ASTM E748-19

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Designation: E748 − 16 E748 − 19
Standard Guide for
Thermal Neutron Radiography of Materials
This standard is issued under the fixed designation E748; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope*
1.1 Purpose—Practices to be employed for the radiographic examination of materials and components with thermal neutrons
are outlined herein. They are intended as a guide for the production of neutron radiographs that possess consistent quality
characteristics, as well as aiding the user to consider the applicability of thermal neutron radiology. Statements concerning
preferred practice are provided without a discussion of the technical background for the preference. The necessary technical
background can be found in Refs (1-16).
1.2 Limitations—Acceptance standards have not been established for any material or production process (see Section 5 on Basis
of Application). Adherence to the guide will, however, produce reproducible results. Neutron radiography, whether performed by
means of a reactor, an accelerator, subcritical assembly, or radioactive source, will be consistent in sensitivity and resolution only
if the consistency of all details of the technique, such as neutron source, collimation, geometry, film, etc., are maintained. This
guide is limited to the use of photographic or radiographic film in combination with conversion screens for image recording; other
imaging systems are available. Emphasis is placed on the use of nuclear reactor neutron sources.
1.3 Interpretation and Acceptance Standards—Interpretation and acceptance standards are not covered by this guide.
Designation of accept-reject standards is recognized to be within the cognizance of product specifications.
1.4 Safety Practices—General practices for personnel protection against neutron and associated radiation peculiar to the neutron
radiologic process are discussed in Section 1718. Jurisdictional nuclear regulations will also apply.
1.5 Other Aspects of the Neutron Radiographic Process—For many important aspects of neutron radiography such as technique,
files, viewing of radiographs, storage of radiographs, film processing, and record keeping, refer to Guide E94, which covers these
aspects for x-rayX-ray radiography. (See Section 2.)
1.6 The values stated in either SI or inch-pound units are to be regarded as the standard.
1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.8 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
E94 Guide for Radiographic Examination Using Industrial Radiographic Film
E543 Specification for Agencies Performing Nondestructive Testing
E545 Test Method for Determining Image Quality in Direct Thermal Neutron Radiographic Examination
E803 Test Method for Determining the L/D Ratio of Neutron Radiography Beams
E1316 Terminology for Nondestructive Examinations
These practices areThis guide is under the jurisdiction of ASTM Committee E07 on Nondestructive Testing and areis the direct responsibility of Subcommittee E07.05
on Radiology (Neutron) Method.
Current edition approved Feb. 15, 2016May 1, 2019. Published February 2016June 2019. Originally approved in 1980. Last previous edition approved in 20082016 as
E748 – 02E748 – 16.(2008). DOI: 10.1520/E0748-16.10.1520/E0748-19.
The boldface numbers in parentheses refer to the list of references at the end of these practices.this standard.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E748 − 19
2.2 ASNT Standard:
Recommended Practice SNT-TC-1A for Personnel Qualification and Certification
2.3 ANSI Standard:
ANSI/ASNT-CP-189 Standard for Qualification and Certification of Nondestructive Testing Personnel
2.4 AIA Document:
NAS-410 Nondestructive Testing Personnel Qualification and Certification
2.5 ISO Standard:
ISO 9712 Non-Destructive Testing—Qualification and Certification of NDT Personnel
3. Terminology
3.1 Definitions—For definitions of terms used in these practices, see Terminology E1316, Section H.
4. Significance and Use
4.1 This guide covers types of materials to be examined, neutron radiographic examination techniques, neutron production and
collimation methods, radiographic film, and converter screen selection. Within the present state of the neutron radiologic art, these
practices are generally applicable to specific material combinations, processes, and techniques.
5. Basis of Application
5.1 Personnel Qualification—If specified in the contractual agreement, personnel performing examinations to this standard shall
be qualified in accordance with a nationally or internationally recognized NDT personnel qualification practice or standard such
as ANSI/ASNT-CP-189, SNT-TC-1A, NAS-410, ISO 9712, or a similar document and certified by the employer or certifying
agency, as applicable. The practice or standard used and its applicable revision shall be identified in the contractual agreement
between the using parties.
5.2 Qualification of Nondestructive Agencies—If specified in the contractual agreement, NDT agencies shall be qualified and
evaluated as described in PracticeSpecification E543. The applicable edition of PracticeSpecification E543 shall be specified in the
contractual agreement.
5.3 Procedures and Techniques—The procedures and techniques to be used shall be as described in these practices unless
otherwise specified. Specific techniques may be specified in the contractual agreement.
5.4 Reporting Criteria/Acceptance Criteria—Reporting criteria for the examination results shall be in accordance with 1.3
unless otherwise specified. Acceptance criteria (for example, for reference radiographs) shall be specified in the contractual
agreement.
6. Neutron Radiography
6.1 The Method—Neutron radiography is basically similar to X-ray radiography in that both techniques employ radiation beam
intensity modulation by an object to image macroscopic object details. X-rays or gamma rays are replaced by neutrons as the
penetrating radiation in a through-transmission examination. Since the absorption characteristics of matter for X-rays and neutrons
differ drastically, the two techniques in general serve to complement one another.
6.2 Facilities—The basic neutron radiography facility consists of a source of fast neutrons, a moderator, a gamma filter, a
collimator, a conversion screen, a film image recorder or other imaging system, a cassette, and adequate biological shielding and
interlock systems. A schematic diagram of a representative neutron radiography facility is illustrated in Fig. 1.
7. Neutron Sources
7.1 General—The thermal neutron beam may be obtained from a nuclear reactor, a subcritical assembly, a radioactive neutron
source, or an accelerator. Neutron radiography has been achieved successfully with all four sources. In all cases the initial neutrons
generated possess high energies and must be reduced in energy (moderated) to be useful for thermal neutron radiography. This may
be achieved by surrounding the source with light materials such as water, oil, plastic, paraffin, beryllium, or graphite. The preferred
moderator will be dependent on the constraints dictated by the energy of the primary neutrons, which will in turn be dictated by
neutron beam parameters such as thermal neutron yield requirements, cadmium ratio, and beam gamma ray contamination. The
Available from the American Society for Nondestructive Testing, 1711 Arlingate Lane, Testing (ASNT), P.O. Box 28518, 1711 Arlingate Ln., Columbus, OH
43228-0518.43228-0518, http://www.asnt.org.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036.10036, http://www.ansi.org.
Available from Aerospace Industries Association of America, Inc., 1250 Eye St., NW, Washington, DC 20005.(AIA), 1000 Wilson Blvd., Suite 1700, Arlington, VA
22209, http://www.aia-aerospace.org.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
E748 − 19
FIG. 1 Typical Neutron Radiography Facility with Divergent Collimator
characteristics of a particular system for a given application are left for the seller and the buyer of the service to decide.
Characteristics and capabilities of each type of source are referenced in the References section. A general comparison of sources
is shown in Table 1.
7.2 Nuclear Reactors—Nuclear reactors are the preferred thermal neutron source in general, since high neutron fluxes are
available and exposures can be made in a relatively short time span. The high neutron intensity makes it possible to provide a
tightly collimated beam; therefore, high-resolution radiographs can be produced.
7.3 Subcritical Assembly—A subcritical assembly is achieved by the addition of sufficient fissionable material surrounding a
moderated source of neutrons, usually a radioisotope source. Although the total thermal neutron yield is smaller than that of a
nuclear reactor, such a system offers the attractions of adequate image quality in a reasonable exposure time, relative ease of
licensing, adequate neutron yield for most industrial applications, and the possibility of transportable operation.
7.4 Accelerator Sources—Accelerators used for thermal neutron radiography have generally been of the low-voltage type which
3 4
utilize the H(d,n) He reaction, high-energy X-ray machines in which the (x,n) reaction is applied and Van de Graaff and other
9 10
high-energy accelerators which employ reactions such as Be(d,n) B. In all cases, the targets are surrounded by a moderator to
12 −1
reduce the neutrons to thermal energies. The total neutron yields of such machines can be on the order of 10 ·n·s ; the thermal
9 −2 −1
neutron flux of such sources before collimation can be on the order of 10 n·cm ·s , for example, the yield from a Van de Graaff
accelerator.
7.5 Isotopic Sources—Many isotopic sources have been employed for neutron radiologic applications. Those that have been
most widely utilized are outlined in Table 2. Radioactive sources offer the best possibility for portable operation. However, because
of the relatively low neutron yield, the exposure times are usually long for a given image quality. The isotopic source Cf offers
a number of advantages for thermal neutron radiology, namely, low neutron energy and small physical size, both of which lead
to efficient neutron moderation, and the possibility for high total neutron yields.
8. Imaging Methods and Conversion Screens
8.1 General—Neutrons are indirectly ionizing particulate radiation that have little direct effect on radiographic film. To obtain
a neutron radiographic image on film, a conversion screen is normally employed; upon neutron capture, screens emit prompt and
delayed decay products in the form of nuclear radiation or light. In all cases the screen should be placed in intimate contact with
the radiographic film in order to obtain sharp images.
8.2 Direct Method—In the direct method, a film is placed on the source side of the conversion screen (front film) and exposed
to the neutron beam together with the conversion screen. Electron emission upon neutron capture is the mechanism by which the
film is primarily exposed in the case of gadolinium conversion screens. The screen is generally one of the following types: (1) a
free-standing gadolinium metal screen accessible to film on both sides; (2) a sapphire-coated, vapor-deposited gadolinium screen
on a substrate such as aluminum; or (3) a light-emitting fluorescent screen such as gadolinium oxysulfide or LiF/ZnS. Exposure
of an additional film (without object) is often useful to resolve artifacts that may appear in radiographs. Such artifacts could result
from screen marks, excess pressure, light leaks, development, or non-uniform film. In the case of light-emitting conversion screens,
it is recommended that the spectral response of the light emission be matched as closely as possible to that of the film used for
TABLE 1 Comparison of Thermal Neutron Sources
Type of Source Typical Radiographic Flux, n/cm ·s Radiographic Resolution Characteristics
5 8
Nuclear reactor 10 to 10 excellent stable operation, not portable
4 6
Subcritical assembly 10 to 10 good stable operation, portability difficult
3 6
Accelerator 10 to 10 medium on-off operation, transportable
1 4
Radioisotope 10 to 10 poor to medium stable operation, portability possible
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TABLE 2 Radioactive Sources Employed for Thermal Neutron Radiography
A
Source Type Half-Life Comments
Sb-Be (γ,n) 60 days short half-life and high γ-background, low neutron energy is advantage for
moderation, high yield source
Po-Be (α,n) 138 days short half-life, low γ-background
Am-Be (α,n) 458 years long half-life, easily shielded γ-background
241 242
Am- Cm-Be (α,n) 163 days short half-life, high neutron yield
Cf spontaneous fission 2.65 years long half-life, high neutron yield, small size and low energy offer advantages in
moderation
A
These comments compare sources in the table.
optimum results. The direct method should be employed whenever high-resolution radiographs are required, and high beam
contamination of low-energy gamma rays or highly radioactive objects do not preclude its use.
8.3 Indirect Method—This method makes use of conversion screens that can be made temporarily radioactive by neutron
capture. The conversion screen is exposed alone to the neutron-imaging beam; the film is not present. Candidate conversion
materials include rhodium, gold, indium, and dysprosium. Indium and dysprosium are recommended with dysprosium yielding the
greater speed and emitting less energetic gamma radiation. It is recommended that the conversion screens be activated in the
neutron beam for a maximum of three half-lives. Further neutron irradiation will result in a negligible amount of additional induced
activity. After irradiation, the conversion screens should be placed in intimate contact with a radiographic film in a vacuum cassette,
or other light-tight assembly in which good contact can be maintained between the radiographic film and radioactive screen. X-ray
intensification screens may be used to increase the speed of the auto-radiographic process if desired. For the indirect type of
exposure, the material from which the cassette is fabricated is immaterial as there are no neutrons to be scattered in the exposure
process. In this case, as in the activation process, there is little to be gained for conversion screen-film exposures extending beyond
three half-lives. It is recommended that this method be employed whenever the neutron beam is highly contaminated with gamma
rays, which in turn cause film fogging and reduced contrast sensitivity, or when highly radioactive objects are to be radiographed.
In short, this method is beam gamma-insensitive.
8.4 Other Imaging Systems—The scope of this guide is limited to film detectors (see 1.2). However, other neutron detector
systems such as track-etch and digital detector systems are available.
9. Neutron Collimators
9.1 General—Neutron sources for thermal neutron radiology generally involve a sizeable moderator region in which the neutron
motion is highly multidirectional. Collimators are required to produce a beam and thereby produce adequate image resolution
capability in a neutron radiology facility. It should be noted that in the definitions of collimator parameters, it is assumed that the
object under examination is placed as close to the detector system as possible to decrease both magnification and image
unsharpness due to the finite neutron source size. Several types of collimators are available. These include the widely used
divergent type, multichannel, pinhole, and straight collimators. The image spatial resolution properties of the beams are generally
set in part by the diameter or longest dimension of the collimator entrance port (D) and the distance between that aperture and the
imaging system (L). An exception is the multichannel collimator in which D is the diameter of a channel and L is the length of
the collimator. It should be noted that the detection system used in conjunction with a multichannel collimator will register the
collimator pattern. Registry can be eliminated by empirically adjusting the distance between the collimator and the imaging system
until the pattern disappears. Ratios of L/D as low as 10 are not unusual for low neutron yield sources, while higher resolution
capability systems often will display L/D values of several hundred or more. Test Method E803 details the method of measuring
the L/D ratio for neutron radiography systems. The actual spatial resolution or image unsharpness in a particular radiologic
examination will depend, of course, on factors additional to the beam characteristics. These include the object size, the geometry
of the system, and scatter conditions. For the typical calculation of geometric unsharpness, the size of the X-radiologic source, F,
would be replaced by the size of the effective thermal neutron radiologic source (D) as discussed in Guide E94.
9.2 Divergent Collimator—The divergent collimator is a tapered reentrant port into the point of highest thermal neutron flux in
the moderator. The walls of the collimator are lined with a thermal neutron absorbing material to permit only unscattered neutrons
from the source to reach the object and the image plane. This type of collimator is preferred when larger objects will be
radiographed in a single exposure. It is recommended that the divergent collimator be lined with a neutron absorber which produces
neutron capture decay products that will not result in background fogging of the film, such as Li carbonate. A typical divergent
collimating system is illustrated in the schematic diagram of Fig. 1.
9.3 Multichannel Collimator—The multichannel collimator is an array of tubular collimators stacked within a larger collimator
envelope. It is recommended as a means of achieving a high degree of collimation within a short collimation length. When this
type of collimator is employed, a suitable collimator to detector distance should be maintained to avoid registry of the collimator
pattern on the radiologic image.
E748 − 19
9.4 Straight Collimator—A straight-tube reentrant port can also be used instead of the tapered assembly described in 9.2.
Although such collimators were widely used in early neutron radiologic work, the need to examine larger objects and to achieve
higher resolution has fostered the use of divergent collimators.
9.5 Pinhole Collimator—Higher resolution can be obtained with a straight collimator when it is employed in conjunction with
a pinhole iris. The pinhole is generally fabricated from a neutron-opaque material such as Cd, Gd, or B. The resolution attainable
will be dependent on the pinhole diameter D. A schematic diagram of this system is illustrated in Fig. 2.
10. Beam Filters
10.1 Thermal Neutron Radiography—In general, filters may not be necessary. However, it may be desirable to employ Pb or
Bi filters in the neutron beam to minimize beam gamma-ray contamination. Whenever Bi gamma-ray filters are employed in a high
neutron flux environment, the filter should be encased in a sealed aluminum can to contain alpha particle contamination due to
210 209
the Po produced by the neutron capture reaction in Bi. Gamma rays can cause film fogging and reduced contrast sensitivity.
In particular, some scintillator converter screens exhibit sensitivity to beam gamma-ray contamination. This effect can be
minimized by careful selection of the screen/film combination.
11. Beam Uniformity
11.1 The only true measurement of the beam uniformity is with a radiograph made without objects. Ideally the intensity of
neutrons should be uniform across the entire radiograph.
11.2 A common approach for measuring the beam uniformity is to expose a film without any objects present on it to an optical
density of 2.0 to 3.0. Optical density measurements are then taken at the center of the film and 25 to 30 mm towards the center
from each corner of the film, and the numerical mean of these measurements calculated. If the beam diameter is smaller than the
film, the four outside measurements are taken 25 to 30 mm from the edge of the beam at 90 degree intervals. A beam is considered
to be uniform if the optical density does not vary by more than 65 % from this numerical mean.
12. Masking
12.1 General—In general, masking is not often used in thermal neutron radiology. Where it is desirable to reduce scatter or to
reduce unusual contrasts, the choice of masking materials should be made carefully. Materials that scatter readily, such as those
containing hydrogen or materials that emit radiation that
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