible in Fig. 6-13. It is located directly in back of the batteries. Make it from a 5-inch length of lucite tubing, with an inside diameter slightly greater than the outside diameter of the brass probe shell which houses the Geiger tube. Use two U-shaped brackets to hold the lucite tube in the case. Tighten these brackets so that there is a slight pressure against the outside of the shell which will hold it in place. Cut a hole through the end of the metal case through which you can insert the probe in the lucite holder. Use a 10-inch length of thin-walled brass tubing with a 1-inch inside diameter for the probe. Drill seven half-inch diameter holes through one side of the brass tube in a 117

Fig. 6-14.

Rear view of the counter Interior.

straight line. Two of these holes can be seen in the portion of the probe, which is shown in Fig. 6-12. Float the Geiger tube inside the brass shell by wrapping several layers of Elastoplast, an elastic adhesive bandage material, around the tube at each end in order to provide a snug sliding fit. Use rubber stoppers to seal the ends of the probe. Use a solid stopper at the free end, and a one-hole stopper at the cable end of the probe. Press the stoppers in for a tight fit and then strip them off even with the ends of the probe. Make the cable about 4 feet long of flexible and well insu lated wire. Insert it through the hole in the stopper and se cure it to the stopper to provide strain relief to the Geiger tube. Push the unused cable down inside the case when the probe is not in use. When the unit is not in use, the search probe fits inside the holder in the case. For normal use, pull the probe out about three-quarters of its length. When desired, for ex ploring otherwise inaccessible areas, remove the probe com pletely from the case. COUNTER WITH

INTERRUPTER-TYPE

POWER

SUPPLY

Several Geiger counter circuits are suggested in Bulletin No. NYO-103 written by H. D. LeVine and published by the New York Operations Office of the Atomic Energy 118

EP30G PHONE

JACK

CRYSTAL PHONE

_„

,

0-20 A OR UICROAMP

METER

TWO

TYPE D BATTERIES

(A) Schematic.

2

DESCRIPTION

ITEM

QUANTITY

(Thordarson 20A00 or equiv.). Relay (Advance Type 5002, 104AM-2A; Allied Control Type BC coil #28, F Coll #32, SK coll #28A; Sigma Type 5F or equiv.). Voltage regulator (Electronic Products Type EP 30 RS or equiv.). Rectifier tube (Raytheon CK-1013 Transformers

11, T2

1

1

VR

1

Red.

or equiv.). (B)

Partial parts list.

Fig. 6-1 5. A simple Geiger-counter

All

circuit with interrupter-type

power supply.

power supply to provide the high voltage. One of the circuits is reproduced in Fig. 6-15. The high-voltage pulses are rectifier by the CK-1013 rectifier tube and regulated by VR. A step-down audio transformer with the high-impedance winding in series with the high voltage to the Geiger tube provides a high-current low-voltage pulse for the meter and headphone circuit. A turns ratio of about 20 to 1 is satisfactory for this trans former. A germanium diode rectifies the pulses, which are Commission1.

use an interrupter-type

with the high inductive kick in a step-up transformer

1Available from U.S. Department of Commerce, Office Services, Washington 25, D.C. ; Price 10?

of Technical

119

out by a 1000 mfd capacitor and passed through the indicating meter. This instrument is very versatile in that three different indications are possible: the flashing of a neon light, clicks in the headphones, or a reading on a meter. As with other counters of this general type, the instrument serves pri marily as a detector of gamma rays. smoothed

COUNTER

WITH TRANSISTORIZED

POWER

SUPPLY

In Chapter 3, a commercial survey meter in which the high voltage for the counter tube was obtained by means of a transistorized audio oscillator and step-up transformer was discussed. This type of design lends itself to home con struction. The circuit and parts list for such a unit is shown in Fig. 6-16. The heart of the audio oscillator in this unit is transistor X1, which is connected in a Hartley oscillator circuit. The constants of this circuit have been selected to produce an audio frequency between 30 and 100 cycles per second. Potentiometer R2 controls the output-voltage level by changing the oscillator frequency. R1 limits the minimum resistance in the transistor base circuit, and thus establishes a limit for the maximum collector current. The value of R1 was selected to limit the collector current to about 10 ma when fresh batteries are employed (about 9 volts), and at the same time provide adequate voltage for the counter tube when the battery voltage has dropped to as little as six volts. A miniature, uncased audio transformer is employed for T1. The audio voltage developed across terminals 1 and 3 is stepped up by a factor of approximately 16. A voltagemultiplier-type rectifier circuit gives a further gain of about 10, and thus the overall voltage multiplication for the entire circuit is approximately 160. The rectifier circuit is such that the voltage across each of the associated capacitors, C4 through C13, is one-fifth the output voltage. This permits the use of capacitors with a lower voltage rat ing than might otherwise be necessary. In addition, the peak inverse voltage across each of the rectifiers is well be low the maximum permissible value, thus contributing to the long life of these components. A filter consisting of R3 and C14 removes most of the ripple from the DC output. The filtered voltage is then fed 120

®

CK1

©@©@©©©@@©@@

HEADPHONES

?14)i.035

4,

t"

/CJ\

]4

(§Pir■

It-!

■'m"

mU

II

©

■'■"®

(A) Schematic.

QUANTITY

ITEM

DESCRIPTION

Rl

4.7K, 'A -watt resistor.

R2

100K potentiometer.

R3

220K, Vi-watt resistor. 4.7 meg, Vi-watt resistor.

R4

10

25-volt electrolytic capacitor.

CI

10-mfd,

C2

0.5-mfd, 200-volt capacitor.

C3

1 00-mfd, 1 2-volt capacitor.

C4

0.1-mfd, 400-volt capacitors.

through

CI 3

C14

0.035-mfd,

C15

500-mmf, 600-volt ceramic capacitor.

Tl

Audio transformer

1 000-volt capacitor.

(Triad T-2, T-2X

or equiv.).

SI 12

SPST toggle

11 through

XI 10

6

X2

switch.

NE-2 neon lamps.

112

CK722 transistor. through

XI

1

20-ma selenium rectifiers (Federal International 5U1, or equiv.).

1 1 59;

VI

Geiger tube (Raytheon CK1026 or equiv.).

B1

Flashlight cells (Size C).

PI

Phone plug (Switchcraft

Jl

Phone

jack.

220 or equiv.).

24,000-ohm headphones (Trimm "Featherweight" or equiv.). (B) Parts list. Fig. 6-1 6. Schematic and parts list for the transistorized

Geiger

counter.

121

to Geiger tube V1 and to the voltage regulator, consisting of a string of neon lamps connected in series. These neon lamps break down at about 75 volts so that practically any desired output voltage can be obtained by inserting the proper number of lamps. For example, if a 300volt Geiger tube is used, four neon lamps would be re quired. With a 900-volt tube, such as the one specified in the parts list, it is necessary to use 12 neon bulbs in series, as shown in Fig. 6- 16 A. The Geiger tube (V1) is of the halogen-quenched type and has a long life. It is not appreciably affected by the number of counts which it has delivered nor by accidental application of overvoltage, as it can go into a state of con tinuous discharge without being damaged. When the Geiger tube is triggered by ionizing radiation, the output pulse is loud enough so that an amplifier is not needed for head phone monitoring. An amplifier and a miniature speaker could be added

if

desired.

Six small flashlight cells (Bl) serve to power this count er. Tests show that battery life will exceed 600 hours when

the instrument is used 40 hours per week or less. If a reduc tion in size is desired, the unit could be built around the size Z penlight cells, which should give fairly long life. It is sometimes desirable in an instrument of this kind to have the Geiger tube itself mounted in a probe and con nected to the main unit by means of a length of flexible cable. A method for constructing such a probe is sketched in Fig. 6-17. Plastic tubing is used to protect the Geiger tube— metal, such as aluminum, could screen out the radi ation which it is desired to detect and measure. This counter is easy to adjust for normal operation. After the unit is assembled and fresh batteries are installed, the switch is turned on and about 30 seconds are allowed for l"O.D.x7/8"

I.D.

PLASTIC TUBING

■3TURNS 116 BUS WIRE

rVl

r<-«> » l/»" R.H. SCREWS

rl"

■FEMALECOAX RECEPTACLE

CK1026-

r-r

Fig. 6-17.

122

ALUMINUM TUBING WALL)

U"0.D. X. 049"

1-7/8" DIA. PLASTIC R0D-

Construction

details

of the Geiger-counter

probe.

END CAP

stabilization. Potentiometer R2 is then slowly advanced until loud regular clicks are heard in the headphones. These clicks are caused by the neon lamps when they fire and are louder than Geiger tube clicks. The two sounds are easily identified with a little experience. When the neon tubes break down, it indicates that the voltago is slightly above the desired value. R2 is then backed off very slightly until the neon clicks are no longer heard. This is the correct operating voltage for V1. It is possible that the voltage will rise slightly after several min utes of operation. If this occurs, it is merely necessary to back off R2 slightly until clicking due to breakdown of the neon tubes ceases.

The battery voltage will normally decrease somewhat with use. To compensate for this decrease, R2 should be advanced slightly after every 80 to 100 hours of operation, until the battery voltage has dropped to about 6 volts. When fresh batteries are installed, the initial adjusting procedure described previously should be repeated. There will be a normal "background" count with this unit of perhaps 30 to 50 clicks per minute. If the click rate ex ceeds this value by an appreciable amount, it indicates that a source of radioactivity is nearby — the more rapid the clicks, the greater is the indicated radiation intensity. This unit is excellent for prospecting for uranium ore, since it is light in weight and has a long battery life. It can also be used for surveying an area for the presence of radioactive materials, and for checking clothing, food, and other items for radioactivity. COUNTER

OPERATED

FROM

POWER

LINES

The schematic and parts list for a home-built Geiger counter which operates on the 117-volt AC house current is given in Fig. 6-18. Although called a radiation fallout monitor, it can be used to detect nuclear radiation from any source which will activate the Geiger tube. The approxi mately 800 volts which are required for the Geiger tube are obtained from the voltage-multiplier circuit, consisting of diodes X1 through X6 and capacitors C1 through C6. Normally, V1 is not conducting; therefore no current flows through load resistor R4. However, if nuclear radia tion penetrates the walls of the tube, ionization will be 123

©2050

(A) Schematic. ITEM

QUANTITY

DESCRIPTION

1

Rl

270K, Vi-watt,

1

R2

10 meg, Vi-watt,

1

R3

22 ohm,

Vi-watt, 5% resistor.

1

R4

2.2 meg,

'A -watt, 5%

1

R5

47K, 1-watt, 5% resistor.

1

R6

IK, 2-watt, 5%

1

R7

1

R8

7

CI

1

C7

0.1 -mfd, 400-volt paper capacitor. 0.1-mfd, 1000-volt paper capacitor.

1

C8

20-mfd, 200-volt electrolytic capacitor.

1

C9

500-mmf mica capacitor.

1

T1

1

T2

7 1

XI VI

1

V2

2050 thyratron tube.

1

11

NE-2 neon lamp.

1

SP1

1 Vi-in. speaker; 10 ohm voice coil (Lafayette SK-61 or equiv.).

1 1

5% resistor. 5% resistor.

resistor.

1.2 meg, Vi-watt,

470K, Vi-watt, through

C6,

CIO

resistor.

5% resistor.

5% resistor.

Power transformer; pri. 117 volts AC; sec. 12SV@ 15 ma, 6.3V @ 0.6 amp (Stancor PS-8415 or equiv.).

Output transformer; pri. 2000 ohm; sec, 8-10 ohm (Lafayette TR-93 or equiv.). through

X7

1 N2070 diodes.

Geiger tube (Raytheon

CK1 026 or equiv.).

x 7'/4" piece perforated board. Wooden, plastic, or metal box. Screen wire (for speaker opening). Misc. hardware.

3Vi"

(B) Parts list. Fig. 6-18. A Geiger counter which operates from a conventional

124

117-volt line.

will occur, causing a pulse of volt age at capacitor C9. This pulse is sufficient to cause thyra-

produced and conduction

tron tube V2 to conduct, producing a voltage pulse at C1O which is transferred to the speaker through transformer T2. This pulse will produce a loud click in the speaker for each "quantum" of gamma radiation. Normal background radia tion will cause around 30 to 50 clicks per minute ; any sig nificant increase in the click rate will indicate that a source of nuclear radiation is nearby. Because of the relatively low-power drain, this instru ment can be kept plugged in >.nd left on continuously to serve as an around-the-clock monitor, either for civil de fense purposes or for any other application where such con tinuous monitoring is desirable. CONCLUSION

Only a few of the many possible electrical and mechani cal designs have been described in this chapter. The am bitious home constructor can get a good deal of enjoyment out of designing his own instrument, or following one of the designs given here.

125

Chapter 7

Nuclear Radiation

Applications

In the earlier portions of this book, you learned that there are many ways to detect and measure nuclear radi ation. It is fitting, then, that the last chapter be devoted to outlining briefly a few of the many applications of nu clear radiation. The primary concern in this chapter is with nuclear ra diation, rather than the somewhat broader subject of nu clear science. Nuclear radiation is utilized in many different fields —medicine, biology, agriculture, industry, insect con trol, food preservation, and research — to name only a few. The discussion will also include applications of radioiso topes and radioactive materials of various types, because in general, it is the radiation from these isotopes which is important. INSTRUMENTATION

Many applications for nuclear radiation have been found in industry. Among these are gauges for measuring the thickness, density, or level of a substance. Some of the ap plications for the various type instruments are given in the

following pages. Thickness Gauges

The transmission thickness gauge is widely used in in dustry. It is an instrument for continuously monitoring the thickness of almost any material which is made or processed in a continuous strip, such as paper, steel, etc. 126

Basically, the thickness gauge consists of a source of radioactive material and a radiation detecting device such as an ionization chamber. The material to be measured passes between these two, and the amount of radiation striking the detector depends on the thickness of the ma terial, assuming its density is constant. With proper calibra tion, such an instrument can detect very small changes in thickness, and the signal caused by the radiation trans mitted through the material may be used to make auto matic corrections in the production process. This type of continuous, noncontacting gauging is espe cially useful where products are moving rapidly, where temperatures are high, and where products are soft and may be easily marred. The isotope used as the source of radiation depends on a number of factors, including the weight per unit area of the material being measured. For example, thallium 204, with a useful life of about 4 years, can be employed for weights up to 100 mg/cm2. Strontium 90 extends the range up to 500 mg/cm2 and has the ad ditional advantage that it lasts about 25 years. For still higher weights, up to 2300 mg/cm2 (140 ounces per sq. yd.), radium can be used. Cobalt 60 is also employed. One of the recurrent problems in industry is that of measuring or controlling the thickness of the coating being applied to a material. Such a problem arises in applying a coating to paper, and in depositing tin on steel in the manu facture of tin plate. This problem can be tackled in two ways — by the use of two transmission gauges or by the use of a reflection gauge. The principle involved in the two-gauge technique is illustrated in Fig. 7-1. Here, one gauge (head No.1) is emDIFFERENCE BETWEEN HEADS No.lBNo.2

CD

®

n

(SECONDARY THICKNESS)

®

SECONDARY LAYER se (OR COATING)

\2

PRIMARY

LAYER

HEAD No. I

HEAD No. 2

Fig. 7-1 . Block diagram of a system for measuring thickness,

or the thickness

differential

of a coating.

127

to determine the thickness of the material before the coating is applied, and the second (head No. 2) after it is applied. The difference in the two readings indicates the thickness of the coating. ployed

IONIZATION CHAMBER REFLECTION GAGE

MATERIAL

DETAIL OF GAGE

ADVANTAGES: I - CAN MEASURE THICKNESS OF COATING AND/OR MATERIAL 2- MEASUREMENT MADE FROM ONE ACCESSIBLE SIDE 3- CAN MEASURE A VARIETY OF MATERIALS WITH ONE CALIBRATION

Fig. 7-2. Arrangement

for measuring

reflection,

the thickness of a coating

or backscatter,

by the

technique.

In the reflection, or backscattering gauge, radiation from a radioactive source is directed at a coated metal strip and

the reflected beam is measured by means of an ionization chamber (Fig. 7-2) . The change in intensity of the reflected beam is proportional to the coating thickness. If a steel roller is used, this technique can be employed to measure the thickness of paper, rubber, a sheet of plastic, or other nonmetellic material. Density Gauges

Fig. 7-3 shows an instrument for measuring the density and moisture content of construction materials, roads, soil, etc. In reality it consists of two instruments in one— one section for measuring density and the other for indicating moisture content. The probe, which is placed on the material being meas ured, is the heart of the instrument. A radioactive material in the probe (radium-beryllium) emits gamma rays and neu trons. The gamma rays are utilized for density measure ments.

When the rays are directed from the probe downward into the material being measured, some are backscattered, or reflected, to a gamma-ray detector in the probe, such as a Geiger tube. The intensity of this backscatter is inversely 128

(Courtesy Tellurometer, Inc.) Fig. 7-3.

Hidrodensimeter

for measuring

content of construction

the density materials.

and moisture

proportional to the density of the material. A tinier on the instrument makes it possible to measure the number of gamma rays which are recovered during a 60-second inter val, thus giving a direct indication of density. Slow neutrons given off by the radium-beryllium source are employed in measurements of moisture content. As mentioned in Chapter 1, neutrons are not readily absorbed by most materials, but are strongly affected by hydrogen atoms, such as those in the water molecules in the soil. Neu trons from the source in the probe are directed downward, and, if hydrogen atoms are present, some of the neutrons will be deflected back to the Geiger tube neutron detector in the probe. Again, by timing the detected neutrons for a 60-second interval, a precise indication of soil moisture content can be obtained. Another commercial instrument which operates on sim ilar principles is shown in Fig. 7-4. It can be used for the rapid determination of the density of asphalt, cement-stabi lization of soils, and other materials which require sur face compaction. Precise results of both density and 129

(Courtesy Nuclear-Chicago Corp.) Fig. 7-4.

d/M

gauge

system for determining

density

and moisture content.

(Courtesy Nuclear-Chicago Corp.) Fig. 7-5. One of the Qualicon Model 502 series of instruments density and moisture content of material

130

for measuring

on a conveyor

belt.

the

moisture content can be obtained by a simple, two-minute measurement. These basic principles can be further applied to an in strument for measuring both the density and the moisture content of material on a conveyor belt. Such an instrument is shown in Fig. 7-5. The basic operating principles of this instrument are given in Fig. 7-6. The material first passes under a scraper to rough-level it before it is measured. It is then irradiated with both neuCABLf TO MEASURING & RECORDING INSTRUMENTS

FAST NEUTRON SOURCE

^3- X5™X3

(Courtesy Nuclear-Chicago Corp.)

Fig. 7-6. The basic operating

principles

of the Qualicon

series of instruments.

trons and gamma rays, and the reflected, or backscattered, radiation is detected by suitable detection devices. With proper electronic equipment, continuous readings of density and moisture content can be obtained. These readings can then be employed in process controls of various kinds. A long cylindrical probe which can be inserted directly into the contents of a bin or hopper is also available as a measuring head for this instrument. Liquid-Level Gauges

A

liquid-level gauge can be made which operates on the basic principle of absorption to alter the intensity of a beam might or courru «w soma hdjustmu on nta. SHIELDEDC.60 , ■ tgEjfJ

Fig. 7-7. Use of a radioactive material and a suitable detector for indicating liquid height.

131

of radiation from a fixed source. Such a gauge is shown in Fig. 7-7. This adaptation has, for example, been found ad vantageous in measuring the height of molten metal in a cupola, since problems of corrosion and heat damage can be eliminated. This scheme is also adaptable to automatic recording and to the control of liquid level. MEDICAL

APPLICATIONS

Nuclear radiation is very widely used for both medical diagnosis and therapy. Sometimes it is the radiation itself that is useful, such as in cancer therapy. At other times, radioactive isotopes are used as tracers to locate diseased parts very accurately — such as brain tumors. Nuclear radi ation is sometimes employed indirectly, as in the steriliza tion of drugs. Body Scanners

A number of

companies

have developed instruments

for

scanning portions of the body, or even the entire body, to determine concentrations of radioactivity. These instru

Flg. 7-8.

132

The Nuclear-Chicago

Model 1700A isotope scanner.

are usually arranged to produce a plot of relative radioactivity over those portions of the body which are of interest. Usually the scanner (or scanners) is basically a scintilla tion counter with a large NaI (Tl) crystal for high sensitiv ity. The probe is usually arranged to collimate the incoming radiation so that only the radiation from a certain direction is detected. Many times, more than one scanner is used. Whole-body scanners, or monitors, come in a variety of shapes and sizes. One such unit, with a single scanning head, is shown in Fig. 7-8. This unit will scan an area up to 14 inches by 17 inches, and has a built-in printer to pro vide a permanent record of radioisotope distribution in the scanned area. It will handle any of the collimated scintilla tion heads manufactured by the same company and can be rolled to the patient's bedside. ments

(Courtesy Nuclear Enterprises, Fig. 7-9. The Nuclear

Enterprises

Ltd.)

NE8102B human-body radiation monitor.

A

multiple-head human body radiation monitor is shown utilizes four nondirectional scintillation-counter heads, each with a 5-inch NaI(Tl) crystal. A collimated detector (top center) which has a in Fig. 7-9. This instrument

133

(Courtesy Nuclear Enterprises, Fig. 7-10.

The Nuclear

Enterprises

Ltd.)

NE8102A human body radiation monitor.

3-inch crystal is also included. It can be used to determine the direction from which radiation is coming. This instru ment is very useful for detecting low-level radiation from the human body, either from naturally occurring radioiso topes or from ones introduced naturally or for medical purposes.

(Courtesy Landsverk Fig

134

7-11. The Landsverk

Electrometer

Model DDS-1A dual-scan instrument.

Co.)

A somewhat different type of whole-body radiation moni tor is shown in Fig. 7-10. This unit has four collimated scintillation counter heads, each with a NaI(Tl) crystal in diameter and 2 inches thick. The scanning heads are provided with circumferential and radial move ment, thus permitting the localization of specific activity with high counting efficiency. Still another type of body scanning instrument is shown in Fig. 7-11. Here, it is being used for simultaneous thyroid uptake and thigh count. This instrument is extremely versa tile because of the two separately adjustable counting heads. These dual detectors may be intercoupled through a coin cidence circuit to provide positron scanning, as applied in brain tumor diagnosis and localization. 4Vi inches

II

X

In

i

M

ill

i

[iiii

II

*

111

X

f~~

1

X!

fflliP Hiliu

Ii

iii lllll

X

ii

i

I

^

llil

"1

„j

1

I

1

Hi11

w

< (Courtesy Landsverk

Electrometer

Co.)

Fig. 7-1 2. A typical scan, showing the back-and-forth morion of the scanner. The short vertical lines indicate radiation intensity.

Fig. 7-12 shows how the scanning system on the fore

is

is

is

a

going instruments operate, and how picture of the radi ation pattern of the scanned area obtained. The scanner moved back and forth across the desired area, and the radiation intensity recorded by short vertical lines, as 135

shown. As the radiation intensity increases, the vertical lines become closer and closer together. These lines are drawn on the chart by means of a stylus. Cancer Treatment

The radiation from radioisotopes has been used exten sively in the treatment of various forms of cancer. Treat ment may be carried out by external application of radiation from sources such as cobalt 60, or the radioisotope may be taken internally or injected. In the latter case, the isotope concentrates at the desired spot in the body, and the radi ation is thus applied at the point where it will do the most good.

(Courtesy Brookhaven National

Lab.)

Fig. 7-13. Solution containing radioactive chlorine 38 being tunneled into the glass container (top right) for admin istration to a patient suffering from cancer.

An

of cancer treatment by injection of short lived radioactive isotopes is shown in Fig. 7-13. Here, a 136

example

solution containing radioactive chlorine is being injected into a patient at the Brookhaven National Laboratory hos pital. This particular isotope has a half-life of only 38 min utes, so great speed, precision, and care are necessary in getting it from the nuclear reactor to the hospital. In gen eral, only 15 minutes elapse from the time the isotope is removed from the reactor until it is injected into the patient. Surgery

Fig. 7-14 shows an interesting example of the use of a radiation-detecting instrument in surgery. Here a miniature scintillation probe as described in Chapter 4 is being used in a thyroid operation. The patient has been administered a dose of radioiodine, which tends to concentrate in the thyroid. By probing with the radiation detector, it is pos-

(Courtesy Nuclear-Chicago Corp.) Fig. 7-14. The Nuclear-Chicago used In surgery to localize

DS8 miniature surgical scintillation probe being accumulated radioiodine in thyroid remnants.

sible to determine if all of the diseased thyroid has been removed. In this particular operation, a 6-millimeter scin tillation probe which is sensitive to both beta and gamma radiation is employed. 137

The probe in this instrument uses a small NaI(Tl) sealed crystal at the tip, coupled to the photomultiplier tube in the handle by means of a light pipe. It is sensitive only at the end, with an equal distribution of sensitivity around the tip. The probe is marked in centimeters for determina tion of penetration depth. In this very short section, we have discussed only a few of the many applications of nuclear radiation and radio active isotopes in the field of medicine. Much research ef fort is being expended at present in this area, and promising results are being announced continually. As radioisotopes become cheaper and more readily available, it may be ex pected that they will be more and more widely used in everyday medical diagnosis and therapy. AGRICULTURE

Nuclear science has been applied in many different ways in the field of agriculture. It has been used to study fertili zer uptake and other plant nutrition studies, insect control, food preservation, plant mutations, and many other applica tions. Here again, applications are expanding rapidly and new uses are being found constantly. Insect Control One of the most fascinating applications

of nuclear radi

ation in the field of insect control was the eradication of the screwworm fly in Florida and Curacao. This was ac complished by flooding the infested areas with male screwworm flies which had been sterilized by large doses of radiation. During the course of this program, more than 3 billion sterilized flies were released over an area of about 70,000 square miles, resulting in the nearly complete eradication of the fly in this area. The irradiation to produce steriliza tion was usually carried out on the pupae, a stage in which the insects can be handled in bulk without adverse effects. The radiation dosage was adjusted to produce sterility but not large enough to affect longevity, mating, and other behavioral characteristics. Gamma rays from cobalt 60 were used to produce sterilization, the required dose being about 2,500 138

r.

Many experiments have been carried out with other in sects, some with promising results. If such programs are successful, they can perhaps reduce the necessity of using large amounts of insecticides, as such use has met with dis approval in certain areas. Much work has been done on the use of radiation to kill insects in stored grains, packaged foodstuffs, etc. One drawback appears to be the large doses necessary; for ex ample, a dose of 400 roentgens will cause 50% mortality in humans, whereas certain insects may require over 200,000 r within 48 hours to produce death. This amount of radiation can be very costly. Somewhat smaller doses may produce sterility. For example, 2,500 r produces ste rility in the male screwworm fly, and it has been found that 5,000 r is sufficient to sterilize a certain type of wasp. How ever, a massive dose of 180,000 r only caused sluggishness in the wasp, from which it recovered in a day or so. The use of radioactive tracers in insecticides has been of great value in the field of insect control to provide a better understanding of their physiological action, and to de termine the amount of residue left by the insecticides when used in normal and abnormal manners. Carbon 14 has been used extensively with DDT in such studies. Food Preservation

Much experimental work has been conducted in the field of food preservation by irradiation. The objective is to kill the food-destroying bacteria without altering the enzymes or producing harmful effects on palatability. If proper tech niques could be worked out, many types of foods now re quiring refrigeration could be stored indefinitely at room temperature. The various military services are particularly interested in this field. Great progress has been made, but much work remains. Cost is a big hurdle to overcome. The U.S. Army has been conducting extensive tests on high-level radiation for a number of years, with generally good results. They have reached the point where they ex pect to process bacon on a fairly large scale, and perhaps other foods in the near future. The Atomic Energy Commis sion has been concerned primarily with low-level radiation to determine if storage time of foods can be extended by this method. Results are very encouraging, and tests are continuing. 139

^^% (Courtesy Brookhaven National Fig. 7-15.

8'/j months after varying Potatoes photographed exposure to gamma rays.

Lab.)

An interesting experiment in the use of gamma radia tion to suppress sprouting in potatoes is illustrated in Fig. 7-15. The sample at the upper left was not irradiated. Others received dosages as follows: top center, 1,250 r;

top right, 5,000 r; bottom left, 20,000 r; bottom center, 80,000 r; bottom right, 106,250 r. This test would seem to indicate that the middle range of exposure best controls sprouting, thus extending storage life. The Canadian Government has done a great deal of work in this field, and is launching an extensive program of ir radiation for prolonging the storage life of potatoes. They have developed an irradiation trailer with a 15,000-curie source of cobalt 60. Irradiation is carried out right at the warehouse. The dose is about 7,500 rad, which will inhibit

sprouting for at least eight months. Total cost is estimated to be 100 to 300 per hundred pounds. Effects on Plants

The effects of various amounts of radiation on plant growth and mutations have been studied extensively at the 140

Brookhaven National Laboratory, as well as other locations. A view at the center of the experimental radiation field at Brookhaven is shown in Fig. 7-16. A 2,000-curie source of radioactive cobalt is stored underground in the 7-foot iron pipe. When no one is in the field, the source is raised by remote control and irradiates the field with high-energy gamma rays.

(Courtesy Brookhaven National

Lab.)

Fig. 7-1 6. Center of a radiation Held whem studies are being made of the effects of radiation on plant growth.

Irradiation, in general, stunts plant growth, as can be seen in the photograph. However, it may be that long-range effects, including genetic changes, are also produced. Mu tations are greatly speeded up by irradiation, particularly of seeds.

A majority of the mutations produced by radiation are definitely deleterious; however, improved strains are pro duced often enough to make the irradiation experiments worthwhile. For example, studies of irradiation-induced mu tations resulted in the introduction of a new pea bean in Michigan (called the Sanilac) in 1956. By 1960, 330,000 acres were devoted to this new bean, which had a yield of 15% greater than the Michelite variety previously pro duced ; and it was much more resistant to disease. 141

Radiation can also be introduced internally in plants by fertilizing them with material containing radioactive iso topes. Fig. 7-17 shows a scientist injecting radioactive phos phorus into a nutrient solution in which the roots of a barley seedling are growing. This work is part of the studies under way at Brookhaven National Laboratories on the nu trient uptake by root systems.

(Courtesy Brookhaven Fig. 7-17. Radioactive phosphorus is being nutrient solution feeding a barley seedling.

injected

National

Lab.)

into a

Many flowering plants will produce flowers of a different color following irradiation. Some results of experiments with carnations are shown in Fig. 7-18. This particular var iety, called White Sim, usually produces only white flowers. However, following irradiation of the young plant, 60% of the branches produced completely red or partially red flowers, as shown at the center and right. The new redflowering branches may be propagated indefinitely through cuttings, but not through seed. These mutations in carnations are not mere oddities, they have definite commercial possibilities. Professor Mehlquist, a breeder of ornamentals at the University of Connecticut, reports that he now has 21 radiation-induced mutants of the White Sim, six of which have been tested by the trade 142

(Courtesy Brookhaven Notional

Lab.)

Fig. 7-1 8. Effect of irradiation of a young carnation plant which ordinarily would produce only white blooms.

on a limited scale, and three of which are now generally available. Thus, work in the field of seed and plant irradi ation is not just a nonproductive scientific pursuit; it has

already resulted in many improved plants and strains which are in large-scale production. INDUSTRIAL

APPLICATIONS

We have already discussed a number of industrial appli cations of nuclear radiation in the instrumentation field, but there are a number of applications of interest outside this field. Perhaps first and foremost is the field of industrial radiography. Gamma rays have great penetrating power. They also expose photographic film. These two characteristics are combined in the nondestructive inspection of such materials as castings, welds, and the like in order to detect hidden flaws. A piece of photographic film is placed in back of the object being inspected, and a source of gamma rays, such as cobalt 60, is placed in front. The gamma rays penetrate the object and strike the film, producing an "X-ray" image of the object. Study of the developed film can then disclose any hidden defects. 143

Implementation of this technique is shown in Fig. 7-19. Here a pipe joint is being inspected. The film can be seen propped in place to the left of the pipe. The operator has removed the shield from in front of the gamma ray source, which can be radioactive cobalt, thulium, iridium, or ces-

(Courtesy Brookhaven National

Lab.)

Fig. 7-19. A source of gamma rays being used for the nondestructive inspection of a pipe joint.

ium. Iridium 192 is widely used for thicknesses up to the equivalent of three inches of steel, and cobalt 60 for up to six inches. This photograph is also interesting in that it indicates some of the precautions that must be taken by personnel in working with radioactive materials. A large warning sign is present, and the operator is standing well away from the beam of rays emerging from the shielded container under his left arm. This is a point which cannot be stressed too strongly — radioactive isotopes can be extremely useful in many areas of human endeavor, but they must be handled carefully and treated with respect. Nuclear radiation is being used commercially in many other areas. For example, irradiation of certain plastic ma terials, such as polyethylene, can provide it with valuable properties not otherwise attainable. Such irradiated material resists physical deformation and change at temperatures as 144

high as 230° C. Irradiated cross-linked polyethylene in the form of tape and film has found extensive application as an insulating material. Radioactive isotopes are widely employed in industry as static eliminators. Static electricity can be a hazard to pro duction processes and personnel; it can even cause fires and explosions. By ionizing the air at selected points, the static can be discharged. Krypton-85, a radioactive gas, is being studied as a very effective material for producing the ionization. Solid materials such as radium and polonium can also be employed. Radiotracing is an application which has expanded great ly in recent years. In this technique, radioactive isotopes are employed in wear and lubrication tests to find the best lub ricants and operating conditions involving piston rings, gears, and machine parts ; in wear tests on paint, varnishes, wax coatings, and other coatings ; in the study of detergents and various cleaning agents on cloth and fabrics ; in finding leaks in complicated systems and underground gas storage depots; and in tracing flow in pipelines, streams, catalytic crackers, chemical processing plants, and fluid or slurry systems. POWER

GENERATION

Considerable research has been carried out in recent years on the development of small, light, reliable sources of elec trical power. Nuclear science has played a large role in this search, and many promising developments are now under way. A few of them will be mentioned in this section. Working with the AEC, the Martin Company has devel oped a simple ceramic rod which spontaneously generates electricity. In this rod, two forms of the element strontium are combined into a one-piece thermoelectric generator which serves as its own heat source. The device consists of a strontium titanate rod with strontium 90 concentrated at one end as a heat source. This heat produces electricity through the thermoelectric effect in the strontium titanate. Although the efficiency and power output of this device are small, it is indicative of the direction in which research in this area is being pointed. The navigation buoy shown in Fig. 7-20 is a practical application of a nuclear power source. It contains a light which flashes every 5 seconds. Power for this light comes 145

(Courtesy Martin Marietta Corp.) Fig. 7-20. A nuclear-powered

navigation buoy.

from a thermonuclear generator called the SNAP-7A. SNAP stands for Systems for Nuclear Auxiliary Power. In this buoy about 40,000 curies of strontium 90 in the form of strontium nitrate pellets is sealed in a container, and generates heat because of its radioactive decay. The temperature differential between the hot junction of a thermocouple near the fuel block and the cold junction pro duces electricity. Sixty pairs of thermocouples, wired in series, are used in the energy conversion process. This power system is designed to operate for ten years or more without refueling. The thermocouples consist of lead-telluride thermoelectric elements which produce an output of 10 watts at 5 volts. After 10 years of operation, the hot junction temperature will be about 800° F and the cold junction about 115° F. On June 28, 1961, the U.S. Navy Transit 4-A satellite was launched into orbit containing a SNAP nuclear power source. The basic operating principle of this power source is similar to the one described previously, but plutonium 238 is used as the heat source. Since this radioactive isotope has a half-life of about 90 years, such a generator has a 146

very long potential life for use in satellites, space vehicles, etc. A similar generator is in Transit-4B. Improved versions of these generators will be used in future operational Transit systems, and perhaps in other satellite systems as well. An artist's sketch of an operational Transit navigational satellite is shown in Fig. 7-21. It will be powered by a SNAP-9A generator.

(Courtesy Martin Marietta Corp.) Fig. 7-21.

sketch of an operational Transit navigational satellite.

Artist's

An atomic-powered weather station is now in operation in a barren area north of Canada. It uses a thermoelectric generator with a strontium 90 heat source, which operates very much like the unit in the navigation buoy described

previously. It uses 17,500 curies of strontium 90, and gen erates 5 watts of power at 4 volts DC. This 4 volts is con verted to 28 volts DC to supply trickle charge to nickel cadmium batteries. MISCELLANEOUS

There are many other applications in which nuclear sci ence is playing an important part. Many additional applica tions are being explored and will be operational in a few years. Some of these uses will be described in the following. Radiocarbon Dating

Radiocarbon dating is a science which has been developed to a high degree of precision in recent years. It is a test 147

for indicating

of organic remains, such as wood, mummies, and the like. A number of companies provide radiocarbon dating service for a nominal fee. the age

The basic assumption underlying this technique is that the cosmic rays striking the atmosphere have always been similar to those which we experience at present. These rays produce neutrons which react with the nitrogen in the air to produce radioactive carbon-14. This is quickly oxidized to produce carbon dioxide. Thus, a small percentage of the carbon dioxide in the air contains radioactive carbon. The entire biological world receives its carbon either di rectly or indirectly from the C02 in the atmosphere, so the concentration of radioactive carbon in living materials can be expected to be the same as that in the atmosphere. When life ceases, the assimilation stops and gradual deterioration of the radioactive carbon begins. This carbon has a half-life of about 5,600 years; therefore, knowing that its original concentration equalled that in the atmosphere, the relative percentage of carbon which is still radioactive is an index of the age of the remains. Light Source

As might be expected, the radiation from radioactive ma terials can be made to produce visible light. Thus, it is pos

(Courtesy U.S. Radium Fig. 7-22. Railroad signal lamp which uses radioactive at the energy tcource.

148

Corp.)

krypton-85

sible to produce a light source which has long life and which does not require an external source of power. A switch lamp for railroad use is shown in Fig. 7-22. It contains four separate light sources for the four colors re quired. Light is produced when the beta rays from radio active krypton-85 strike specially selected radiation-respon sive zinc sulfide phosphor crystals. The lamp is carefully shielded so that the external radiation intensity is well be low Federal Radiation Council recommendations. Since krypton-85 has a half-life of about 10 years, these lamps should provide long, reliable service. The same basic principles can be applied in the design of other light sources, such as EXIT signs. These signs re main lit continuously, regardless of failure of conventional power sources, and have an anticipated life of many years. A wide variety of signs, light sources, etc. is available. Negative Ions

Another interesting

manufactured by the U.S. Radium Corp. is the Ionaire negative ion source. Some medical researchers report that negative ions can be helpful for individuals with symptoms of bronchial asthma, hay fever, and similar respiratory conditions. The Ionaire con tains a small amount of radioactive tritium sealed in a titan ium metal foil. Weak beta emission from the tritium ionizes the air in the immediate vicinity. The positive ions are absorbed within the element's head, and the negative ions are forced out into the room. device

Neutron Activation Analysis

The technique known as neutron-activation analysis has proved to be a very powerful tool in analytical chemistry, permitting the identification and concentration of unknowns when they exist in extremely minute quantities. In this technique the sample is irradiated with neutrons. This makes some atoms of each element present radioactive. Each element can then be identified and measured by its radiation characteristics. The technique can be used when the concentration of the unknown element is too low to be identified by chemical or spectroscopic methods, or where standard analysis is balked by contaminants. In the Pre face, reference was made to the use of this technique for determining the amount of arsenic in Napoleon's hair. 149

For industrial

purposes,

it appears that neutrons from

a

is entirely adequate. such as polonium-beryllium measuring instru the of the sensitive radiation With use ments now available, the amount of induced radioactivity necessary to accomplish an activation analysis can be kept

source

very low.

150

Appendix

I

Abbreviations The following is a list of some of the abbreviations which may be encountered when dealing with atomic radiation. For definitions see Appendix II. TERMS c— curie

cc—cubic centimeter cm — centimeter cm2 — square

centimeter

cpm — counts per minute

dps —disintegrations per second ev — electron volt g — gram

m — meter

MPC —maximum permissible concentration

MPE — maximum permissible exposure r— roentgen RBE —relative biological effectiveness rd — rutherford rem — roentgen equivalent man rep— roentgen equivalent physical r/hr — roentgens per hour a — alpha p— beta y—gamma

PREFIXES

b— billion

(109)

k — thousand (103) m — milli (one thousandth, 10-3) or million (106) f—micro (one millionth, 10-6) w —micromicro (one trillionth, 10-12) For example, mc is the abbreviation for millicurie, mc for microcurie and wc for micromicrocurie. mr is the abbrevi ation for milliroentgen, mev represents million electron volts, and bev is billion electron volts.

151

Appendix II

Definitions Working definitions of some of the terms commonly en countered when dealing with nuclear radiation are given in the following pages. alpha rays — A stream of helium nuclei. The helium nu cleus (alpha particle) has a mass number of 4 and an atomic number of 2. It consists of two protons and two neutrons. anticoincidence — Nonsimultaneous occurrence of two or more events — usually refers to ionizing events. atom — The smallest particle into which an element can be subdivided and still retain its chemical properties. atomic battery — A battery which obtains its energy entirely from nuclear reactions. atomic number — The characteristic of an element repre senting the net positive charge on the nucleus. It is the number of protons in the nucleus. atomic radiation — Radiation resulting from nuclear reac tions. The most common forms of such radiation are alpha, beta, and gamma rays, and neutrons. atomic weight — The relative weight of the atom of an ele ment compared to some standard. Usually, the standard is oxygen with an atomic weight of 16. The atomic weight is approximately equal to the sum of the neutrons and protons in the nucleus. background counts — Counts caused by any source other than the one which it is desired to detect. backscattering — Reflection of incident radiation back to wards the source. beta rays — A stream of high-velocity minute elementary particles ejected from the nuclei of a radioactive element. A beta particle carries the smallest negative charge found in nature and is identical with an electron. calorimeter — A device which measures the heat energy re leased in a reaction by measuring the temperature rise in a known quantity of water as a result of the reaction. chain reaction — A nuclear reaction in which the energy in the products of a single disintegration (usually neutrons) is sufficient to produce more than one new disintegration, thus resulting in a cumulative effect. 152

— Occurrence of two or

more events simultane ously — usually refers to ionizing events. cosmic rays — Very high-energy ionizing radiation entering the earth's atmosphere from outer space. count — An impulse generated by the detection of an ioniz ing event, resulting in a momentary aural or visual indi cation (click or flash). curie — A quantity of radioactive material which produces 3.700 X 1010 disintegrations per second. dead time — The time immediately following a discharge when a detector is unable to respond to ionizing radia tion. detector — A device for indicating the presence of ionizing radiation. deuterium — Heavy hydrogen, or the isotope of hydrogen having a mass number of 2. dose — The total received quantity of ionizing radiation. dosimeter — An instrument or device for detecting and measuring the accumulated dose of ionizing radiation. dynode — One of the intermediate electrodes in a photomultiplier tube. electrometer — An instrument for measuring differences in electrical potential between two points without drawing appreciable current. electron— The elementary charge of negative electricity — one of the basic building blocks of matter. electron avalanche — A condition wherein the applied volt age is sufficient to accelerate ionized particles enough to produce further ionization. electron volt — The amount of energy acquired by an elec tron when it moves through a potential difference of 1 volt. electroscope — An instrument for detecting the presence of an electric charge on a body. film badge — A badge containing a sensitized film which, when developed, indicates the total dose of ionizing radi ation to which the badge has been subjected; one type of Dosimeter. fission — Breaking up into parts; in atomic fission, an atom breaks up, releasing a great deal of energy in the process. Fission occurs only in heavy elements such as uranium and plutonium. coincidence

153

fusion — Joining of atomic nuclei to form a heavier nucleus. If light atoms, such as hydrogen or lithium, are caused to fuse, a great deal of energy is released. gamma rays — Electromagnetic radiation having an ex tremely short wavelength and great penetrating power. The wavelength is shorter than that of X rays. gas amplification — Ratio of the charge collected to the charge produced by the original ionizing event. Geiger counter — A complete instrument for indicating or measuring atomic radiation in which the detecting por tion is a Geiger tube. Geiger plateau — A range of voltages which result in a rela tively flat operating characteristic in the Geiger tube to which they are applied; fairly large changes in the ap plied voltage in this range result in only small changes in the output. Geiger tube — A two-electrode tube containing a small amount of gas which can be ionized by incident radia tion. The normal shape is cylindrical. It has a center con ductor which is operated at a positive potential. The conducting cylinder is negative. half-life — Time required for the activity of a radioactive material to be reduced by half. halogen — A general name which applies to four chemical elements with some similar chemical properties. The ele ments are flourine, chlorine, bromine, and iodine. halogen quenching — Quenching the discharge in a counter tube by introducing a small quantity of one of the halo gens. (See halogen and quenching.) heavy water — Water in which the hydrogen of the water molecule is in the form of the isotope deuterium. ion — An atom or molecule which has gained or lost one or more electrons and has an electric charge. ionization — The process of adding or removing one or more electrons to or from an electrically neutral atom or molecule so that it becomes charged. isotope —Variation of an element which has the same ex ternal electron configuration in the atom and therefore has the same chemical properties, but which has a dif ferent mass number. mass number —

A

number assigned to an atom, equal to the sum of the protons and neutrons in the nucleus.

154

meson — An elusive particle which may have a

unit positive

or negative charge or no charge at all. It may have any of a number of different weights. Life of the meson is very short. neutrino — A particle having the same mass as an electron but no electrical charge. neutron — An elementary building block of matter having the same mass as a proton (hydrogen nucleus) but con taining no charge. nuclear accelerator — A device for accelerating nuclear par ticles such as electrons or protons. nuclear reactor — A device in which controlled nuclear re actions take place. phosphor — A material, such as zinc sulfide, which gives off visible light when struck by nuclear radiation. The inside face of a television picture tube is coated with a phosphor. photomultiplier tube —A tube in which the electrons from a photoemissive cathode are multiplied by secondary emission from a series of dynodes. photon — A unit bundle of electromagnetic energy. planchet — A small metal container or sample holder that is usually used to hold radioactive materials that are being checked for the degree of radioactivity. positron — An atomic building block having the same mass as an electron, but carrying a unit positive charge. proton — An elementary building particle carrying a unit positive charge and having a mass about 1,840 times that of an electron. The nucleus of a normal hydrogen atom is a proton. quenching — The process of preventing a continuous dis charge in a counter tube which uses gas amplification. rad — A unit of absorbed dose of radiation. It amounts to 100 ergs of energy imparted to matter by any ionizing radiation per gram of irradiated material. This term is gradually replacing a somewhat similar term called the rep. (See Roentgen Equivalent Physical.) radioactivity — Spontaneous disruption of an atomic nucleus with the resultant emission of atomic radiation. radiophotoluminescence — A property of a material whereby its ability to give off visible light when irradiated with 155

ultraviolet light depends on its previous exposure to nu clear radiation. ratemeter — An instrument for indicating the rate at which counts are received — usually in counts per minute. relative biological effectiveness (RBE) — A measure of the effectiveness of various types of radiation on human tis sue. For example, alpha rays have an RBE of 20, indi cating that they are 20 times as damaging as an equivalent dose of gamma rays which have an RBE of 1. roentgen (r) — Unit of quantity of radiation, defined as that quantity which will produce, in 0.001293 grams of air, ions carrying 1 electrostatic unit of electricity. This amount of air is equal to 1 cc at 0° C and atmospheric pressure. roentgen equivalent man (rem) — Relative effectiveness of radiation on the human body. Rem = rad X RBE. roentgen equivalent physical (rep) — The quantity of radi ation which produces energy absorption of 93 ergs per gram of tissue. This term is being replaced by the rad. rutherford (rd) — A quantity of radioactive material which will produce one million (106) disintegrations per second. saturation — Condition in an ionization chamber when the applied voltage is sufficiently high to collect all the ions formed by the incident radiation, but is insufficient to produce ionization by collision. scaler —A device for indicating the total number of counts produced by a detector, such as a Geiger or scintillation counter. scintillation — Flash of light produced in a phosphor or crys tal by ionizing radiation. survey — A critical examination of the radiation near a source.

time-constant — A measure of the time required for a ca pacitor to charge or discharge in a resistance-capacity circuit. It is numerically equal in seconds to the product of the resistance in megohms and capacity in microfarads. tracer — A radioactive material used to trace the progress of a reaction or process of some kind. tritium — A radioactive isotope of hydrogen having an atomic number of 3. X rays — Electromagnetic rays having a wavelength between ultraviolet and gamma rays. 156

INDEX

Abbreviations,

151

Agricultural applications, nu clear radiation,

138-143 48-49 Alpha-beta-gamma probe, 43 Alpha rays, 14 Alpha-ray probe, 76-77 Alpha scintillation detector, 78-79 Atom, 7 Atom smasher, 11-12 Atomasters-Buntaine, G-301 low-level counter, 54-66 GSM5 survey meter, 42-44 Atomic building blocks, 7-8

Alarm, radiation,

Atomic Atomic Atomic Atomic

number, 9 radiation, 14-16 structure, 8-9 weight, 9

Badge dosimeters, 93-94 414 survey meter, 56-57 818B scintillation probe, 74-76

Baird-Atomic, 845

liquid scintillation

spectrometer, 85 870 alpha-ray probe, 76-77 Barium titanate, 39 Beta-gamma probe, 42 Beta rays, 15 Beta scintillation detector, 78-80 Body scanners, 132-135 Bomb, atomic, 13 thermonuclear, 14 Boron Trifluoride-filled tubes, 33-34 Building blocks, atomic, 7-8

Calorimeter, use of, 39 Cancer treatment, 136-137

Chamber, cloud, 24-25 ionization, 26-30 Chemical, dosimeters, 95-96 indicators, 37-38 Chloroform dosimeter, 95 Cloud chamber, 24-25 Coating, measuring thickness of, 127-128 Commerical Geiger-counter instruments, 42-56 Commercial ionization-counter instruments, 56-65 Commercial proportional counter, 65-68 Commercial scintillation detectors, 79-84 Commercial scintillation probes, 73-79 alpha-ray, 76-77 construction, 78-79 medical, 76-77 universal, 74-76 Conducting crystals, 37 Construction, hints, 104-105 scintillation probes, 78-79 Continuous discharge range, Geiger tube, 28-29 Counter, home-built, 104-125 low-level, 54-56 operated from power line, 123-125 portable transistorized, 42-46 simple, 105-106 solid-state, 90-91 with amplifier, 106-109 with interrupter-type power supply, 118-120 with meter indicator, 113-118

with transistor

amplifier, 112-113

with transistorized

power supply, 120-123 Crystals, conducting, 37 scintillation, 36-37 Curie, definition of, 18-19 157

Dating, radiocarbon, 147-148 Definitions, 152-156 Depletion layer, ionization in, 89 Density gauges, 128-131 Detector, dual, 54-56 gamma-radiation, 61-64 solid state, 38, 87-91 Deuterium, 8, 10 Diamond, as radiation detector, 37 Dosages, maximum, 21 Dosimeter, chemical, 95-96 definition of, 92 film, 93-95 ionization, 98-101 radiophotoluminescence, 96-97 Dual detector, 54-56

DuBridge-Brown circuit,

Gelman 31100 recording-rate meter, 46-48 Glossary, 156 Gold-leaf electroscope, 34-35 Gun-type survey meter, 56-57

Half-life, theory of,

16-17

Halogen quenching, 30-31 Helium atom, 8-9 Hidrodensitometer, 128-129 High voltage, need for in Geiger tube, 41 Home-built dosimeters, 104-125 Humans, effect of radiation on, 21-23

Hydrogen atom, 8-9

36

Indicators,

chemical, 37-38

Industrial applications, nuclear

radiation, 143-145 Insect control, 138-139 Instrumentation, nuclear, 126-132 Ionization, 8-9 chambers, 26-30 Ionization counter instruments, commercial, 56-65 Ionization-type dosimeter, 98-101 Ions, negative, 149 Isotope analysis kit, 64-65 Isotopes, 9-10 radioactive, 19-20

Einstein's equation,

10-11 35-36, 58-61 vibrating reed, 63 Electron, 7-8 Electron volt, 10-11 Electroscopes, 34-35

Electrometer,

Emulsions, photographic, 38

Energy,

10-11

Equation, Einstein's, 10-11

Fail safe relay, operation of,

54

Ferrous sulphate dosimeter, 38 Film, as radiation detector, 38 Film badges, 93-94

Film dosimeters,

93-95 13, 18 Fissionable materials, 18 Food preservation, 139-140 Franklyn Systems 60-4

Kahl FH40TV radiameter, Kilorutherford, 19

49-51

Fission, nuclear,

scintillation detector, Fricke dosimeter, 38 Fusion, nuclear, 13-14

84

Gauges, density, 128-131 liquid-level, 131-132 thickness, 126-128 Gamma-radiation detector, 61-64 Gamma rays, 15 Gas amplification, 28 Geiger counter instruments, commercial, 42-56 Geiger plateau, 30 Geiger range, 28-29 Geiger tube, 30-34 construction, 31-33 relationship between applied voltage and output, 26-28 158

Landsverk L-75D isotope analysis kit, 64-65 Lauritsen electroscope, 35 Light source, using radioactive material for, 148-149 Liquid-level gauge, 131-132 Liquid scintillometers, 85-86 Low-level counter, 54-56

M Mass number,

9

Materials, scintillation,

characteristics, of, 71-72 Medical applications, nuclear radiation, 132-138 Medical scintillation probes, 77-78

Megarutherford,

19

Meson, 7, 15, 16 Meter, pocket, 48-49 recording-rate, 46-48 survey, 42-46

Microcurie,

19

Millicurie, 19 Monitor, radiation, Mutations,

51-54 plant, 140-143

Proportional counter,

commercial, 65-68

Proportional range,

Geiger tube, 28, 29

N Neon-light flasher,

addition of, 112 Negative ions, 149 Neptunium, 13 Neutrino, 7 Neutron, 8 activation analysis, 149-150 detector, 72, 90-91 dosimeter, 102 generator, 17-18 measurement, 33 Novel power supply, 110-112 Nuclear-Chicago, 720 liquid scintillometer, 86 2112 survey meter, 65-68 2660 survey instrument, 44-46 Nuclear fission, 13, 18 Nuclear fusion, 13-14 Nuclear instrumentation, 126-132 Nuclear reactions, 11-14, 18, 33 Nuclear standards, 18-21 Number, atomic, 9 mass 9 N. Wood SC-1U scintillation counter, 74

Organic crystals, 72-73 Organic quenching, Geiger tube, 30

Particles, alpha, beta, 15 Photographic

14

emulsions, 38

Photomultiplier tube, operation of, 70

Plants, effect of radiation on, 140 Plastic scintillator, 73 Plateau, Geiger, 30 Plutonium, 13, 18

P-N junction,

38-39

Pocket meters, 48-49 Portable transistorized counters, 42-46

Positron,

7, 15-16

Potatoes, sprout suppression, 140 Power generation, 145-147 Power supply, interruptertype, 118-120 power line, 123-125

transistor,

120-123

Prefixes, 151 Preservation, food, 139-140 Probe construction, 78-79

Quenching, Geiger tubes, 30-31

Rad, 20 Radiameter, 49-51 Radiation, alarm, 48-49 atomic, 14-16 damage effects, 88-89 effect on humans, 21-23 monitor, 51-54 Radioactive elements, 14, Radioactivity, 16-18

artificial,

16

17-18

Radiocarbon dating, 147-148 Radiophotoluminescence, 96-97 Radiotracing, 145 Ratemeter, 99 definition of, 41

RBE,

20-21

Reactions, nuclear, 11-14 Recording-rate meter, 46-48 Relative biological effectiveness, 20-21 Rem, 20 Rep, 20 Roentgen, definition of, 20 meter, 100 Rutherford, definition of, 19

Satellite, power source for, 146-147 Saturated region, Geiger tube, 27 Scaler, use of, 41-42 Scanners, body, 132-135 Scintillation crystals, 36-37 Scintillation detectors, commercial, 79-84 Scintillation materials, characteristics of, 71-72 Scintillation probes, commercial, 73-79 alpha-ray, 76-77 construction, 78-79 medical, 76-77 universal, 74-76 Scintillometers, solid, 69-73 Shorthand, atomic, 9 Silicon solar battery, 39 Silicon solar cells, 87-88 Silver cloride crystals, 37 Simple counter, 105-106

SNAP,

146-147

Sodium iodide crystals, 37 Solar cells, silicon, 87-88 Solid scintillometers, 69-73 159

Solid-state, counters, commercial, 90-91 detectors, 38-39, 87-91 devices,

Underwater detector,

102-103

Standards, nuclear, 18-21 Structure, atomic, 8-9 Stylus, for chart, 48 Surgical applications, 137-138 Survey meter, 42-46 electrometer-type, 58-60 gun-type, 56-57

Technical Associates, Cutie Pie Model CP-3 survey meter, 58-60 Juno Model 7 survey meter, 60-61 Temperature range, halogenquenched tubes, 31 Thickness gauges, 126-128 Transistorized counter, 42-46

Tritium,

12

8U

20231*5

160

V Victoreen, 440 survey meter, 61-64 589 scintillation detector, 80-83 Vamp 808 radiation monitor, 51-54 Volt, electron-, 10-11 Voltage ranges, ionization chamber, 26-28

W Wall thickness, Geiger tubes, Weight atomic, 9 Wilson cloud chamber, 24-25 Window thickness, Geiger tubes, 32-33

8, 10

Tubes, electrometer, 36 Geiger, 30-34 photomultiplier, 70

0

I

3

X rays,

BN

84

21

on n OZOH

r*

32