ASTM E1931-97

Standard Guide for X-Ray Compton Scatter Tomography

SCOPE
1.1 Purpose--this guide covers a tutorial introduction to familiarize the reader with the operational capabilities and limitations inherent in X-ray Compton Scatter Tomography (CST). Also included is a brief description of the physics and typical hardware configuration for CST.

General Information

• Status: Historical
• Publication Date: 09-Dec-1997
• Technical Committee: E07 - Nondestructive Testing
• Drafting Committee: E07.01 - Radiography (X and Gamma) Method

Relations

• Replaced By: ASTM E1931-97(2003) - Standard Guide for X-Ray Compton Scatter Tomography
• Effective Date: 10-Mar-2003

Standards Content (Sample)

NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: E 1931 – 97
Standard Guide for
X-Ray Compton Scatter Tomography
This standard is issued under the fixed designation E 1931; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope NDE methods. Chief among the limitations is the difficulty in
performing CST on thick sections of high-Z materials. CST is
1.1 Purpose—This guide covers a tutorial introduction to
best applied to thinner sections of lower Z materials. The
familiarize the reader with the operational capabilities and
following provides a general idea of the range of CST
limitations inherent in X-ray Compton Scatter Tomography
applicability when using a 160 Kv constant potential X-ray
(CST). Also included is a brief description of the physics and
source:
typical hardware configuration for CST.
Material Practical Thickness Range
1.2 Advantages—X-ray Compton Scatter Tomography
(CST) is a radiologic nondestructive examination method with
Steel Up to about 3 mm ( ⁄8 in.)
several advantages that include:
Aluminum Up to about 25 mm (1 in.)
Aerospace composites Up to about 50 mm (2 in.)
1.2.1 The ability to perform X-ray examination without
access to the opposite side of the test object;
The limitations of the technique must also consider the
1.2.2 The X-ray beam need not completely penetrate the test
required X, Y, and Z axis resolutions, the speed of image
object allowing thick objects to be partially examined. Thick
formation, image quality and the difference in the X-ray
test objects become part of the radiation shielding thereby
scattering characteristics of the parent material and the internal
reducing the radiation hazard;
features that are to be imaged.
1.2.3 The ability to image test object subsurface features
1.5 The values stated in both inch-pound and SI units are to
with minimal influence from surface features;
be regarded separately as the standard. The values given in
1.2.4 The ability to obtain high-contrast images from low
parentheses are for information only.
subject contrast materials that normally produce low-contrast
1.6 This standard does not purport to address all of the
images when using traditional transmitted beam X-ray imaging
safety concerns, if any, associated with its use. It is the
methods; and
responsibility of the user of this standard to establish appro-
1.2.5 The ability to obtain depth information for test object
priate safety and health practices and to determine the
features thereby providing three-dimensional examination. The
applicability of regulatory limitations prior to use.
ability to obtain depth information presupposes the use of a
2. Referenced Documents
highly collimated detector system having a narrow angle of
acceptance.
2.1 ASTM Standards:
1.3 Applications—This guide does not specify which test
E 747 Test Method for Controlling Quality of Radiographic
objects are suitable, or unsuitable, for CST. As with most
Testing Using Wire Penetrameters
nondestructive examination techniques, CST is highly applica-
E 1025 Practice for Hole-Type Image Quality Indicators
tion specific thereby requiring the suitability of the method to
Used for Radiography
be first demonstrated in the application laboratory. This guide
E 1255 Practice for Radioscopy
does not provide guidance in the standardized practice or
E 1316 Standard Terminology for Nondestructive Examina-
application of CST techniques. No guidance is provided
tions
concerning the acceptance or rejection of test objects examined
E 1441 Guide for Computed Tomography (CT) Imaging
with CST.
E 1453 Guide for the Storage of Media that Contains
1.4 Limitations—As with all nondestructive examination
Radioscopic Data
methods, CST has limitations and is complementary to other
E 1475 Guide for Data Fields for Computerized Transfer of
Digital Radiological Test Data
E 1647 Practice for Determining Contrast Sensitivity in
Radioscopy
This guide is under the jurisdiction of ASTM Committee E-07 on Nondestruc-
tive Testing and is the direct responsibility of Subcommittee E07.01 on Radiology
(X and Gamma) Method.
Current edition approved Dec. 10, 1997. Published June 1998. Annual Book of ASTM Standards, Vol 03.03.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E1931–97
2.2 ANSI/ASNT Standards: actual test object are the best means for CST performance
ASNT Recommended Practice No. SNT-TC-1A Personnel monitoring. Conventional radiologic performance measuring
Qualification and Certification in Nondestructive Test- devices, such as Test Method E 747 and Practice E 1025 image
ing quality indicators or Practice E 1647 contrast sensitivity gages
ANSI/ASNT CP-1 89 Standard for Qualification and Certi- are designed for transmitted X-ray beam imaging and are of
fication in Nondestructive Testing Personnel little use for CST. With appropriate calibration, CST can be
2.3 Military Standard: utilized to make three-dimensional measurements of internal
MIL-STD-410 Nondestructive Testing Personnel Qualifica- test object features.
tion and Certification
5. Significance and Use
3. Terminology
5.1 Principal Advantage of Compton Scatter Tomography—
The principal advantage of CST is the ability to perform
3.1 Definitions:
three-dimensional X-ray examination without the requirement
3.1.1 CST, being a radiologic examination method, used
for access to the back side of the test object. CST offers the
much that the same vocabulary as other X-ray examination
possibility to perform X-ray examination that is not possible by
methods. A number of terms used in this standard are defined
any other method. The CST sub-surface slice image is mini-
in Terminology E 1316. It may also be helpful to read Guide
mally affected by test object features outside the plane of
E 1441.
examination. The result is a radioscopic image that contains
4. Summary of Guide information primarily from the slice plane. Scattered radiation
limits image quality in normal radiographic and radioscopic
4.1 Description—Compton Scatter Tomography is a
imaging. Scatter radiation does not have the same detrimental
uniquely different nondestructive test method utilizing pen-
effect upon CST because scatter radiation is used to form the
etrating X-ray or gamma-ray radiation. Unlike computed
image. In fact, the more radiation the test object scatters, the
tomography (CT), CST produces radioscopic images which are
better the CST result. Low subject contrast materials that
not computed images. Multiple slice images can be simulta-
cannot be imaged well by conventional radiographic and
neously produced so that the time per slice image is in the
radioscopic means are often excellent candidates for CST. Very
range of a few seconds. CST produces images that are thin with
high contrast sensitivities and excellent spatial resolution are
respect to the test object thickness (slice images) and which are
possible with CST tomography.
at right angles to the X-ray beam. Each two-dimensional slice
5.2 Limitations—As with any nondestructive testing
image (X–Y axes) is produced at an incremental distance along
method, CST has its limitations. The technique is useful on
and orthogonal to the X-ray beam (Z–axis). A stack of CST
reasonably thick sections of low-density materials. While a 1
images therefore represents a solid volume within the test
in. (25 mm) depth in aluminum or 2 in. (50 mm) in plastic is
object. Each slice image contains test object information which
achievable, the examination depth is decreased dramatically as
lies predominantly within the desired slice. To make an
the material density increases. Proper image interpretation
analogy as to how CST works, consider a book. The test object
requires the use of standards and test objects with known
may be larger or smaller (in length, width and depth) then the
internal conditions or representative quality indicators (RQIs).
analogous book. The CST slice images are the pages in the
The examination volume is typically small, on the order of a
book. Paging through the slice images provides information
few cubic inches and may require a few minutes to image.
about test object features lying at different depths within the
Therefore, completely inspecting large structures with CST
test object.
requires intensive re-positioning of the examination volume
4.2 Image Formation—CST produces one or more digital
that can be time-consuming. As with other penetrating radia-
slice plane images per scan. Multiple slice images can be
tion methods, the radiation hazard must be properly addressed.
produced in times ranging from a few seconds to a few minutes
depending upon the examined area, desired spatial resolution
6. Technical Description
and signal-to-noise ratio. The image is digital and is typically
6.1 General Description of Compton Scatter Tomography—
assembled by microcomputer. CST images are free from
Transmitted beam radiologic techniques used in radiography,
reconstruction artifacts as the CST image is produced directly
radioscopy and computed tomography have dominated the use
and is not a calculated image. Because CST images are digital,
of penetrating radiation for industrial nondestructive examina-
they may be enhanced, analyzed, archived and in general
tion. The transmitted beam technique depends upon the pen-
handled as any other digital information.
etrating radiation attenuation mechanisms of photoelectric
4.3 Calibration Standards—As with all nondestructive ex-
absorption and Compton scattering. For low-Z materials at
aminations, known standards are required for the calibration
energies up to about 50 keV, the photoelectric effect is the
and performance monitoring of the CST method. Practice E
dominant attenuation mechanism. As X-ray energy increases,
1255 calibration block standards that are representative of the
Compton scattering becomes the dominant attenuation mecha-
nism for large scattering angles in low-Z materials. Pair
production comes into play above 1.02 MeV and can become
Available from American National Standards Institute, 11 W. 42nd St., 13th
the dominant effect for higher X-ray energies. Photoelectric
Floor, New York, NY 10036.
4 absorption is strongly dependent upon the atomic number and
Available from Standardization Document Order Desk, Bldg. 4 Section D, 700
Robbins Ave. Philadelphia, PA 19111-5094, Ans:NPODS. also the electron density of the absorbing material. Compton
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E1931–97
scattering also depends upon the Z of the scattering material, relationships may be seen in Fig. 1. The following relationships
but to a lesser degree than is the photoelectric effect. These show the approximate dependence of the photoelectric effect
NOTE 1—Hubbell, J.H. and Seltzer, S.M., Tables of X-Ray Mass Attenuation Coefficients and Mass Energy-Absorption Coefficients, 1 keV to 20 MeV
for ElementsZ=1to92and 48 Additional Substances of Dosimetric Interest, NISTIR 5632, 1996. Available from National Institute of Standards and
Technology (NIST), Gaithersburg, MD 20899.
FIG. 1 Linear Absorption and Scatter Coefficients for Polyethylene, Aluminum and Iron
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E1931–97
and Compton scattering upon target material Z and incident noise CST image and faster examination speed. For this reason
X-ray energy E:
an X-ray source is often a better choice than a radioisotope for
5 7/2
Photoelectric Effect Z / E CST. Radiation detection and other image forming consider-
Compton Scattering Z / E
ations may also differ substantially from other radiologic
Pair Production: Z (lnE - constant)
imaging methods.
6.1.1 CST is best suited for lower Z materials such as
6.3 Theory of Compton Scatter Tomography—In the energy
aluminum ( Z=13 ) using a commercially available 160 Kv
range appropriate for CST (roughly 50 keV to 1 MeV), the
X-ray generating system. Somewhat higher Z materials may be
primary interaction mechanisms between electromagnetic ra-
examined by utilizing a higher energy X-ray generator rated at
diation and matter are photoelectric absorption and inelastic
225, 320, or 450 Kv. It is useful to envision the CST process as
(Compton) scatter. Fig. 2 illustrates the principles of photo-
one where the X-rays that produce the CST image originate
electric absorption and Compton scattering. As an X-ray
from many discrete points within the inspected volume. Each
having an energy E collides with an electron, the electron
Compton scatter event generates a lower energy X-ray that 0
absorbs energy from the incoming X-ray photon and is ejected
emanates from the scattering site. Singly scattered X rays that
from its shell. In the case of photoelectric absorption, the
reach the detector carry information about the test object
material characteristics at the site where it was generated. The incoming photon’s energy is totally absorbed. As the energy E
scatter radiation is also affected by the material through which of the incoming photon increases, the probability of photoelec-
it passes on the way to the detector. The external source of tric absorption decreases while the probability of Compton
primary penetrating radiation, that may be either X rays or scattering increases. The Compton scattering creates a new
gamma rays, interact by the Compton scattering process. The
X-ray having and energy E , and travelling at an angle u with
primary radiation must have adequate energy and intensity to
respect to the direction of the original primary X-ray.
generate sufficient scattered radiation at the examination site to
6.3.1 Fig. 1 shows how material linear attenuation coeffi-
allow detection. The examination depth is limited to that depth
cients due to photoelectric absorption and Compton scattering
from which sufficient scattered radiation can reach the detector
vary with energy for polyethylene, aluminum and iron. The
to form a useable image. The test object is therefore effectively
linear absorption coefficient μ for all three materials falls
a
imaged from t
...