Radiographic Testing

The ASNT PERSONNEL TRAINING

PUBLICATIONS

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RADIOGRAPHIC TESTING STUDENT

GUIDE

Compiled for ASNT by Bahman Zoofan The Ohio State University

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The American Society for Nondestructive Testing, Inc.

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Published by The American Society for Nondestructive Testing, Inc. 1711 Arlingate Lane Columbus, OH 43228-0518 Copyright © 2007 by The American Society for Nondestructive Testing, Inc. All rights reserved. ASNT is not responsible for the authenticity or accuracy of information herein, and published opinions or statements do not necessarily reflect the opinion of ASNT. Products or services that are advertised or mentioned do not carry the endorsement or recommendation of ASNT.

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IRRS~ Materials Evaluation®, NDT Handbook, Nondestructive Testing Handbook®,

The NDT Technician and <www.asnt.org> are trademarks of The American Society for Nondestructive Testing, Inc. ACCP, ASNT, Level III Study Guide, Research in Nondestructive Evaluation and RNDE are registered trademarks of The American Society for Nondestructive Testing, Inc.

ASNT exists to create a safer world by promoting the profession and technologies of nondestructive testing. Edited by Ann Spence ISBN-13: 978-1-57117-160-3 Printed in the United States of America First printing 04/07 Second printing with revisions 11/09

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Nondestructive Testing Training Program

Student Guide

I. Introduction to the Radiographic Testing Student Guide The materials in this training package are designed to provide basic knowledge of the fundamentals of radiographic testing. The training program that you are participating in will contain the following classroom hours of instruction to present the information suggested in the ASNT publication Recommended Practice No. SNT-TC-1A. Level I training will include lectures on chapters 1 through 5, with an average of approximately one hour per lesson. Level II training will include lectures on all eight •

chapters with an average of approximately one hour per lesson, with emphasis on chapters 6 through 8. The student shall assume the responsibility for reading all assignments, including the Radiographic Testing Classroom Training Book, additional reference materials associated

with the Student Guide, attend all lectures, and participate in classroom discussions. Short exams will be administered after each lesson to provide the student with an indicator of their progress and to stimulate study.

II. Contents of Training Package Your training package contains the following materials, with specific instructions and assignments to be given by the course instructor.

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1. Student Guide A.

Student Guide Introduction that outlines the purpose, content, and use of the training material.

B.

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Radiographic Testing Classroom Training Book that serves as the major text

for the training course. C.

Printed copy of the electronic lecture Radiographic Testing consisting of eight individual lessons on the fundamentals of radiographic testing. The copy of the electronic lecture is identical to the presentation used by the instructor during the lectures on each chapter. During the lecture the student should use the Student Guide to make additional notes, and it will also be valuable to study at a later date.

D.

Quizzes. The instructor may elect to remove the quizzes from your packet prior to starting the course and administer them as each lesson is completed. A quiz will be furnished for each of the lessons in the training course.

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2. Outline of Lessons and Related Reading Assignments The reading assignments will be made by the instructor and will correlate with the lectures. The Radiographic Testing Classroom Training Book published by ASNT follows the lessons/lectures in the training course in the following order. Lesson 1 - Introduction to Radiographic Testing. Lesson 2 - Radiographic Testing Principles. Lesson 3 - Equipment. Lesson 4 - Radiographic Film. Lesson 5 - Safety. Lesson 6 - Specialized Radiographic Applications. Lesson 7 - Digital Radiographic Imaging. Lesson 8 - Special Radiographic Techniques.

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Personnel Training Publications

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III. Optional Reference Material The following materials are available from ASNT and are suggested for students looking for additional information on radiographic testing. 1.

Nondestructive Testing Handbook, third edition: Volume 4. Radiographic Testing.

2.

ASNT Level II Study Guide: Radiographic Testing Method.

3.

ASNT Level III Study Guide: Radiographic Testing Method, second edition.

4.

Supplement to Recommended Practice No. SNT-TC-1A (Q & A Book): Radiographic Testing Method.

5.

Supplement to Recommended Practice No. SNT-TC-1A (Q & A Book): Neutron Radiographic Testing Method.

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6.

Radiographic Interpretation, Revised.

7.

Gamma Radiation Safety Study Guide, second edition.

8.

Working Safely in Radiography, second edition.

• Student Guide: Radiographic Testing

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Table of Contents Nondestructive Testing Training Program: Student Guide Introduction to the Radiographic Testing Student Guide Contents of Training Package Student Guide Outline of Lessons and Related Reading Assignments Optional Reference Material Table of Contents

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Lesson 1 - Introduction to Radiographic Testing Radiography Advantages of Radiographic Testing Limitations of Radiographic Testing Test Objective Safety Considerations Qualification Certification Lesson 1 - Quiz Lesson 2 - Radiographic Testing Principles Penetration and Differential Absorption Geometric Exposure Principles Film/Detector Image Sharpness Image Distortion X-radiation and Gamma Radiation X-rays Electron Source Electron Target Electron Acceleration Radiation Intensity Inverse Square Law X-ray Quality Characteristics Interaction With Matter Photoelectric Absorption Compton Effect Pair Production Scatter Radiation Internal Scatter Sidescatter Backscatter Gamma Rays Natural Isotope Sources Artificial Sources Gamma Ray Intensity Specific Activity Half Life Gamma Ray Quality Characteristics Lesson 2 - Quiz

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j j ii ji jii v

3 3 3 3 4 .4 4 5 7 11 11 11 12 13 13 13 14 14 14 14 15 15 16 16 16 17 17 17 17 17 18 18 .18 19 19 .19 19 23 v

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Lesson 3 - Equipment X-ray Equipment Portable X-ray Units X-ray Tube Tube Envelope Cathode Filament Heating Anode Focal Spot Linear Accelerators X-Ray Beam Configuration Accelerating Potential Iron Core Transformers Heat Dissipation Equipment Shielding Control Panel Gamma Ray Equipment Gamma Ray Sources Radium Artificial Radioisotopes Isotope Cameras Lesson 3 - Quiz

27 27 27 27 28 28 29 29 29 30 30 30 30 30 31 31 31 31 32 32 32 35

Lesson 4 - Radiographic Film Introduction Usefulness of Radiographs Radiographic Contrast Subject Contrast Film Contrast Film Characteristic Curves Film Speed Graininess Film Selection Factors Film Processing Tank Processing Tank Processing Procedures Developing Stop Bath Fixing Washing Drying Automatic Film Processing Darkroom Facilities and Equipment Lesson 4 - Quiz

39 39 39 39 40 41 .41 42 42 43 43 44 44 45 45 45 46 46 46 .47 49

Lesson 5 - Safety Introduction Units of Radiation Dose Measurement Roentgen (R) Radiation Absorbed Dose (rad) Quality Factor Roentgen Equivalent Mammal (rem) International System of Units (SI) Measurements Becquerel Replaces Curie Coulomb per Kilogram Replaces Roentgen Gray (Gy) Replaces Rad

53 53 53 54 54 54 55 55 55 55 56

Personnel Training Publications

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Sievert (Sv) Replaces Rem Maximum Permissible Dose Protection Against Radiation Allowable Working Time Working Distance Shielding Exposure Area Radiation Protective Construction Gamma Ray Requirements United States Nuclear Regulatory Commission Occupational Radiation Exposure Limits Levels of Radiation in Unrestricted Areas Personnel Monitoring Caution Signs, Labels and Signals Exposure Devices and Storage Containers Radiation Survey Instrumentation Requirements Radiation Surveys Detection and Measurement Instruments Pocket Dosimeters Personal Electronic Dosimeters Film Badges and Thermoluminescent Dosimeters Optically Stimulated Luminescence (OSL) Badges Ionization Chambers Geiger-Mueller Counters Area Alarm Systems Electrical Safety Lesson 5 - Quiz

56 56 57 57 57 58 58 59 60 60 60 61 61 61 62 62 62 63 63 64 64 65 65 65 66 66 69

Lesson 6 - Specialized Radiographic Applications Introduction Selection of Equipment Accessory Equipment Diaphragms, Collimators and Cones Filters Screens Fluorescent Screens Lead Screens Masking Materials Image Quality Indicators (IQI) Shim Stock Film Holders and Cassettes Identification and Location Markers Area Shielding Equipment Densitometer X-Ray Exposure Charts Preparation of an Exposure Chart Film Latitude Gamma Ray Exposure Chart Dated Decay Curves Film Characteristic Curves Radiographic Equivalent Factors Exposure Variables Movement Source Size Source-to-Film Distance (SFD) Radiographic Applications

73 73 .73 74 74 75 75 76 77 78 78 80 80 81 82 82 83 84 85 85 86 87 87 87 88 88 89 90 Student Guide: Radiographic Testing

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Radiography of Welds Tube Angulation Incident Beam Alignment Discontinuity Location Critical and Noncritical Criteria Improper Interpretation of Discontinuities Elimination of Distortion Proper Identification and Image Quality Indicator Placement Radiography of Welded Flat Plates Radiography of Welded Corner Joints Single-Wall Radiography of Tubing Double-Wall Radiography of Tubing Tubing up to 3.5 in. (9 em) Outside Diameter (OD) Radiography of Closed Spheres Radiography of Closed Tanks Radiographic Multiple Combination Application Radiographic of Hemispherical Sections Panoramic Radiography Radiography of Large Pipe Welds Radiographic Techniques of Discontinuity Location Alignment Discontinuity Depth Location Techniques Radiography of Brazed Honeycomb Radiography of Semiconductors Techniques of Semiconductor Radiography Alignment of Semiconductors Lesson 6 - Quiz

90 90 90 91 91 91 92 92 92 93 93 93 93 94 94 95 95 95 95 96 96 96 97 97 97 98 101

Lesson 7 - Digital Radiographic Imaging Introduction Development Detectors for Digital Imaging Principles of Digital X-ray Detectors Charge Coupled Devices Thin Film Transistor Light Collection Technology Radiation Conversion Material Storage Phosphors Linear Arrays Scanning Beam, Reversed Geometry Detection Efficiency Spatial Resolution Modulation Transfer Function (MTF) Gain and Offset Correction Radiation Damage Selection of Systems to Match Application X-ray Detector Technology Amorphous Silicon Detectors Amorphous Selenium Detectors Charge Coupled Device Radiographic Systems Linear Detector Arrays Lesson 7 - Quiz

105 105 105 106 107 107 108 108 108 108 109 109 110 110 110 110 111 111 112 112 112 112 113 115

Personnel Training Publications

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Lesson 8 - Special Radiographic Techniques Introduction Fluoroscopy Image Amplifier Television Radiography Xeroradiography Stereoradiography and Double Exposure Stereoradiography Double Exposure (Parallax Radiographic Technique) Flash Radiography In-Motion Radiography Conclusion Lesson 8 - Quiz

119 119 119 120 120 120 121 121 121 122 122 122 125

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• Student Guide: Radiographic Testing

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• Chapter 1: Radiographic Testing Principles In this lesson you will learn about: • Concepts of radiography. • Advantages and limitations of radiographic testing. • Test objectives. • Personnel qualifications and certifications .

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Lesson 1

Introduction to Radiographic Testing RADIOGRAPHY 1.

In radiography, test objects are exposed to X-rays, gamma rays or neutrons, and an image is produced.

2.

Radiography is used to test a variety of products, such as castings, forgings and weldments. It is also used heavily in the aerospace industry for the detection of cracks in airframe structures, detection of water in honeycomb structures and detection of foreign objects.

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Advantages of Radiographic Testing 1.

Radiography can be used on most materials.

2.

Radiography provides a permanent record of the test object.

3.

Radiography reveals discontinuities within a material.

4.

Radiography discloses fabrication errors and often indicates the need for corrective action.

Limitations of Radiographic Testing

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1.

The radiographer must have access to both sides of the test object.

2.

Planar discontinuities that are not parallel to the radiation beam are difficult to detect.

3.

Radiography is an expensive testing method.

4.

Film radiography is time consuming.

5.

Some surface discontinuities or shallow discontinuities may be difficult, if not impossible, to detect.

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TEST OBJECTIVE The objective of radiographic testing is to ensure product reliability. Perfonning the actual

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radiographic test is only part of the procedure. The test results must then be interpreted to acceptance standards by qualified personnel, and an evaluation of the results must be made.

Safety Considerations Radiation can cause damage to the cells of living tissue, so it is essential that personnel be aware and protected. Compliance with state and federal safety regulations is mandatory.

QUALIFICATION 1.

It is important that personnel responsible for radiographic testing have adequate

training, education and experience.

2.

Guidelines are for the qualification and certification of nondestructive testing personnel.

3.

ASNT has published guidelines for training and qualifying nondestructive testing

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(NDT) personnel since 1966. These are known as: Recommended Practice No. SNT-TC-1A: Personnel Qualification and Certification in Nondestructive Testing.

4.

Recommended Practice No. SNT-TC-1A describes the knowledge and capabilities of nondestructive testing personnel in tenns of certification levels.

5.

Per SNT-TC-1A, there are three basic levels of qualification applied to NDT personnel: a.

Level I.

b.

Level II.

c.

Level III.

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Personnel Training Publications

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CERTIFICATION 1.

The formal certification of a person in NDT to a Level I, Level II and Level III is a written testimony that the individual has been properly qualified.

2.

Certification is meant to document the actual qualification of the individual in a specific NDT method.

3.

Proper qualification and certification are extremely important in modern manufacturing, fabrication and inservice inspection due to the impact on the health and safety of the public .

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• Student Guide: Radiographic Testing

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Notes

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Lesson 1

Quiz Please answer true or false to the following statements. 1.

Radiography can reveal all types of discontinuities within a material.

2.

Radiography cannot be used in aerospace due to radiation safety

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constraints .

3.

In radiography, access to both sides of the test object is not necessary.

4.

Radiography provides a permanent visual record of internal discontinuities.

5.

Every radiographer can decide acceptance or rejection of a test object.

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• Chapter 2: Radiographic Testing Principles In this lesson you will learn about: • Penetration and differential absorption. • Geometric exposure principles. • Film!detector image sharpness. • Characteristics of X-radiation and gamma radiation.

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• X-ray tubes . • Inverse square law. • X-ray quality characteristics. • Interaction of radiation with matter. • Gamma rays (natural isotope sources, artificial sources and gamma ray intensity) .

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Lesson 2

Radiographic Testing Principles PENETRATION AND DIFFERENTIAL ABSORPTION 1.

X-rays and gamma rays have the ability to penetrate materials, including materials that do not transmit light.

2.

Depending on the thickness and density of the material, and the intensity of the source being used, the amount of radiation that is transmitted through the test object will vary.

3.

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The radiation transmitted through the test object produces the radiographic image.

Figure 2.1 in the Radiographic Testing Classroom Training Book illustrates the partial absorption characteristics of radiation. Thicker portions of the test object or dense inclusions will appear lighter because of more absorption of the radiation.

GEOMETRIC EXPOSURE PRINCIPLES 1.

A radiograph is a shadow picture of a test object placed between the film/detector and the X-ray or gamma radiation source.

2.

If the film/detector is placed too far from the test object, the image will be enlarged.

3.

If the test object is too close to the source, the image will be greatly enlarged, resulting in the loss of resolution.

4.

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The degree of enlargement will vary according to the relative distances of the test object from the film/detector.

5.

As shown in Figure 2.2 in the Radiographic Testing Classroom Training Book, the D d . . d' f Image en1argement: Df .IS equa1 to th e ratIo: o

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Film/Detector Image Sharpness 1.

2.

The sharpness of a radiographic image is determined by: a.

The size of the radiation source.

b.

The ratio of the object-to-film/detector distance.

c.

The source-to-object distance.

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The unsharpness or fuzziness around an image is called geometric unsharpness (penumbra), as shown in Figure 2.3 in the Radiographic Testing Classroom Training Book.

3.

To minimize the geometric unsharpness (Ug) in the image, the test object should be placed as close to the film/detector as possible.

4.

Most radiographic codes recommend the maximum acceptable values for geometric unsharpness.

5.

Geometric unsharpness can be calculated by using the following formula: U =Fd g D

a.

Ug is the geometric unsharpness (in millimeters or inches).

b.

F is the source size (the maximum projected dimension of the radiation

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source, or effective focal spot size). c.

D is the distance from the source of the radiation to the object being

radiographed. d. 5.

d is the distance from the source side of the test object to the film/detector.

Optimum geometric unsharpness of the image is obtained when: a.

The radiation source is small.

b.

The distance from the source to the test object is relatively large.

c.

The distance from the test object to the film/detector plane is small.

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Personnel Training Publications

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Image Distortion Two possible causes of radiographic image distortion are: 1.

The test object and the film/detector plane are not parallel.

2.

The radiation beam is not directed perpendicular to the film/detector plane.

X-RADIATION AND GAMMA RADIATION 1.

X-rays and gamma rays are part of the electromagnetic spectrum.

2.

These rays have high energy and short wavelengths.

X-rays The conditions required to generate X-rays are:

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1.

A source of electrons.

2.

A suitable target for electrons to strike.

3.

A means of speeding the electrons in the desired direction.

Characteristic X-rays: When an electron from a higher energy level interacts with an electron in a lower orbit of an atom, characteristic X-rays may be generated. Continuous radiation: The generated X-rays have a continuous energy spectrum and are not entirely dependent on the disturbed atom's characteristics. Bremsstrahlung radiation: This is a German name for braking or continuous radiation. KeV (Kilo-electron volts): This unit corresponds to the amount of kinetic energy that an electron would gain when moving between two points that differ in voltage by 1 kV.

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MeV (1 000000 electron volts): This unit corresponds to the amount of kinetic energy an electrons gains when moving between two points that differ in voltage by 1 MV. Student Guide: Radiographic Testing

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Electron Source 1.

When a suitable material is heated, some of its charged negative particles (electrons) become agitated and escape the material as free electrons.

2.

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Cathode: In an X-ray tube, a coil of wire or filament (known as the cathode) serves as the electron source.

Electron Target For industrial radiography applications, a solid material of high atomic number, usually tungsten, is used as the target in the tube anode.

Electron Acceleration 1.

By placing a positive charge on the anode of an X-ray tube and a negative charge on the cathode, free electrons are accelerated from the cathode to the anode.

2.

The electron path should occur in a vacuum.

Radiation Intensity 1.

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The number of X-rays created by electrons striking the target is one measure of the intensity of the X-ray beam.

2.

Intensity depends on the number of electrons available at the X-ray tube cathode.

3.

Keeping the other factors constant, an increase in the current through the tube filament will increase the cathode temperature, causing emission of more electrons and consequently increasing the intensity of the X-ray beam.

4.

Similarly, though to a lesser degree, an increase in the applied tube voltage will increase the beam intensity.

5.

The output rating of an X-ray tube is expressed in volts (kV or MeV).

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Personnel Training Publications

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Inverse Square Law 1.

The intensity of an X-ray beam varies inversely with the square of the distance from the radiation source.

2.

The relationship is known as the inverse square law:

where I] and 12 are the received radiation intensities at distances D] and D2 .

X-Ray Quality Characteristics 1.

The spectrum of continuous X-rays covers a wide band of wavelengths, as shown in Figure 2.9 in the Radiographic Testing Classroom Training Book.

2.

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An increase in applied voltage in an X-ray tube increases the intensity (quality) of X-rays. This produces higher energy rays with greater penetrating power.

3.

X-rays with higher energy (shorter wavelengths) are called hard X-rays.

4.

X-rays with lower energy (longer wavelengths) are called soft X-rays.

5.

Variation in tube current changes the intensity of the beam, but the spectrum of wavelengths produced remains unchanged. (See Figure 2.11 in the Radiographic Testing Classroom Training Book.)

6.

Effects of changes in kilovoltage and tube current on the produced X-rays are summarized in Table 2.1 in the Radiographic Testing Classroom Training Book.

• Student Guide: Radiographic Testing

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INTERACTION WITH MATTER 1.

Any action that disrupts the electrical balance of an atom and produces ions is

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called ionization. 2.

X-rays passing through matter cause ionization in their path.

3.

X-rays are photons (bundles of energy) traveling at the speed of light.

4.

In passing through matter, X-rays lose energy to atoms by ionization processes known as: a.

Photoelectric absorption.

b.

Compton effect.

c.

Pair production.

Photoelectric Absorption 1.

In photoelectric absorption, when X-rays (photons) with relatively low energy pass through matter, the photon energy may be transferred to an orbital electron (see Figure 2.12 in the Radiographic Testing Classroom Training Book).

2.

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Part of the energy is expended in ejecting the electron from its orbit, and the remainder gives velocity to the electron.

3.

This phenomenon usually takes place with low energy photons of 0.5 MeV or less.

4.

This absorption effect is what makes radiography possible.

Compton Effect 1.

When higher energy photons (0.1 to 3 MeV) pass through matter, part of the photon energy is expended in ejecting an electron. The remaining slower energy photons travel at different angles compared to the original photon path (see Figure 2.13 in the Radiographic Testing Classroom Training Book).

2.

This process is repeated, progressively weakening the photon, until the photoelectric effect completely absorbs the last photon.

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Personnel Training Publications

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Pair Production Pair production occurs only with higher energy photons of 1.02 MeV or more (see Figure 2.14 in the Radiographic Testing Classroom Training Book).

Scatter Radiation 1.

The major components of scatter radiation are the low energy rays represented by photons weakened in the Compton process.

2.

Scatter radiation is low-level energy content of random direction.

Internal Scatter 1.

Internal scatter is the scattering that occurs in the object being radiographed (see Figure 2.15 in the Radiographic Testing Classroom Training Book).

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2.

It affects image definition by blurring the image outline.

3.

The increase in radiation passing through matter caused by scatter in the forward direction is known as buildup.

Sidescatter 1.

Sidescatter is the scattering from walls and the surrounding of the object in the vicinity of the test object that cause rays to enter the sides of the test object.

2.

Sidescatter obscures the image outline just as internal scatter does.

Backscatter 1.

Backscatter is the scattering of rays from the surface or from objects beneath or behind the test object (see Figure 2.17 in the Radiographic Testing Classroom

Training Book).

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2.

Backscatter also obscures the test object.

Student Guide: Radiographic Testing

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GAMMA RAyS 1.

Gamma rays are produced by the disintegration of the nuclei of a radioactive

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isotope. 2.

Isotopes are varieties of the same chemical element having different atomic weights.

3.

The wavelength and intensity of gamma waves are determined by the source isotope characteristics and cannot be controlled or changed.

Natural Isotope Sources 1.

Some heavy natural elements disintegrate because of their inherent instability.

2.

Radium is the best known and most used natural radioactive source.

3.

Natural radioactive sources release energy in the form of: a.

Gamma rays.

b.

Alpha particles: Positively charged particles having mass and charge equal in

magnitude of a helium nuclei. c.

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Beta particles: Negatively charged particles having charge and mass equal in

magnitude to those of the electron. 4.

The penetrating power of alpha and beta particles is relatively negligible.

Artificial Sources 1.

There are two ways of manufacturing radioactive isotopes, or so-called radioisotopes: a.

By using the by-product of nuclear fission in atomic reactors, such as cesium-I37 (Cs-I37).

b.

By bombarding certain elements with neutrons to make them unstable. Examples include cobalt-60 (Co-60), thulium-I70 (Tm-I70), selenium-75 (Se-75) and iridium-I92 (Ir-I92).

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These artificial isotopes emit gamma rays, alpha particles and beta particles. Personnel Training Publications

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Gamma Ray Intensity 1.

The activity of a gamma ray source determines the intensity of its radiation.

2.

The measure of activity is the curie, which is 3.7 x 10 10 becquerel (Bq) or disintegrations per second.

Specific Activity 1.

Specific activity is defined as the degree of concentration of radioactive material

within a gamma ray source. 2.

Specific activity is expressed in terms of curies per gram or curies per cubic centimeter.

3.

Specific activity is an important measure of radioisotopes because the smaller the source, the sharper the radiographic image that can be produced (as shown in

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Figure 2.4 in the Radiographic Testing Classroom Training Book) .

Half Life 1.

The length of time required for the activity of a radioisotope to decay to one half of its initial intensity is called its half life.

2.

The half life of a radioisotope is a basic characteristic and depends on the particular isotope of a given element.

3.

Dated decay curves (similar to the one shown in Figure 2.18 in the Radiographic Testing Classroom Training Book) are supplied by source suppliers for each

particular radioisotope and should be used by radiographers to determine the exact source intensity.

Gamma Ray Quality Characteristics

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1.

Radiation from a gamma ray source consists of rays whose wavelengths and energy are determined by the nature of the source.

Student Guide: Radiographic Testing

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2.

Each of the commonly used radioisotopes has a specific application because of the fixed gamma energy characteristics.

3.

Table 2.3 in the Radiographic Testing Classroom Training Book lists the most

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common radioisotopes for radiography and their equivalent energy. 4.

Gamma rays and X-rays have identical propagation characteristics, and both conform to the inverse square law.

5.

The mechanism of interaction of gamma rays with matter is identical to those discussed for X-rays.

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Personnel Training Publications

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Notes

Notes

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Lesson 2

Quiz

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• Chapter 3: Equipment In this lesson you will learn about: • X-ray equipment. • Gamma ray equipment. • Equipment protection devices. • Radioisotopes .

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Lesson 3

EquipIllent

x-RAY EQUIPMENT There are three basic requirements for the generation of X-rays: 1.

A source of free electrons.

2.

A means of rapidly accelerating the beam of electrons.

3.

A suitable target material to stop the electrons.

Portable X-ray Units

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In field radiography, such as inspection of pipelines, bridges, vessels, and ships, portable

X-ray units are very important. The characteristics of these tubes are: 1.

Lightweight.

2.

Compact.

3.

Usually air-cooled.

X-ray Tube 1.

The main components of X-ray equipment include: a.

Thbe: Enclosed in a high-vacuum envelope of heat-resistant glass or ceramic.

b.

Cathode: To produce free electrons.

c.

Anode: Target which the electrons strike.

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2.

Associated with the tube are the following parts: a.

Equipment that heats the filament, accelerates, and controls the resultant free electrons.

3.

b.

Equipment to remove the heat generated by the X-rays.

c.

Shielding of the equipment.

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There are many varieties in the size and shape of X-ray tubes.

Tube Envelope 1.

2.

The tube envelope is constructed of glass or ceramic that has: a.

A high melting point.

b.

Sufficient strength.

For the following reasons, a high-vacuum environment for the tube element is necessary. a.

Prevents oxidation of the electrode material.

b.

Permits ready passage of the electron beam without ionization of gas within

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the tube. c.

Provides electrical insulation between the electrodes.

Cathode The cathode of an X-ray tube consists of: 1.

Focusing cup: Functions as an electrostatic lens.

2.

Filament: A coil of tungsten wire that produces a cloud of electrons by flowing an

electrical current through it.

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Personnel Training Publications

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Filament Heating 1.

A small flow of current through the filament is enough to heat it to a temperature that causes electron emission.

2.

A change in the number of emitted electrons varies with the current flow through the filament.

3.

The tube current is measured in milliamperes (rnA), and it controls the intensity of X-rays.

Anode

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1.

The anode of an X-ray tube is usually made of copper.

2.

Copper and tungsten are the most common anode materials.

3.

A dense target material is required to ensure a maximum number of collisions.

4.

Material with a high melting point is necessary for a target to withstand the excessive heat.

Focal Spot 1.

The image sharpness is partly determined by the size of the focal spot.

2.

The electron beam is focused so that it bombards a rectangular area of the target.

3.

The projected area of the electron beam is the effective focal spot (see Figure 3.2 in the Radiographic Testing Classroom Training Book).

4.

The size to which the focal spot can be reduced is limited by the heat generated by target bombardment.

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Linear Accelerators There are two types of linear accelerators: 1.

Standing wave linear accelerator for energy up to 200 MeV.

2.

Traveling wave linear accelerator for energy up to 30 GeV (giga-electron volts or

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billion electron volts).

X-ray Beam Configuration 1.

Once the X-rays are created, they cannot be focused or otherwise directed.

2.

The direction of useful X-radiation is determined by the positioning of the target and the lead shielding.

Accelerating Potential 1.

The applied potential between the cathode and anode determines the penetrating effect of the produced X-ray.

2.

The higher the voltage, the greater the electron velocity along with shorter wavelengths and more penetrating power for the generated X-rays.

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Iron Core Transformers 1.

The majority of X-ray equipment for industrial radiography (up to 400 kV) use iron core transformers.

2.

Their basic limitations are their size and weight.

Heat Dissipation 1.

X-ray generation is a very inefficient process as most of the electron energy is expended in producing heat.

2.

Heat dissipation in the X-ray tube is achieved by a flow of oil, gas or water.

3.

Efficiency of an X-ray tube cooling system is the main factor in determining the duty cycle of the tube.

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Personnel Training Publications

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EQUIPMENT SHIELDING 1.

To prevent unwanted radiation, lead is used to shield the X-ray tube.

2.

The shielding design varies with different X-ray tubes, but in all cases, it serves to absorb that portion of the radiation that is not traveling in the desired direction.

CONTROL PANEL 1.

The control panel of an X-ray system is designed to permit a radiographer to set the desired exposure parameters.

2.

The control panel also provides critical indications for tube performance, such as the flow of oil or water in the cooling system.

GAMMA

•

1.

RAy

EQUIPMENT

Handling and storage of gamma ray sources are extremely important since they cannot be shut off.

2.

The United States Nuclear Regulatory Commission (NRC) and various state agencies recommend safety standards for proper transportation, storage and handling of radioisotopes.

3.

Every inspection firm should prepare a comprehensive safety procedure for the storage and handling of all their radioisotopes. More information on this can be found in Lesson 5.

Gamma Ray Sources 1.

•

2.

There are two types of gamma ray sources: a.

Natural isotopes.

b.

Artificial isotopes.

Most isotopes used in industrial radiography are round wafers encapsulated in a stainless steel cylinder. Student Guide: Radiographic Testing

31

Radium 1.

Radium is a natural radioactive substance having a half life of about 1600 years.

2.

Most radium sources consist of radium sulfate packaged in either spherical or

•

cylindrical capsules. 3.

Because of its low specific activity and its long half life, radium is rarely used in industrial radiography.

Artificial Radioisotopes 1.

2.

The artificial radioisotopes used in industrial radiography for gaging purposes are: a.

Cobalt-60 (Co-60).

b.

Iridium-I92 (Ir-I92).

c.

Selenium-75 (Se-75).

d.

Thulium-I70 (Tm-I70).

e.

Cesium-I37 (Cs-I37).

Table 3.2 in the Radiographic Testing Classroom Training Book gives a summary of the main characteristics of the most used isotopes.

•

Isotope Cameras 1.

The equipment to accomplish safe handling and storage of radioisotope sources is called a camera or exposure device.

2.

These cameras are self-contained units, meaning no external power supply is required.

3.

The exposure devices contain self-locking mechanisms ensuring safety in accordance with ANSI and ISO requirements, in addition to NRC and IAEA requirements.

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•

•

Notes

Notes

•

•

•

•

Lesson 3

Quiz Please answer true or false to the following

6.

statements. 1.

referred to as a camera or projector.

A high vacuum environment for an X-ray tube is to make it lighter for easy transportation.

2.

Typical isotope equipment is often

7.

Compared to cobalt-60, iridium-192 has a shorter half life.

The function of a focusing cup in the cathode of an X-ray tube is to focus the

•

produced X-radiation.

3.

The isotopes used in industrial radiography are usually natural isotopes.

4.

A major disadvantage of isotope radiography is the high cost of isotope equipment and sources.

5.

•

After a radioactive material is stored, its gamma radiation shuts off.

35

• Chapter 4: Radiographic Film In this lesson you will learn about: • Radiographic contrast. • Film density. • Film characteristic curves. • Film graininess.

•

• Film selection factors . • Film processing (manual and automatic). • Darkroom facilities .

• 37

•

Lesson 4

Radiographic Filll1 INTRODUCTION 1.

Radiographic film consists of: a.

Base: A thin, transparent plastic sheet.

b.

Emulsion coat: A coat of an emulsion of gelatin about 0.001 in. (0.003 cm) thick on one or both sides. The emulsion coat contains very fine grains of silver bromide (AgBr).

2.

•

Latent (hidden) image: Exposure of radiation on the film that cannot be detected until chemical processing occurs .

3.

Visible image: Image on the film after developed by chemical processing.

Usefulness of Radiographs 1.

Film density: Degree of darkening on the developed film.

2.

Radiographic contrast: Difference between two film areas. The darker area (higher density) has received more radiation compared to the area of light density.

3.

Definition: Sharpness of any change in film density.

4.

Contrast and definition are important for a successful interpretation of radiographs.

RADIOGRAPHIC CONTRAST 1.

•

The film density D is a logarithmic value defined as: D = 10glO 10 1

where (10) is the intensity of the incident light and 1 is the intensity of the transmitted light through the film. The higher the number, the darker the film. 39

2.

If the intensity of light is 1000 units and the film allows only one unit of that intensity to pass through, the film density based on the previous equation will be: 1000 D = 10glO - - = 3 1

3.

•

Radiographic contrast (as shown in Figure 4.2 in the Radiographic Testing Classroom Training Book) is defined as the difference in the film density between

two selected areas of the exposed and developed film. 4.

Higher contrast is better for film interpretation.

5.

Radiographic contrast is a combination of:

6.

a.

Subject contrast.

b.

Film contrast.

Radiographic contrast depends on: a.

Applied radiation energy (penetrating quality).

b.

Contrast characteristics of the film.

c.

Amount of exposure (the product of radiation intensity and exposure time).

d.

Film screen.

e.

Film processing.

f.

Scattered radiation.

•

Subject Contrast 1.

Subject contrast is the relative radiation intensities passing through any two selected portions of material. Subject contrast depends on the following factors:

2.

a.

Type and shape of the test object.

b.

Energy of the applied energy radiation (wavelength, type of source).

c.

Scattered radiation.

Subject contrast decreases as the wavelength of the incident radiation decreases.

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3.

Higher subject contrast can be achieved by: a.

Larger thickness variation.

b.

Use of different X-ray or gamma ray energies.

c.

Masks.

d.

Diaphragms.

e.

Filters or screens.

Film Contrast 1.

The ability of film to detect and record different radiation exposures as differences in film density is calledfilm contrast.

2.

The relationship between the amount of exposure and the resulting film density is expressed in the form of film characteristic curves and is determined by the following factors:

•

a.

Film grain size .

b.

Chemistry of the film processing chemicals.

c.

Concentration of the processing chemicals.

d.

Development time.

e.

Development temperature.

f.

Agitation in the developer solution.

Film Characteristic Curves 1.

Figure 4.3 in the Radiographic Testing Classroom Training Book shows a film characteristic curve.

•

a.

The vertical axis is the resulting film density.

b.

The horizontal axis is expressed in a logarithm of relative exposure.

c.

The minimum point of the curve on the vertical axis is calledfog density.

d.

Based on this curve, as the exposure increases, film contrast increases .

Student Guide: Radiographic Testing

41

2.

3.

A film characteristic curve has two different sections: a.

A tail of lower densities.

b.

A straighter portion (with a higher slope on the curve).

•

High radiographic contrast is achieved with densities along the straight portion of a characteristic curve. This is the reason that films should always be exposed for a density of at least 1.5.

4.

Most radiographic codes, standards and specifications usually give upper and lower density limits within a range of 1.8 to 4.0.

Film Speed 1.

Film speed is an important consideration in determining the proper exposure time to obtain the desired film density.

2.

Figure 4.4 in the Radiographic Testing Classroom Training Book illustrates films with high, medium and low speeds.

3.

Knowing film speed is important when selecting film for each particular radiographic testing task.

•

Graininess 1.

Graininess is the visible evidence of the grouping into clumps of the silver particles that form the image on the radiographic film.

2.

Figure 4.5 in the Radiographic Testing Classroom Training Book shows the effect of grain variation on the image definition.

3.

42

The degree of graininess of an exposed film depends on the following factors: a.

Grain size.

b.

The quality of the radiation.

c.

Film processing conditions.

d.

Type of film screens.

Personnel Training Publications

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•

FILM SELECTION FACTORS 1.

When not otherwise specified by the customer or governing standards, the selection of film is made by the radiographer. Most of the time, the selection of film is based on the following factors:

2.

•

a.

Need for certain contrast and definition quality.

b.

Thickness and density of the test object.

c.

The type of indication or discontinuity normally associated with the object.

d.

Size of an acceptable indication.

e.

Accessibility, location and configuration of the test object.

f.

Customer requirements.

In film selection, remember that: a.

Film contrast, film speed and graininess are interrelated.

b.

Faster films need shorter exposure time but usually have larger grains and poor resolution/sensitivity.

c.

Slower films need longer exposure time but have finer grain and good resolution/sensitivity.

d.

Film manufacturers' recommendations for film selection are a useful tool in selecting the proper film for a given application.

FILM PROCESSING 1.

Film processing makes the latent image visible.

2.

The following general precautions must be observed during film processing: a.

Follow manufacturer recommendations for chemical concentrations, temperature and processing time.

b.

•

Use equipment, tanks, trays and holders that can withstand the chemical action.

c.

Ensure tanks are clean. Student Guide: Radiographic Testing

43

d.

Use recommended safelights and checked them regularly.

e.

Maintain cleanliness in the darkroom to avoid any artifacts on developed radiographs.

f.

•

Avoid any contamination of different solutions.

TANK PROCESSING The arrangement of a tank processing (manual processing) unit is shown in Figure 4.6 in the Radiographic Testing Classroom Training Book. 1.

The tanks for processing solutions and wash water should be deep enough for the film to be submerged.

2.

The chemicals in the tanks must be stirred and the temperature must be checked with a calibrated thermometer before turning off the ambient light.

3.

All required equipment should be arranged before turning off the ambient light.

4.

All unnecessary materials should be kept away from the processing area.

5.

Test the safelights and arrange them for easy viewing. Follow the standard

•

recommendations for regular checking.

6.

Lock the darkroom door to prevent accidental exposure to ambient light.

7.

To load the film inside the hangers, grasp it by its edges or comer to avoid fingerprints, bending, wrinkling or crimping during handling.

8.

Keep the loading area completely dry.

9.

Follow the tank processing procedures.

Tank Processing Procedures There are five separate steps in tank processing:

44

1.

Developing.

2.

Stop bath.

3.

Fixing. Personnel Training Publications

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•

4.

Washing.

5.

Drying.

Developing Developing is the chemical process of reducing silver bromide particles in the exposed area of the film emulsion to metallic silver. 1.

Follow the manufacturers' recommendations for developing temperature and time.

2.

Agitate the film during developing to obtain a uniform development and to avoid any air bubbles from attaching to the film.

3.

Use strips of exposed radiographs to control the developer activity as a method of regular quality control checking.

4.

•

Follow the manufacturers' recommendations to replenish the solution.

Stop Bath The stop bath, a solution of acetic acid and water, serves to remove the residual developer solution from the film. 1.

Running uncontaminated water for at least 2 min. can be used as an alternative to the stop bath.

2.

The manufacturers' directions should be used to make the stop bath solution.

3.

A fresh stop bath solution is yellow in color and clear under safelight.

Fixing 1.

Fixer, an acidic solution, has two functions on the film: a.

It dissolves and removes the silver bromide from the undeveloped portions of

the film without affecting the developed portion.

•

b.

It hardens the emulsion gelatin.

Student Guide: Radiographic Testing

45

2.

The minimum time required for fixing is twice the amount of time necessary to clean the film.

3.

Fixing time should not exceed 15 min.

4.

Improper fixing shortens the archival length of the film.

5.

Film should be agitated in fixing solution at 2-min. intervals.

6.

The replacement of fixing solution should be determined by checking the acidity of

•

the solution.

Washing After fixing, washing is necessary to remove the fixer from the emulsion. 1.

Each film is washed for a period of time equal to twice the fixing.

2.

Hypo clearing agent may be used to speed up film washing.

3.

Best results for washing are obtained with a water temperature between 65 and 70 of (18.3 and 21.1 °C).

4.

To avoid any watermarks, film is immersed in a wetting agent that also aids in reducing the drying time.

•

Drying The final stage of film processing is drying.

Automatic Film Processing Automatic film processing systems are used whenever the volume of work makes them economical. 1.

The entire processing cycle is completed in less than 15 min.

2.

Automatic film processing units consistently produce radiographs of much higher quality than those obtained using a manual process.

3.

Loading the film inside the unit should be done in a dark environment.

4.

Properly maintaining the system is the key for high performance of an automatic system.

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Personnel Training Publications

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DARKROOM FACILITIES AND EQUIPMENT Some requirements that must be satisfied in the design and construction of a darkroom: 1.

It must be lighted with suitable and tested safelights.

2.

It must be protected against ambient light from outside sources.

3.

The walls and ceiling must be painted with lightly colored, semigloss paint.

4.

Darkroom floors are usually covered with chemical resistant, waterproof and slip-proof materials.

5.

Cleanliness is of great importance during the entire film processing procedure .

•

• Student Guide: Radiographic Testing

47

Notes

•

•

•

•

Lesson 4

Quiz Please answer true or false to the following

6.

statements. 1.

A film should always be exposed for at least a density of 1.0.

The emulsion gelatin coating is only applied to one side of industrial films.

7.

Slower films need longer exposure because of larger grain size.

2.

If light with intensity of 10 000 units is used to see a radiograph film, and

8.

only 100 units of light pass through it,

•

Time in developing solution is always fixed.

the density of that film is 3.0 . 9. 3.

4.

Subject contrast cannot have any effect

agitation inside the developing solution

on the radiographic contrast.

should be avoided.

The type of the radiation source has an effect on the subject contrast.

5.

To attain acceptable film quality, film

10. Wetting agents help to speed up the fixing procedure.

Developing conditions do not have any effect on film characteristic curves .

• 49

• Chapter 5: Safety In this lesson you will learn about:

• Units of radiation dose measurement. • International system of units (SI) measurements. • Maximum permissible dose. • Protection against radiation.

•

• Radiation protective construction. • United States Nuclear Regulatory Commission. • Occupational radiation exposure limits. • Levels of radiation in unrestricted areas. • Personnel monitoring. • Exposure devices and storage containers. • Detection and measurement instruments. • Electrical safety.

• 51

•

Lesson 5

Safety INTRODUCTION This lesson is designed to present some of the basic radiographic safety procedures. 1.

Radiographers must be aware of the latest effective safety regulations.

2.

Radiation safety practices are based on the effects of radiation on the human body and the characteristics of radiation.

3.

Personnel protection is dependent on detection devices, as well as the proper use of time, distance and shielding.

•

4.

Agreement States are states that observe the regulations covering use, handling and transportation of radioactive materials approved by the Nuclear Regulatory Commission (NRC).

5.

All of the safety regulations are designed to limit exposure to the radiographer and to provide protection to the general public.

6.

The radiographer, who is employed by a licensee of NRC or who is employed by a licensee of an Agreement State, must have knowledge of, and comply with, all applicable regulations.

UNITS OF RADIATION DOSE MEASUREMENT 1.

The damaging effects of radiation are dependent on both the type and the level of energy of the radiation.

•

2.

For different types of radiation, a relative biological effectiveness is applied.

3.

For radiation safety purposes, the cumulative effect of radiation on the human body is of primary concern. S3

Roentgen (R) 1.

The roentgen (R) or sievert (Sv) is the physical unit measure of the ionization of air by X-radiation or gamma radiation.

2.

•

R is defined as the quantity of radiation that will produce one electrostatic unit (esu) of charge in one cubic centimeter of air at standard temperature and pressure (STP).

3.

1 R of radiation equals absorption by ionization of about 83 ergs (unit of work or energy in physics) of radiation energy per gram of air.

4.

For practical purposes, mR is often used, which is: 1 mR = 111000 R.

Radiation Absorbed Dose (rad) 1.

Radiation absorbed dose (rad) is the unit of measurement of radiation absorption by humans.

2.

It represents an absorption of 100 erg of energy per gram of irradiated tissue.

3.

Whereas the roentgen applies only to X-rays and gamma rays, rad applies to any type of radiation.

4.

For X-ray and gamma radiation, exposure to 1 R results in 1 rad.

5.

The unit gray (Gy) has been introduced as: 100 rad

•

= 1 Gy.

Quality Factor 1.

The quality factor takes into account the biological effect of different radiations on the human body.

2.

Quality factor values are determined by the National Committee on Radiation Protection. They are summarized in Table 5.1 in the Radiographic Testing Classroom Training Book.

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Roentgen Equivalent Mammal (rem) 1.

Roentgen equivalent mammal (rem) represents the radiation absorbed dose (rad) multiplied by the quality factor of the type of radiation.

2.

Radiation safety levels are established in terms of roentgen equivalent mammal (rem).

3.

Since the quality factor of X-radiation and gamma radiation is 1, then: 1 rad = 1 rem.

INTERNATIONAL SYSTEM OF UNITS 1.

(SI)

MEASUREMENTS

The Nuclear Regulatory Commission, state regulations and radiographers in the U.S. often still use the old English units: curie, roentgen, rem and rad.

2.

Different organizations, such as the following, support the replacement of older units with SI units: The National Institute of Standards & Technology (NIST), The

•

American National Standards Institute (ANSI), The American Society for Testing and Materials (ASTM), The Institute of Electrical and Electronics Engineers (IEEE), the International Organization for Standardization (ISO) and The American Society for Nondestructive Testing (ASNT).

Becquerel Replaces Curie 1.

Curie (Ci) is the original unit for radioactivity, which is defined as: 3.7 x 10 10 disintegrations per second.

2.

In SI, the replacement unit for radioactivity is the becquerel (Bq), which is one disintegration per second.

3.

1 Ci

= 37 GBq (gigabecquerel), where giga = 109 .

Coulomb per Kilogram Replaces Roentgen

•

1.

Coulomb (C) is the unit of electrical charge, where: 1 C = 1 ampere xIs Student Guide: Radiographic Testing

55

2.

1 R = 258 microcoulombs per kilogram of air (258 jlC'kg- 1 of air).

Gray (Gy) Replaces Rad

•

In the SI system, the unit of radiation dose is the gray (Gy), and 1Gy =100 rad.

Sievert (Sv) Replaces Rem In the SI system, the unit of radiation absorbed by the human body is the Sievert (Sv), and 1 Sv = 100 rem.

MAXIMUM PERMISSIBLE DOSE 1.

Permissible dose is defined by NIST as the dose of radiation that is not expected to cause appreciable bodily injury to a person.

2.

The following restrictions for the maximum annual permissible dose limits for classified workers should be observed: a.

Total effective dose equivalent being equal to 5 rem (0.05 Sv).

•

Or b.

The sum of the deep dose and the committed dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rem (0.5 Sv).

c.

The maxmum dose absorbed by the lens of the eye being 15 rem (0.15 Sv).

d.

A shallow dose equivalent of 50 rem (0.5 Sv) to the skin of the whole body or to the skin of any extremity.

3.

The maximum annual radiation dose is limited to 5 rem (0.05 Sv).

4.

The absorbed dose shouldn't exceed 0.5 rem (5 mSv) during an entire pregnancy.

5.

Dose limits to the general public shall not exceed 0.002 rem or 2 mrem (0.02 mSv) per hour or exceed 0.5 rem or 500 mrem (5 mSv) annually.

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Personnel Training Publications

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PROTECTION AGAINST RADIATION Safe radiographic techniques and radiographic installation design are achievable by applying these principles: 1.

Time: Keep the time close to a radiation source as low as possible.

2.

Distance: Keep the distance from a radiation source as high as possible.

3.

Shielding: Keep adequate shielding from the radiation source.

Allowable Working Time 1.

The amount of absorbed radiation by the human body is directly proportional to the time that the body is exposed to radiation. Example: 2 rem (0.2 mSv) in 60 s = 10 mrem (1 mSv) in 5 min.

•

2.

Allowable working time for working with gamma sources is calculated by measuring radiation intensity and substituting it in the following equation: Allowable working time in h/wk

permissible exposure in Ci/wk =

exposure rate in Ci/h

Working Distance 1.

The greater the distance from a radiation source, the lower the radiation intensity.

2.

The inverse square law is used to calculate radiation intensities at various distances from a radiation source:

!.l- _D; 12

-

D~

where 1] and 12 are intensities at distances D] and D 2 , respectively. Practice the examples (1 to 4) on pages 62 and 63 in the Radiographic Testing Classroom

•

Training Book.

Student Guide: Radiographic Testing

57

3.

The same principles hold for X-radiation. The intensity at a known distance with predetermined current and voltage setting (usually given by the X-ray tube's manufacturer) can be determined by applying the inverse square law.

4.

•

Radiation intensity at any point is the sum of the primary radiation and the secondary (scattered) radiation at that point.

Shielding 1.

Materials commonly used for shielding to reduce personnel exposures are lead, steel, water and concrete.

2.

Shielding cannot stop all of the energy of X-radiation or gamma radiation; therefore, it is practical to measure shielding efficiency in terms of half value layers.

3.

Half value layer (HVL) is that amount of shielding that will stop half of the radiation of a given intensity.

4.

Similarly, shielding efficiency is often measured in tenth value layers. A tenth value layer is that amount of shielding that will stop nine tenths of the radiation of

•

a given intensity. (Look at Tables 5.4 and 5.5 in the Radiographic Testing Classroom Training Book.)

5.

Follow the examples in the Radiographic Testing Classroom Training Book on page 65.

Exposure Area 1.

The exposure area should consist of a room with concrete or block walls, lined with lead or other suitable shielding materials.

2.

An exposure area can be an enclosed shielding cabinet large enough for the test objects and with reliable safety features.

3.

58

Controls should be located outside the exposure area.

Personnel Training Publications

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•

4.

In field radiography or temporary job sites, safe distance in relation to exposure must be determined and be secured by: a.

Guard rails or ropes.

b.

Legible radiation warning signs.

c.

Sufficient shielding.

5.

Only monitored radiographers are permitted in the radiation area.

6.

Keeping a safe distance from the radiation source is the simplest and most effective safety consideration in field radiography.

Radiation Protective Construction

•

1.

Lead and concrete are the most common materials used to protect against radiation.

2.

Shielding measurements are usually expressed in terms of thickness.

3.

Ensuring a leak-proof shielding is very important.

4.

Sheets of lead must be overlapped, and nails and screws in the walls must be covered with adequate lead.

5.

Pipes, conduits and air ducts passing through the walls of the shielding must be completely shielded (see Figure 5.1 in the Radiographic Testing Classroom Training Book).

6.

7.

The thickness of lead is dependent on two factors: a.

Energy of the radiation source.

b.

Occupancy of the surrounding areas.

Other than lead, structural materials such as concrete and brick are often used as shielding materials.

8.

•

At voltages greater than 400 kV, concrete is used as shielding because: a.

Installing very thick lead can be difficult.

b.

Thick sheets of lead are cost-prohibitive.

c.

Concrete is the best alternative material because of its property of radiation protection and its simplicity of construction. Student Guide: Radiographic Testing

59

Gamma Ray Requirements 1.

2.

Special radiation protection is required for gamma radiation based on two factors: a.

Gamma radiation cannot be shut off.

b.

Gamma radiation has considerable penetrating ability.

•

A combination of shielding and distance is usually used during gamma radiography.

3.

Specially labeled storage containers are necessary to store gamma sources when not in use.

4.

After every use, readings with survey meters are taken to ensure the source is safely stored.

5.

Special projectors (called pigs) or isotope cameras containing heavy shielding made of lead or depleted uranium should be used for handling radioisotope sources.

UNITED STATES NUCLEAR REGULATORY COMMISSION 1.

The NRC regulates handling, storage and use of radioisotopes.

2.

Figures 5.2 and 5.3 in the Radiographic Testing Classroom Training Book show

•

NRC Form-4 and NRC Form-5, used to monitor the occupational dose history.

Occupational Radiation Exposure Limits Limitations on individual dosage greater than those listed in Table 5.6 in the Radiographic Testing Classroom Training Book may be permitted with the following conditions:

1.

The dose for the whole body does not exceed 5 rem (0.05 Sv) during any calendar year.

2.

The individual's accumulated occupational dose has been recoded on NRC Form-4 and the individual has signed the form.

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Personnel Training Publications

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Levels of Radiation in Unrestricted Areas Table 5.7 in the Radiographic Testing Classroom Training Book shows the exposure limits in an unrestricted area.

Personnel Monitoring There are different personnel monitoring devices required for use by radiographers and their assistants during radiographic operations:

•

1.

Film badges.

2.

Thermoluminescent dosimeters (TLDs).

3.

Optically stimulated luminescence badges (OSL).

4.

Direct reading dosimeters.

5.

Pocket dosimeters.

6.

Electronic personal dosimeters .

The last two types should be capable of measuring exposures from 0 to 200 mR (0 to 2 mSv).

Caution Signs, Labels and Signals 1.

The radiation symbol (as illustrated in Figure 5.4 in the Radiographic Testing Classroom Training Book) should be placed:

a.

In exposure areas.

b.

On containers for transporting and storing radioactive materials.

2.

The words caution or danger must appear.

3.

The words radioactive material should be marked on containers of radioactive materials and in the areas housing such containers.

4.

•

Exposure devices should have a radiation symbol and the phrase Danger radioactive material - do not handle. Company information and a 24-hr. phone

number must be mentioned on the sign.

Student Guide: Radiographic Testing

61

Exposure Devices and Storage Containers Based on the radiation regulations: 1.

•

Exposure devices must have the name of the company or laboratory and the location of the office placed in a noticeable site on the device.

2.

All of the labels and signs must be legible.

Radiation Survey Instrumentation Requirements 1.

Radiographers should have operable and calibrated radiation survey meters.

2.

Each exposure device should be accompanied by a survey meter.

3.

The meters should have a range of 2 mR (0.02 mSv) per hour through 1 R (0.1 Sv) per hour.

Radiation Surveys 1.

An operable and calibrated radiation survey instrumentation should be available at

•

an exposure area. 2.

When working with radioisotopes, a radioactive survey should be made around the camera to ensure the source has been returned to its shielded condition. This is known as a 360 0 sweep.

3.

Before storing each sealed source, a radiation survey should be made to ensure that the source is in its shielded position.

4.

All these readings should be recorded on a radiation report survey.

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DETECTION AND MEASUREMENT INSTRUMENTS There are different instruments that measure the radiation based on the ionization produced in a gas. These instruments fall into two categories: 1.

2.

•

Instruments that measure total dose exposure. a.

Pocket dosimeters.

b.

Personal electronic dosimeters.

c.

Thermoluminescent dosimeters (TLDs).

d.

Optically stimulated luminescence (OSL) badges.

Instruments that measure dose rate (radiation intensity) are called survey meters. a.

Ionization chambers.

2.

Geiger-mueller counters.

Pocket Dosimeters 1.

The pocket dosimeter is a small device, about the size of a fountain pen (see Figure 5.5 in the Radiographic Testing Classroom Training Book). Its operation is based on two main principles:

2.

a.

Radiation causes ionization in a gas.

b.

Similar electrical charges repel each other.

The dosimeter should be properly charged (the indicator on zero scale) before use.

3.

Pocket dosimeters are designed with a sensitivity that permits them to be scaled in doses from 0 to 200 mR (0 to 2 mSv).

4.

Pocket dosimeters must be calibrated annually, per NRC regulation, and the calibration date should be labeled on them.

• Student Guide: Radiographic Testing

63

Personal Electronic Dosimeters 1.

2.

Personal electronic dosimeters (or electron dosimeters) have different features: a.

Easy to use.

b.

Sensitive.

c.

Different dosimeter functions can be enabled or disabled.

•

The electronic dosimeter provides dose, dose rate and set point check, and usually operates with an AA battery.

3.

The set points can be preset to definitive alarm points.

4.

The pocket-sized monitors provide three-digit digital display.

5.

The energy responses of the pocket-sized monitor for gamma rays and X-rays are 40 keV to 1.2 MeV.

6.

They should be calibrated annually.

Film Badges and Thermoluminescent Dosimeters 1.

The film badge (see Figure 5.6 in the Radiographic Testing Classroom Training Book) consists of a small film holder equipped with thin lead on cadmium filters.

2.

•

The badge is designed to be worn by an individual only when working in a radiation area.

3.

After a period of time, the film is removed and developed by standard techniques.

4.

Film badges and dosimeters each record total radiation received and serve as check on each other.

5.

Thermoluminescent dosimeters (TLDs) contain a special crystal of lithium fluoride (rather than a sheet of film) which can store the energy.

6.

The TLD is sent to a laboratory where the crystals are processed to extract the amount of absorbed energy.

7.

Compared to film badges, they are not as sensitive to heat, moisture and rough handling, but they are more expensive.

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Optically Stimulated Luminescence (OSL) Badges 1.

OSL badges measure beta (P), gamma, neutron and X-radiation exposures.

2.

The OSL is a thin strip of specially formulated aluminum oxide crystalline material.

3.

They detect energies from 5 keV to 40 MeV for photons, 150 keV to lOMeV for beta particles and 40 keV to 35 MeV for neutrons.

4.

The dose measurements range from 1 mrem to 1000 rem.

Ionization Chambers 1.

Ionization chambers measure the radiation intensity (dose rate) in milliroentgen per hour or millisievert per hour.

2.

•

Ionization chambers typically attain an accuracy of ±15%, except in low-intensity radiation areas .

3.

Radiation intensity measurements in low-intensity radiation areas are usually made with geiger-mueller counters.

4.

Ionization chambers should be calibrated annually.

Geiger-Mueller Counters 1.

Geiger-mueller counters are highly sensitive radiation detection devices.

2.

Geiger-mueller counters are typically accurate to ±20% for the quantity of radiation to which they are calibrated.

3.

They should be calibrated annually.

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Area Alarm Systems 1.

These systems consist of one or more sensing elements, usually ionization chambers, whose output is fed to a central alarm meter.

2.

•

The meter can be preset so that an audible alarm is sounded and a visual indication is displayed when permissible radiation levels are exceeded.

ELECTRICAL SAFETY 1.

Because X-ray machines use high-voltage circuits, the radiographer must comply with safe electrical procedures.

2.

This is more serious specifically for portable X-ray equipment, which requires certain electrical precautions.

3.

During operation or service of X-ray equipment, the following precautions, applicable to both permanent and portable installations, should be observed carefully: a.

Do not tum power on until setup for exposure is completed.

b.

Ensure that grounding instructions are followed.

c.

Regularly check power cables for signs of wear, and replace them when

•

necessary. d.

Avoid handling power cables when the power is on. The machine's operational key should be removed when not in use.

e.

If power cables must be handled with the power on, use safety equipment such as rubber gloves, rubber mats and insulated high-voltage sticks.

f.

Be sure that water and moisture are not in close contact with power cables.

g.

Ensure that capacitors are completely discharged before checking an electronic circuit.

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Notes

Notes

•

•

•

•

Lesson 5

Quiz Please answer true or false to the following

6.

statements.

I.

The effects of all types of radiation on humans are the same.

The main process by which damage occurs to human tissue is through a

7.

Sievert is the new SI unit for roentgen.

8.

Radiation safety levels are established

process called ionization.

2.

•

Because of penetration power, X-rays

in terms of roentgen equivalent

are more damaging to humans than

mammal (rem) dose.

gamma rays. 9. 3.

The roentgen is the physical unit

The new unit for radioactivity is the Becquerel (Bq).

measure of the ionization of air by X-radiation or gamma radiation. 10. Each organization should establish its own occupational annual dose limits for 4.

Ionization radiation can produce an

personnel.

electric charge.

II. Maximum radiation dose in any period 5.

•

In industrial radiography, rad stands for

of one calendar year for an individual in

radiation.

a restricted area is normally limited to 5 rem (0.05 Sv) .

69

12. Some parts of the human body, such as arms and feet, can receive a higher radiation dose.

•

13. A given amount of radiation dose will have less effect on the body if the exposure occurs gradually over a long period of time.

14. The half value layer is that amount of shielding that will stop half of the radiation of a given intensity.

15. NRC Form 5: Occupational Dose Records must be completed at the end

•

of each shift.

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• Chapter 6: Specialized Radiographic Applications In this lesson you will learn about:

• Accessory equipment (diaphragms, collimators, cones, filters, screens, masking materials, image quality indicators, shim stock, film holders, cassettes, linear and angular measuring devices, positioning devices,

•

identification and location markers, area shielding equipment, densitometers). • X-ray exposure charts. • Gamma ray exposure charts. • Film characteristic curves. • Exposure variables (movement, source size, source-tofilm distance, film contrast, speed, graininess, control of scattered radiation, kilovoltage, milliamperage and time, source energy, strength, exposure time, absorption and contrast). • Exposure calculations .

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Lesson 6

Specialized Radiographic Applications INTRODUCTION 1.

2.

•

A quality radiograph should have the following properties: a.

Low distortion.

b.

High definition.

c.

High contrast.

d.

Adequate density.

This chapter presents information obtained in the field and laboratory from different exposure techniques .

3.

With a basic knowledge and understanding of the radiographic process, radiographers can produce effective radiographic procedures for different test objects.

4.

Proper film processing is an essential aspect of proper radiographic practice.

SELECTION OF EQUIPMENT 1.

2.

Equipment selection for a radiographic test depends on the following factors: a.

Selection of X-radiography or gamma radiography.

b.

Selection of specific X-ray or gamma ray equipment.

Before selecting radiographic equipment for a specific task, it must be determined that radiography will produce the desired test results.

3.

•

The test should be thoroughly analyzed to be sure that the results of a radiographic test justify the time, effort and cost.

4.

By knowledgeable choice of film, exposure and radiographic techniques, any particular equipment can be used for a variety of tasks. 73

5.

6.

The following are reasons for selecting gamma radiography: a.

Necessity of high radiation energy.

b.

Field tests in areas where electrical power is difficult to obtain.

c.

Areas where X-rays cannot be used due to physical restrictions.

d.

Simultaneous exposures of many test objects.

•

Before selecting radiographic equipment for a specific test, the radiographer must consider all aspects of the job. a.

Availability of the equipment.

b.

The time allocated for the test.

c.

The number or frequency of similar object tests.

ACCESSORY EQUIPMENT 1.

A radiation source, a test object and film are the main elements needed to make a radiograph.

2.

To create acceptable radiographs, additional equipment is required, which will be discussed in this chapter.

•

Diaphragms, Collimators and Cones 1.

Diaphragms, collimators and cones (shown in Figure 6.1 in the Radiographic Testing Classroom Training Book) are designed to limit the area of radiation.

2.

They are made of lead or other dense materials, like tungsten, fitted to the X-ray tube or built to contain a gamma ray source.

3.

They decrease the amount of scatter radiation by limiting the beam to the desired test object area.

4.

Many X-ray machines have built-in adjustable diaphragms designed so that the beam covers a standard film size at a fixed distance.

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Filters 1.

The role of a radiation filter is to absorb the soft radiation of the beam (i.e., the radiation with longer wavelengths and less penetration power).

2.

Filters accomplish two purposes: a.

They reduce subject contrast, permitting a wide range of test object thickness to be recorded with one exposure.

b. 3.

They eliminate scatter caused by soft radiation.

Filters are made of sheets of metal having high atomic numbers - usually brass, copper, steel or lead, as shown in Figure 6.2 in the Radiographic Testing Classroom Training Book.

4.

Filters are particularly useful in radiography of objects with small object contrast and thin sections.

•

5.

The material and thickness of the test object, especially its range of thickness, determines the necessary filter.

6.

In radiography of steel, good results have been obtained by the following methods: a.

Using lead filters, 3% of the maximum test object thickness.

b.

Using copper filters, 20% of the maximum test object thickness.

Screens 1.

Screens are used in most radiographic techniques because they reduce the exposure time, improve the quality of the image and increase contrast. Two types of radiographic screens are used: a.

Fluorescent: Usually calcium tungstate with lead, these types of screens are used when exposure time is factor.

b.

•

Lead: Lead screens produce high quality radiographs. A 0.005 in. (0.013 cm) thickness is used for the front of the film (top screen), and a 0.01 in. (0.025 cm) is used for the back of the film (bottom screen).

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Fluorescent Screens 1.

Fluorescent screens usually consist of calcium tungstate bound to a plastic or cardboard base.

2.

The screens are used in pairs with the film placed between them in a film holder.

3.

During the exposure, the photographic action on the film is the result of the

•

radiation and the light emitted by the screens impinging on the film. This is called the intensification effect. 4.

Because the emitted light is diffused, image definition is less sharp when these screens are used.

5.

To avoid a blurred image, a close contact between the screens and the film is necessary.

6.

Intensification factor of screens is defined as:

· fi Exposure without screens IntenslificatLOn actor = --.::....-------Exposure with screens 7.

The only advantage of using fluorescent screens is that they have a high intensification factor of 95 % .

8.

•

Due to their inherent poor image definition characteristic, fluorescent screens are used only in special applications.

9.

Practically, their use is limited to those occasions when a short exposure is required, for example radiography of concrete looking for rebar or wire position.

10.

Fluorescent screens cause excessive film graininess when exposed to high-energy radiation; thus, their use is largely restricted to the application of low-energy radiation.

11.

For higher energy applications on thicker materials, they are used to reduce exposure time.

12.

When loading films with screens inside a film holder, dust, dirt, stains and scratches should be avoided.

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13.

Touching of the sensitive surface of the screen should be avoided, and manufacturer recommendations should be followed for cleaning.

14.

To prolong their useful life, direct exposure to ultraviolet radiation must be avoided.

Lead Screens 1.

These screens are usually made of an antimony and lead alloy that is more wear resistant than pure lead.

2.

These screens are used in pairs on each side of, and in close contact with, the film.

3.

The front screen in most applications in thinner than the back screen. Front screens 0.005 in. (0.013 cm) thick and back screens 0.01 in. (0.025 cm) thick are commonly used.

4.

Lead screens are particularly efficient because of their ability to absorb scattered radiation (soft radiation), in addition to increasing the photographic action on the

•

film due to the release of electrons in the test object. 5.

The intensification factor of lead screens is much lower than that of fluorescent ones, but the resulting improvements in image contrast and definition make them very popular.

6.

They are used in almost all gamma ray applications.

7.

In use with X-rays, the photographic effect will start from a certain kilovoltage, and the film manufacturer recommendations should be followed.

8.

To ensure the intensification action of lead screens, they must be kept free from dirt, grease, lint, deep scratches, wrinkles or depressions that affect their flatness.

9.

The best results can be achieved with ready-packed films and screens that are in completely close contact in a vacuum.

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Masking Materials 1.

Masking is the practice of covering or surrounding portions of the test object with highly absorbent materials (usually lead) during exposure.

2.

•

Masking eliminates much of the scatter radiation and improves the image definition at the boundary and sharp edges of the test object.

3.

Commonly used masking materials are lead, barium clay and metallic shot (see Figure 6.4 in the Radiographic Testing Classroom Training Book).

Image Quality Indicators (IQIs) 1.

A standard image quality indicator (IQI) must be included in every radiograph (with some exceptions) as a check on the performance of the selected radiographic technique.

2.

The purpose of using an IQI is not to judge the size or establish acceptance limits of discontinuities in the test object.

3.

There are a variety of IQls.

4.

The hole type IQI (shown in Figure 6.5 in the Radiographic Testing Classroom

•

Training Book) has three holes in it. If the thickness of the IQI is T, then the

diameters of the holes are IT, 2T, and 4T.

5.

Each hole type IQI is identified by an identification number, which represents the thickness of the IQI (or, in some designs, the thickness of the test object).

6.

Figure 6.5 in the Radiographic Testing Classroom Training Book shows an ASME image quality indicator with a thickness of 0.025 in. (0.064 em); therefore: a.

The diameter of 1Thole = 0.025 in. (0.064 cm).

b.

The diameter of 2T hole = 0.05 in. (0.13 em).

c.

The diameter of 4T hole = 0.1 in. (0.25 em).

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7.

In some standards, the selection of IQI will call for exactly a 2% sensitivity, which means: Thickness of IQI

- - - - - - - - - ' - - - - = 2%

Thickness of test object

8.

Table 6.1 in the Radiographic Testing Classroom Training Book indicates the ASME IQI designations and sizes.

9.

Being able to see the outline of the IQI on the radiograph confirms that the image contrast is sufficient to see the required change in the thickness of the test object.

10.

The IQI should be placed on the source side of the test object during radiography.

11.

Being able to see the required hole indicates that the film has the required sensitivity.

12.

The IQI is designed to determine the radiographic quality level, usually referred to as sensitivity of a radiograph (see Table 6.2 in the Radiographic Testing

•

Classroom Training Book) .

Example: Calculate the size of 2T hole in an IQI which is designed for 2% sensitivity of a radiograph of a test object with 0.75 in. thickness. Solutions:

Thickness of IQI = 2% Thickness of test object

T --=2% 0.75 T = 0.015 in. 2T = 0.03 in .

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Shim Stock 1.

Shim stock is used in radiography of test objects where the area of interest is thicker than the nearby test object thickness (such as welds).

2.

•

Shim stock comprises thin pieces of material radiographically similar to test object materials.

3.

The thickness of shim stock must be equal to the thickness added to the test object by the weld (as shown in Figure 6.6 in the Radiographic Testing Classroom Training Book).

4.

The shim is placed underneath the IQI, between the IQI and the test object.

5.

The length and width of the shims should always be greater than the dimensions of the IQI.

Film Holders and Cassettes 1.

Film holders are designed to shield film from light and to protect it from any damage.

2.

They are made from a variety of materials including rubber, plastic and cardboard.

3.

Film holders are flexible and permit molding the film to the contours of the test

•

object. 4.

Cassettes are specially designed; some are two-piece hinged, rigid film holders that spring clamp tightly together.

5.

Cassettes are of use when flexibility is not required because they can hold film and screens together firmly in place.

6.

Rigid cassettes usually are made of aluminum, a low-absorbent material.

7.

Other types of cassettes are more flexible and are usually closed and secured with masking tape or elastic bands.

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Identification and Location Markers 1.

Location markers are lead numbers and letters that help correlate the radiograph and the exposure location on the test object.

2.

Location markers also provide proper coverage of the test object during radiography.

3.

Permanent markers or paint sticks are commonly used to mark the part or weld.

4.

Using lead letters and numbers identical to the part number or weld's identification number eliminates any possibility of wrong identification.

5.

Lead letters and numbers are attached with duct tape or masking tape for use during radiography.

6.

Using right lead identification on the test object is mandatory in most radiographic codes.

•

7.

Using location marks and lead numbers is particularly important in radiography of pipes in field inspection.

8.

For pipe varying in outside diameter from 2 to 42 in. (5 to 107 cm), the maximum location marker spacing may be determined by the following formula: . pipe outside diameter x 1! Maxlmum location marker spacing = ------::........::....----------Number of required films for exposure

Example: A pipe with a nominal size of 6 in. (15 cm) has an outside diameter of 6.625 in. (16.8 cm). Based on ASME V, Article 2, a minimum of three films should be used.

Step 1:

Outside diameter = 6.625 in. x

Step 2:

20.8 = 6.93 in. 3

Step 3:

Make a lead number belt with 6.93 in. (17.6 cm) maximum spacing between the

1t

= 20.8 in.

numbers.

Step 4:

•

Place the lead number belt adjacent to the weld to be radiographed, and then mark the placement of the numbers on the pipe .

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9.

For large test objects or large pipes, radiographic codes usually recommend a minimum overlap between each exposure.

10.

For pipes, vessels, etc., that have an outside diameter greater than 42 in. (107 cm),

•

a universal number belt with lead numbers spaced 14 to 15 in. (35.6 to 38 cm) apart can be used. 11.

Another option is the use of lead numbers indicating inches from a starting point.

Area Shielding Equipment 1.

Proper shielding is necessary for control of scatter radiation during radiography.

2.

The area in which radiography is performed must be adequately protected against both side and backscatter.

3.

In permanent installations, this is accomplished using lead-shielded rooms or enclosed cabinets.

4.

In field inspection and when permanent installations are not available, the radiographer uses lead sheets to shield the primary radiation.

5.

The area immediately beneath or behind the film should particularly be covered

•

with an adequate thickness of lead. This is mandatory per radiographic codes.

Densitometer 1.

The densitometer is an instrument that measures the density value or graininess of a developed film.

2.

This is done by measuring the intensity of light transmitted through the film.

3.

Two types of densitometers are commonly available: analog and digital.

4.

Before any reading is done, a densitometer should be calibrated with a density calibration strip provided by the manufacturer to show its linearity and appropriate consistency.

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5.

ASTM standards have recommendations for calibrating and checking the linearity of densitometers after a certain period of time. These recommendations should be followed by radiographers.

X-ray Exposure Charts 1.

X-ray exposure charts, an example of which is shown in Figure 6.7 in the Radiographic Testing Classroom Training Book, are useful for radiography using

X-rays because they show the relationship between material thickness, kilovoltage and exposure. 2.

• 3.

Note that the chart applies only to specific radiographic conditions, such as: a.

X-ray equipment.

b.

Materials.

c.

Source-to-film distance (SFD).

d.

Film.

e.

Processing conditions.

f.

Density upon which the chart is created.

Exposure charts are useful to determine exposures of test objects of uniform thickness.

4.

X-ray tube manufacturers provide exposure charts which are usually accurate within ±10%, because no two X-ray machines are identical, and film developing conditions also playa significant factor.

5.

Each radiographic laboratory should prepare an exposure chart for its specific X-ray equipment, for the type of material most often radiographed, the film most commonly used and its own processing conditions .

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Preparation of an Exposure Chart 1.

To prepare an exposure chart, a series of radiographs is taken using a stepped wedge of the selected object material (usually steel).

2.

•

Radiographs are taken with different kilovoltages (from lower to higher values) at different exposures with a specific SFD.

3.

All resultant films are processed together in accordance with routine work procedure.

4.

After these steps, there are two methods to prepare an exposure chart: a.

Method A (using a densitometer). 1.

The radiographer uses a densitometer to locate the desired density on each stepped wedge thickness on the process films.

11.

At each point the desired density (usually D

= 2.0) appears, a

corresponding value of applied kilovoltage, exposure and wedge thickness will be collected. 111.

When the desired density does not appear on a radiograph, interpolation will be used to find the correct material for the density.

IV.

The extracted data are then plotted on a semi-log paper.

v.

The horizontal line scale is for material thickness (thickness of the

•

stepped wedge). vi.

The kilovoltages corresponding to the exposure for a specific density point (D = 2.0) are then plotted on a chart similar to that shown in Figure 6.7 in the Radiographic Testing Classroom Training Book.

Vll.

A legend should be attached to show all the specific conditions applied to the prepared chart.

b.

Method B (using film characteristic curves). 1.

In this method of preparing an exposure chart, film characteristic curves will be used. This method requires more calculations but fewer exposures.

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11 .

At each selected kilovoltage, one exposure is made for the stepped wedge.

111.

The density of the radiographs for each stepped wedge thickness is measured.

IV.

Then by using characteristic curves, the exposure can be calculated to give the desired density (D = 2.0).

v.

As before, the resulting values of exposures, thicknesses and kilovoltages are plotted.

V1.

Follow the example on page 93 of the Radiographic Testing Classroom Training Book showing this method.

Film Latitude 1.

•

Film latitude is defined as the variation in material thickness that can be radiographed with one exposure while maintaining film density within accepted limits.

2.

Exposure charts can also be prepared to show film latitude.

3.

Either of the above methods can be followed, except that both the lowest and highest acceptable densities are plotted as well.

Gamma Ray Exposure Chart 1.

The variables in gamma radiography, which should be represented on an exposure chart (see Figure 6.8 of the Radiographic Testing Classroom Training Book), are:

•

a.

Type of source.

b.

Source of strength.

c.

Source-to-film distance (SFD).

d.

Thickness of material.

e.

Type of film.

f.

Processing conditions. Student Guide: Radiographic Testing

85

2.

The parameters are related on the chart to each of three types of film.

3.

Note that the exposure factor shown in the vertical axis of the chart is a logarithmic scale derived by the given formula in the chart.

4.

•

The density correction factors (at the bottom of the chart) are obtained from the film characteristic curves. The exposures can be adjusted to get densities like 1, 1.5, 2.5 and 3.0 while an exposure from density 2.0 is the reference exposure.

5.

Gamma ray exposure charts can be easily modified to show material latitude .

6.

As shown in Figure 6.9 in the Radiographic Testing Classroom Training Book for film type A, two other parallel curves have been added for density 2.5 (above) and for density 1.5 (below). These curves are created by using exposure factors of 1.3 for density 2.5, and 0.71 for density 1.5.

7.

The range of material thicknesses that can be radiographed in one exposure resulting in densities between 1.5 and 2.5 can be found from the horizontal difference between the 1.5 and 2.5 density curves.

Dated Decay Curves 1.

•

Dated decay curves are useful for gamma radiography and are usually provided by the radioisotope's supplier (see Figure 6.10 in the Radiographic Testing Classroom Training Book). The vertical axis is in curies and the horizontal axis is by date.

2.

These curves are computer-generated tables of date versus source activities for each specific radioisotope.

3.

By using these curves, a radiographer can find the exact source strength for the exposure calculation.

4.

Knowing the source strength and a half-life value for a specific radioisotope, the source strength versus half-life value can be plotted on a semi-logarithmic paper.

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Film Characteristic Curves 1.

Film characteristic curves, as discussed in Chapter 4 in the Radiographic Testing Classroom Training Book, are necessary curves for the radiographer to find a new

exposure for any desired density. 2.

Film manufacturers usually provide accurate film characteristic curves, but to produce more realistic ones, it is a good practice to make them based on each radiographic laboratory condition.

Radiographic Equivalent Factors 1.

The most common materials for radiography are steel and aluminum, so these two materials are considered as reference materials in radiography. (See Table 6.3 in the Radiographic Testing Classroom Training Book.)

•

2.

In radiography of other materials besides steel and aluminum, radiographic equivalent factors are calculated (as shown in Table 6.4 in the Radiographic Testing Classroom Training Book).

3.

Note that aluminum is typically used as the standard material at 100 kV energy and below. Steel is the standard material at higher voltages.

4.

To find the necessary exposure for materials, the thickness of that material is multiplied by the corresponding factor shown in the table to obtain the approximate equivalent thickness of the standard metal.

EXPOSURE VARIABLES The exposure variables that affect practical radiography techniques are reviewed in the following sections .

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Movement 1.

In radiography, movement of source, test object or film during exposure can cause fuzziness in the radiographs.

2.

•

In high-wind areas, care must be taken to ensure that the film and the exposure/ guide tube (for gamma radiography) do not move during radiography.

3.

In X-radiography, permanently installed equipment is designed to easily set the X-ray tube in a desired position, eliminating any risk of movement.

4.

For portable X-ray tubes, professionally designed, heavy-duty fixtures are used to hold the X-ray tube in a desired position.

5.

In field radiography using radioisotopes, the film, test object and source guide should be held firmly in position with clamps, duct tape, wire, magnetic holders, etc.

6.

In any attempt to hold the source, film and test object firmly in place, care should be taken to keep the scatter radiation as low as possible.

Source Size 1.

•

Source size is a strong factor in producing sharp images by reducing geometric unsharpness (as discussed in Chapter 2 in the Radiographic Testing Classroom Training Book).

2.

In purchasing either X-ray machines or gamma ray sources, consideration of the source size is an important factor.

3.

X-ray focal spots vary from 0.08 in.2 (0.5 cm2) down to fractions of a millimeter, and consequently the prices are higher with finer focal spots.

4.

In purchasing radioisotopes for specific tasks, the source size is another important requirement besides the source strength and half life.

5.

In gamma radiography, if a smaller source size is required for a specific task, source manufacturers can produce a smaller isotope with a resulting lower intensity.

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•

6.

When X-ray equipment is limited to what is available in the field, correct sourceto-film distance (SFD) can produce good images with focal spots of acceptable dimensions. (This will be discussed in the following section.)

Source-to-Film Distance (SFD) 1.

Source-to-film distance (SFD) in X-ray and gamma radiography is an important factor in both image sharpness and exposure time.

2.

A longer SFD creates a sharper image compared to a shorter one, which results in greater geometric unsharpness (penumbra).

3.

The maximum geometric unsharpness that cannot be recognized by human eyes is about 0.02 in. (0.05 cm). Based on this, the following equation can be used to determine a minimum SFD giving an acceptable geometric unsharpness: D=dX! +d 0.02

•

where D is SFD, d is distance from the source side of the test object to the film and f is focal spot size. Example: Find the minimum acceptable SFD for radiography of 1.5 in. (3.8 cm) plate using

an X-ray tube with 0.12 in. (0.3 cm) focal spot size. There is no gap between the test object and the film. D=? d = 1.5 in.

!

= 0.12 in.

D = 1.5 x 0.12 + 1.5 0.02

D = 10.5 in. (26.7 cm)

4.

A second means of determining SFD is this rule of thumb: the SFD should not be less than 8x the test object thickness. For example, in the previous case it gives:

•

1.5 x 8 = 12 in. (30.5 cm)

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RADIOGRAPHIC ApPLICATIONS 1.

The exposure procedures discussed in the following sections are commonly used in

•

radiography of most test objects. 2.

Any of these arrangements may be used with either X-rays or gamma rays.

Radiography of Welds The following information regarding a weld test object should be provided to a radiographer before applying any exposure: 1.

Weld material.

2.

Joint preparation.

3.

Weld procedure.

4.

Related radiographic standards,including critical and noncritical criteria.

5.

Any requirements set by the customer.

Tube Angulation

•

Before performing an exposure of any weld specimen, the radiographer must know the following information in order to set the tube angulations for the best direction of the beam: 1.

Weld penetration.

2.

Weld fusion lines.

3.

Area of interest.

Incident Beam Alignment 1.

The central beam of the radiation field is the direction of the incident beam.

2.

The effective focal spot size of an X-ray tube is projected along this central beam to the area of interest.

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Discontinuity Location 1.

Locating discontinuities in thick test objects is sometimes necessary for repairing purposes.

2.

Correctly locating and removing the discontinuities will save time and material.

3.

Estimation of the depths of discontinuities cannot be done by a single exposure.

4.

Several methods such as stereoradiography and double exposure (discussed in Chapter 8 in the Radiographic Testing Classroom Training Book) can be used for finding discontinuity depths.

Critical and Noncritical Criteria 1.

The radiographer must know the acceptance criteria and area of interest of every test object.

•

2.

The radiographer must select film based on film speed and film sensitivity.

3.

The radiographer must determine the distance and angle of exposure to give the least amount of distortion.

4.

To provide complete coverage, the radiographer must determine the number of necessary exposures.

5.

The radiographer must follow all radiographic requirements.

Improper Interpretation of Discontinuities For a proper interpretation, all factors of the manufacturing or welding process should be known by the interpreters .

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Elimination of Distortion To minimize the distortion in radiographs: 1.

Setting the proper geometry of exposure is necessary.

2.

The source should typically be perpendicular to the surface of an object and the

•

plane of the film (detector).

Proper Identification and Image Quality Indicator Placement 1.

To show the image sensitivity, appropriate image quality indicators are added to a test object.

2.

Appropriate identification is used to correctly identify each exposure.

3.

Information should be provided for each exposure such as:

4.

a.

Test object number.

b.

Weld number.

c.

Area number.

d.

Date of exposure.

e.

Project number.

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Lead location markers are used for large areas that require more than one view. The location markers correlate the radiograph to the location on the weld or component.

Radiography of Welded Flat Plates 1.

This type of weld is easily radiographed because its area of interest is clearly defined (see Figure 6.18 in the Radiographic Testing Classroom Training Book).

2.

It has a small subject contrast.

3.

The exposure calculations are relatively simple.

4.

Proper image quality indicators (IQI) and sufficient shim stock must be selected to ensure the correct degree of sensitivity.

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Radiography of Welded Corner Joints 1.

Refer to Figures 6.19 to 6.21 in the Radiographic Testing Classroom Training Book for correct and incorrect X-ray setups for a corner joint.

2.

The correct setup depends on welding standards, joint configuration and design stress.

Single-Wall Radiography of Tubing 1.

Figure 6.22 in the Radiographic Testing Classroom Training Book illustrates an example of a single-wall radiography technique.

2.

The circular test object should be numbered in a clockwise direction.

3.

Lead numbers should be placed adjacent to the weld and at least 0.125 in. (0.3 cm) from the heat-affected zone.

•

4.

To determine the area with the least amount of distortions, deduct 10% from both sides of the area with the most visual circumferential changes .

5.

Lead arrows can be attached with adhesive backs at the ends of each area. Leave these arrows on the test object until the interpretation of the radiographs is done.

6.

At least 1 in. (2.5 cm) overlap between each film should be observed.

Double-Wall Radiography of Tubing Refer to Figures 6.23 and 6.24 in the Radiographic Testing Classroom Training Book for the setup of double-wall radiography applications.

Tubing up to 3.5 in. (9 em) Outside Diameter (OD) 1.

For tubes with outside diameters in this range, the elliptical exposure technique should be used.

•

2.

By offsetting the location of the radiation source, both the near and far side of the weld can be viewed on a single film.

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3.

The offset angle, which depends on the tube OD and tube wall thickness, should be set in such a way that far side and near side images do not become superimposed.

4.

To ensure full coverage of the weld, a minimum of two exposures at 90° to each

•

other are necessary.

5.

The lower left image of Figure 6.23 in the Radiographic Testing Classroom Training Book illustrates a multiple tube assembly exposure.

6.

The area of interest should be determined based on the diversionary beam.

7.

A relatively large focal film distance of 48 in. (122 cm) or more should be applied.

Radiography of Closed Spheres 1.

Figure 6.25 in the Radiographic Testing Classroom Training Book shows the radiographic setup for a closed sphere.

2.

The technique is similar to those for double-wall tubing.

3.

The image quality indicator should be placed on shim stock to show total double-wall thickness.

4.

The offset angle should be determined by sphere diameter.

5.

The primary beam should be as nearly perpendicular as possible but should not be

•

superimposed. 6.

Equally spaced numbering should be set taking into account the geometric radiation distortion principles.

Radiography of Closed Tanks 1.

Figure 6.26 in the Radiographic Testing Classroom Training Book shows the radiographic setup for a closed tank (when film and X-ray tube are both outside).

2.

A single source is shown at various positions.

3.

A horizontal exposure from the upper left should be taken to cover the circumferential weld at the tank end.

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Radiographic Multiple Combination Application 1.

Figure 6.27 in the Radiographic Testing Classroom Training Book shows a radiographic setup with a single shot for a weld with high degree of latitude.

2.

Film cassettes under the object can have different types of film and screen combinations to provide different film densities.

Radiography of Hemispherical Sections 1.

Figure 6.28 in the Radiographic Testing Classroom Training Book shows a hemispherical section with multiple welds.

2.

The application of a radioisotope located at the geometric center of the hemispherical section can cover all the welds.

Panoramic Radiography

•

1.

Figures 6.29 and 6.30 in the Radiographic Testing Classroom Training Book illustrate panoramic radiography setup for piping whose diameter is great enough to insert a rod anode X-ray.

2.

Note that the exposure calculation is based on single-wall thickness.

3.

A radioisotope source at the center may be used in the same manner as a rod anode X-ray tube.

4.

In the case of pipe welds, enough overlap should exist between films to show the similar marker on each film to ensure 100% coverage.

Radiography of Large Pipe Welds 1.

For pipe welds with large diameters, the double-wall exposure/single-wall viewing technique can be used (see Figure 6.31 in the Radiographic Testing Classroom

•

Training Book) . 2.

An elastic cord holds the radiation source and films in place.

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3.

Location markers at each end of the film markers should be used to show the area of interest or the right coverage of the weld.

4.

A lead letter F should be placed close to the IQI to show that its location is at the

•

film side of the weld.

5.

An appropriate shim stock should be under the IQI.

6.

After each exposure, by rotating the source and using a new film cassette at the opposite side, the full coverage of welds should be ensured.

7.

In the case of thicker pipes, the area of interest at each exposure should be carefully determined with a technique shot.

Radiographic Techniques of Discontinuity Location

Alignment 1.

Alignment of the discontinuities and the path of the X-ray is the key to recording finer discontinuities.

2.

Figure 6.32 (b) in the Radiographic Testing Classroom Training Book shows a

•

discontinuity cross-section with less than 2% subject contrast (along path AA). 3.

Figure 6.32 (c) in the Radiographic Testing Classroom Training Book shows a correct discontinuity alignment with respect to the X-ray beam (AlAI).

Discontinuity Depth Location Techniques There are different techniques to determine the depth of a discontinuity by radiography: 1.

Superimposed single exposures: Exposures on two separate films.

2.

Tube shift method: Exposures on a single film after moving the source locations

(or test object and film location) a certain distance. At each exposure, half of the exposure should be applied to avoid too much radiation on the film.

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Radiography of Brazed Honeycomb Four different radiographic techniques can be used for evaluation of brazed or bonded honeycomb (see Figures 6.33 through 6.36 in the Radiographic Testing Classroom Training Book). 1.

Double surface radiographs.

2.

Single surface radiographs.

3.

Edge member exposures.

4.

Vertical tie exposures.

Radiography of Semiconductors 1.

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2.

In the evaluation of semiconductors, two major areas are of concern: a.

Inconsistent internal construction.

b.

Internal foreign materials .

Specific discontinuities associated with semiconductors include: a.

Loose particles, solder balls, flakes, weld splash and wires.

b.

Loose or open connecting leads between internal elements and external terminals.

c.

Excessive solder or weld extrusions.

d.

Inclusions or voids in seals or around lead connections.

e.

Inadequate clearance.

Techniques of Semiconductor Radiography In radiographic techniques of semiconductors, the following points must be taken into consideration:

•

1.

X-ray systems with beryllium filters at the tube window should be used.

2.

Voltage less than 150 kV should be applied.

3.

Extra fine-grain, single-coated film should be used. Student Guide: Radiographic Testing

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4.

Use 20x optical magnification and sufficient light intensity during film interpretation.

5.

Use correct alignment between semiconductor and X-ray directions.

6.

Correctly locate the radiographic source.

7.

Ensure proper density in area of interest.

•

Alignment of Semiconductors Figure 6.38 in the Radiographic Testing Classroom Training Book demonstrates a fixture designed to hold the film along a curvature for which the X-ray source is located at the center. In this case, an equal SFD can be achieved for both edges and the central points.

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Notes

Notes

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•

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Lesson 6

Quiz Please answer true or false to the following

6.

statements.

The image quality indicator is designed to determine the radiography quality level.

1.

Filters for radiography are usually sheets of very low atomic number metals.

7.

A densitometer is an instrument for measuring the density of the test object prior to performing radiographic testing.

2.

•

Using fluorescent screens makes the image definition sharper.

8.

Masking with lead or metal shot is effective in increasing the energy of the

3.

Due to the high absorption of lead, it X-ray beam through intensification, cannot be used as a screen in therefore decreasing scatter. radiography.

9. 4.

Filters can be used to provide greater

The image quality indicator is a check latitude in recording test object for determining the smallest defects thickness. detected during radiography.

10. A filter is most effective when placed

5.

An identification number of 25 on an between the specimen and the film. ASME IQI shows a thickness of

•

0.025 in.

101

11 . The 2T hole in an image quality

16. The geometric unsharpness depends

indicator is usually 2% of the test object

on the source size and cannot be

thickness.

affected by the source-to-film distance.

12. Masking should not be used in critical

•

17. As a general rule, technicians should try

radiography because of the fuzziness

to obtain the lowest contrast possible

that may be caused on the edges of the

when making a radiograph.

test object. 18. Unsharpness will increase as the source13. X-ray exposure charts are developed for

to-film distance increases.

use with certain film types but can be used for any tube head that has the same 19. The X-ray exposure chart is not affected voltage range. by the use of lead foil screens.

14. Film developing conditions do not have

•

20. The geometric unsharpness effect on the any effect on the exposure chart, and film can be controlled by either raising there is no reason to mention these in or lowering the kilovoltage. preparation of the chart.

15. If the radiographer knows the activity in curies, the source-to-film distance can be found by using the decay chart.

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• Chapter 7: Digital Radiographic Imaging In this lesson you will learn about: • Detectors for digital imaging. • Principles of digital X-ray detectors. • Charge coupled devices (CCDs). • Thin film transistors.

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• Linear arrays . • Scanning beam, reversed geometry. • Detection efficiency. • Spatial resolution. • Modulation transfer function. • Selection of systems to match application. • X-ray detector technology.

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Lesson 7

Digital Radiographic Illlaging INTRODUCTION This lesson describes some of the new developments in digital radiography. The discussion and examples include:

•

1.

Techniques of conversion of X-rays to light and then to electronic images.

2.

Photoconductive conversion of X-rays to electronic images.

3.

Photostimulable phosphors.

4.

Array detectors.

5.

Line detectors .

6.

Line scan imaging.

7.

Scanning electron beams.

Digital systems use discrete sensors with data from each detection pixel being read out into a file structure to form the pixels of the digital image file.

Development 1.

The ability to develop digital imaging technology that would be useful for radiographic testing is largely due to the growth in the speed and memory of computer systems.

2.

Today, large image files are common and can be transported, stored and displayed with relatively inexpensive computer systems.

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3.

The development of X-ray digital systems basically originated from the medical community. 105

4.

In the early 1980s, digital imaging for radiographic purposes was primarily done by electronic digitization of the video signal for a real-time X-ray system.

5.

In the 1970s and 1980s, digital imaging systems using line detector arrays were

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developed.

6.

In the late 1970s to early 1980s, the photostimulable phosphor array was developed for medical use. It was used in the NDT industry in the 1990s.

7.

In the 1990s, the development of large, thin film transistor arrays by using either amorphous silicon or amorphous selenium panels provided the tool that could make large X-ray images possible.

8.

Developments in direct digital image output for charge coupled device (CCD) cameras resulted in CCD arrays that consisted of millions of pixels.

Detectors for Digital Imaging 1.

2.

Digital detectors are used in numerous applications, such as: a.

Airport security scanning.

b.

Medical diagnosis.

c.

Inservice nondestructive testing.

d.

Manufacturing processes.

e.

Online production testing.

f.

Pipeline testing for corrosion damage.

g.

Industrial and medical computer tomography systems.

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Digital images provide numerical results important for metrology and thickness measurements.

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PRINCIPLES OF DIGITAL X-RAY DETECTORS 1.

Detection devices that support larger imaging systems can have either of the following X-ray capture materials:

2.

3.

a.

X-ray phosphor materials combined with a photoelectric device.

b.

X-ray photoconductor materials with an electronic readout device.

The most common detection systems in operation today are: a.

Flat panel detection systems.

b.

Camera systems based on CCD technology.

c.

Storage phosphor systems.

The replacement of any of these systems with film radiographic techniques depends on the following criteria:

•

a.

The size of the particular application.

b.

Spatial resolution.

c.

Image contrast.

d.

Image dynamic range.

e.

Required speed.

Charge Coupled Devices 1.

CCDs are based on crystalline silicon.

2.

Crystalline silicon is cut from silicon wafers available in sizes only as large as 4 to 6 in. (10 to 15 cm) in diameter or less.

•

3.

They are not fabricated in larger arrays.

4.

CCD advantage: A large field of view (FOV) can be accomplished through either:

5.

a.

Tiling of the device.

b.

A lens or a fiber optic transfer device to an X-ray conversion screen.

CCD limitation: Poor light collection efficiency.

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Thin Film Transistor 1.

Amorphous silicon, through large area deposition, offers a solution to the size constraints of CCDs while maintaining good light collection efficiency.

2.

It is commercially available with a pixel pitch smaller than 75 Jlm.

3.

Amorphous silicons show good light collection efficiency from the phosphor

•

photoconductor materials. 4.

Having small pixels may be a limiting factor, however.

Light Collection Technology 1.

On a per pixel basis, the CCD is more efficient in collecting the light produced from the phosphor materials.

2.

For small FOV applications, the directly coupled CCD approach provides high spatial resolution and high light efficiency.

3.

For large FOV applications, the amorphous silicon approach offers excellent light collection efficiency.

•

Radiation Conversion Material 1.

Amorphous selenium devices and amorphous silicon-based detectors are similar in that both use thin film transistor readout circuitry.

2.

The difference between the two devices lies in the X-ray conversion materials.

3.

The selenium layer is typically 0.02 in (0.05 cm) thick and offers direct X-ray collection efficiency in a sturdy, compact package.

Storage Phosphors 1.

The stored charges, due to the entrapment of X-rays, can be released when stimulated by infrared or red laser light.

2.

The emitted photostimulated luminescence can be converted to an electrical signal that is then amplified and sampled.

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3.

These systems have been widely used in production due to their practical spatial resolution and contrast sensitivity.

4.

Their flexibility can be compared to that of industrial films.

5.

They are portable and fully reusable.

6.

The advantages of phosphor screens over film are: a.

The reduction of costly film and developing processes.

b.

The ability to digitally acquire a film quality image.

c.

The high dynamic range.

d.

The corresponding benefits of the digital image file, such as easy archival and retrieval.

Linear Arrays 1.

•

Linear array detectors are much like CCDs, except they typically have pixels in only one dimension.

2.

They may be composed of a small rectangular array, such as a 32 x 1024 pixel array.

3.

The advantage of linear arrays is their scatter rejection capability.

4.

Linear arrays have been successfully used in computed tomography applications.

Scanning Beam, Reversed Geometry 1.

The reverse geometry system goes one step further in reducing X-ray scatter.

2.

In this system, the data are acquired with a small thallium-activated sodium iodide (NaI:Tl) scintillator coupled to a photomultiplier tube.

3.

The X-ray source operates in a manner similar to a video monitor.

4.

The test object is placed on top of the X-ray source (the opposite of conventional radiography) .

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5.

The disadvantage of this approach is the detector size.

6.

The detector size is typically much larger than a typical industrial X-ray focal spot. Student Guide: Radiographic Testing

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Detection Efficiency 1.

Except for photoconductive selenium-based detectors, the detectors discussed above use some sort of phosphor layer to capture and convert the X-ray intensity.

2.

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The signal-to-noise ratio of a detector and the image contrast are dependent on the transfer of information along the imaging chain.

SPATIAL RESOLUTION 1.

2.

The spatial resolution of a detector depends on two main factors: a.

Detector resolution.

b.

Pixel pitch.

The accepted way to measure the spatial resolution is the modulation transfer function (MTF).

Modulation Transfer Function (MTF) 1.

The MTF measures the signal modulation as a function of spatial frequency.

2.

Its computation is based on the Fourier Transform of a line spread function

•

acquired on an angled tungsten edge placed directly on the detector. 3.

Figure 7.1 in the Radiographic Testing Classroom Training Book shows a typical MTFcurve.

Gain and Offset Correction 1.

2.

Images from digital detectors are frequently: a.

Normalized for pixel-to-pixel gain variation.

b.

Adjusted to subtract out the background or offset.

Reforming this gain correction can also be done to flatten the radiation intensity distribution across the detector panel.

3.

Making the radiation beam intensity more uniform across the detector can result in wider latitude in the image.

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Radiation Damage 1.

Each component in the imaging chain of digital imaging devices not shielded appropriately from X-rays or gamma rays can be damaged by radiation.

2.

The term radiation damage, in general, can refer to any range of damage to a component, from a subtle change in performance all the way to failure.

3.

The damage that occurs in the electronic circuitry can result in an increase in the electronic noise of the device.

4.

Each manufacturer uses proprietary circuitry or various forms of shielding elements to prevent these effects.

5.

•

Two types of damage include: a.

Afterglow or lag.

b.

Gain decrease.

Selection of Systems to Match Application Key characteristics to consider in the selection of a digital radiography system include: 1.

Detection precision and accuracy.

2.

System speed.

3.

Detection area.

4.

Volume of the detector for access to tight locations in an assembly.

5.

Presence of artifices that can impact detection capability.

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X-RAY DETECTOR TECHNOLOGY

Amorphous Silicon Detectors 1.

•

Most new amorphous silicon designs are based on a flat glass panel that has undergone a deposition process resulting in a coating on one side that contains several million amorphous silicon transistors.

2.

The transistors are arranged in a precise array of rows and columns.

3.

The system components comprise:

4.

a.

A receptor incorporating a phosphor conversion layer.

b.

The amorphous silicon array.

c.

Readout electronics.

Most flat panel receptors are designed to provide radiographic acquisition capability at a rate of one image every 5 to lOs.

Amorphous Selenium Detectors 1.

•

Amorphous selenium converters produce direct conversion from X-ray to electronic signals.

2.

The amorphous selenium conversion layer exhibits extremely high resolution.

Charge Coupled Device Radiographic Systems 1.

A CCD is an integrated circuit formed by depositing a series of electrodes, called gates, on a semiconductor substrate to form an array of metal oxide semiconductor

(MOS) capacitors. 2.

CCDs, in combination with X-ray phosphors or scintillators, eliminate the need for electronic image intensification.

3.

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The typical scanned speed of CCDs is an exposure per frame of 33 ms.

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4.

Averaging multiple frames in a digital processor can improve the image quality but does not produce film quality images.

5.

Integration of the charge produced by light from the phosphor directly on the CCD cells can improve the signal-to-noise ratio.

6.

CCDs are available with image formats as large as 4096 x 4096 pixels and 16 bits.

7.

Using fiber optic image transfer plates with CCDs can reduce noise.

8.

Application of fiber tapers or lens systems can improve the field of view of CCD X-ray systems.

9.

Compared to a fiber optic taper, a lens system is less efficient by a factor of ten or more.

Linear Detector Arrays

•

1.

Linear detector array systems are ideally suited for production environments.

2.

Examples of applications are in automotive manufacturing, cargo transport, food inspection security and nuclear waste containment.

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Notes

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•

•

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Lesson 7

Quiz

4.

Spatial resolution of a digital detector is a factor of detector resolution and its pixel patch.

5.

Linear detector array systems are ideally suited for a production environment.

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• Chapter 8: Digital Radiographic Imaging In this lesson you will learn about: • Fluoroscopy. • Image amplifier. • Television radiography. • Xeroradiography.

•

• Stereoradiography and double exposure . • Flash radiography. • In-motion radiography.

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Lesson 8

Special Radiographic Techniques INTRODUCTION In this chapter, the following special radiographic techniques will be discussed:

•

1.

Fluoroscopy.

2.

Television radiography.

3.

Xeroradiography.

4.

Stereoradiography and double exposure.

5.

Flash radiography.

6.

In-motion radiography.

FLUOROSCOPY 1.

Fluoroscopy is an imaging system in which a real-time X-ray image can be observed on a fluorescent screen (see Figure 8.1 in the Radiographic Testing Classroom Training Book).

2.

It is a relatively low-cost, high-speed process.

3.

It can easily adapt to production line requirements.

4.

This system of imaging has the following disadvantages: a.

It has limitations on producing bright images for thick or very dense

materials. b.

It has relatively poor sensitivity compared to film radiography.

c.

It does not produce a permanent record .

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5.

Despite these limitations, it shows potential in the following applications: a.

Rapid scanning of test objects for gross internal discontinuities.

b,

Saving time and money by preliminary inspection of huge numbers of test

•

objects before sending them for further nondestructive evaluation.

IMAGE AMPLIFIER 1.

The image amplifier (or image intensifier) is designed to overcome the disadvantages of fluoroscopy, such as its low luminance.

2.

It also serves to protect the technician from radiation exposure (see Figure 8.2 in the Radiographic Testing Classroom Training Book).

3.

It consists of two main parts: an image tube and an optical system.

TELEVISION RADIOGRAPHY 1.

In converting X-rays into light, the television technique is relatively inefficient due to a large energy loss.

2.

•

The X-ray sensitive vidicon tube (see Figure 8.3 in the Radiographic Testing Classroom Training Book) is an advanced technique specifically designed for

radiographic application. 3.

The system is designed for radiographic testing of small test objects, such as electronic assemblies, as well as in-motion X-ray testing.

XERORADIOGRAPHY 1.

Xeroradiography is a dry radiographic process that uses a thin layer of selenium bonded to a backing plate of aluminum to record an X-ray image.

2.

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A permanent record can be obtained on paper.

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STEREORADIOGRAPHY AND DOUBLE EXPOSURE In the case of thick test objects, two radiographic methods are available to determine the depth of a discontinuity: 1.

Stereoradiography.

2.

Double exposure (or parallax radiographic technique).

Stereoradiography 1.

This technique provides a three-dimensional effect using two radiographs of the test object and a stereoscope.

2.

Stereoradiography is not common in radiography but is of value in discontinuity location and structure visualization (see Figure 8.5 in the Radiographic Testing Classroom Training Book).

•

Double Exposure (Parallax Radiographic Technique) 1.

This technique is more practical than stereoradiography because it does not depend on human depth perception.

2.

The technique is presented in Figure 8.6 in the Radiographic Testing Classroom Training Book.

3.

The distance of the discontinuity from the film plane (d) or depth is determined by the following formula:

d=~ a+b

where a is tube shift distance, b is image shift distance of the discontinuity on the film and c is SFD .

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FLASH RADIOGRAPHY 1.

Flash radiography permits the observation of high-speed events in opaque

•

materials. 2.

Flash radiography freezes the motion of a high-speed event by extremely short time duration exposures (microseconds).

3.

High voltages with high currents (as high as 200 A) are used in this technique.

IN-MoTION RADIOGRAPHY 1.

With in-motion radiography, the movement of the test object and the film is synchronized.

2.

In many cases, motion picture cameras are loaded with X-ray films.

CONCLUSION 1.

The Radiographic Testing Classroom Training Book provides background on

•

critical practices for implementation and control of applied radiographic technology. 2.

It provides basic understanding of processes for conducting uniform and repeatable

radiographic tests.

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Notes

•

Lesson 8

Quiz Please answer true or false to the following statements.

6.

Flash radiography permits the observation of high-speed events in opaque materials.

1.

Fluoroscopy is widely used in applications where rapid scanning of test objects is desirable.

2.

Stereoradiography is a radiographic technique to provide information about

•

the depth of a discontinuity in a thick test object.

3.

The double exposure method depends on human depth perception.

4.

In flash radiography, the duration of exposure is a millisecond.

5.

In an image amplifier, X-rays convert to electrons and are then accelerated by

•

electron lenses .

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