ASTM E748-02(2008)
(Practice)Standard Practices for Thermal Neutron Radiography of Materials
Standard Practices for Thermal Neutron Radiography of Materials
SIGNIFICANCE AND USE
These practices include 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.
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 (radiology, radiographic, and related terms are defined in Terminology E 1316). 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 practices will, however, produce reproducible results that could serve as standards. 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., is maintained through the practices. These practices are 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 these practices. 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 17. For further information on this important aspect of neutron radiology, refer to current documents of the National Committee on Radiation Protection and Measurement, the Code of Federal Regulations, the U.S. Nuclear Regulatory Commission, the U.S. Department of Energy, the National Institute of Standards and Technology, and to applicable state and local codes.
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 E 94. (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 and health practices and determine the applicability of regulatory limitations prior to use. (For more specific safety information see 1.4.)
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Designation: E748 − 02(Reapproved 2008)
Standard Practices 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.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope tee on Radiation Protection and Measurement, the Code of
Federal Regulations, the U.S. Nuclear Regulatory
1.1 Purpose—Practicestobeemployedfortheradiographic
Commission, the U.S. Department of Energy, the National
examination of materials and components with thermal neu-
Institute of Standards and Technology, and to applicable state
trons are outlined herein. They are intended as a guide for the
and local codes.
production of neutron radiographs that possess consistent
qualitycharacteristics,aswellasaidingtheusertoconsiderthe 1.5 Other Aspects of the Neutron Radiographic Process—
applicability of thermal neutron radiology (radiology, For many important aspects of neutron radiography such as
radiographic, and related terms are defined in Terminology technique, files, viewing of radiographs, storage of
E1316). Statements concerning preferred practice are provided radiographs, film processing, and record keeping, refer to
without a discussion of the technical background for the Guide E94. (See Section 2.)
preference. The necessary technical background can be found
1.6 The values stated in either SI or inch-pound units are to
in Refs (1-16).
be regarded as the standard.
1.2 Limitations—Acceptancestandardshavenotbeenestab-
1.7 This standard does not purport to address all of the
lished for any material or production process (see Section 5 on
safety concerns, if any, associated with its use. It is the
Basis of Application). Adherence to the practices will,
responsibility of the user of this standard to establish appro-
however, produce reproducible results that could serve as
priate safety and health practices and determine the applica-
standards. Neutron radiography, whether performed by means
bility of regulatory limitations prior to use. (For more specific
ofareactor,anaccelerator,subcriticalassembly,orradioactive
safety information see 1.4.)
source, will be consistent in sensitivity and resolution only if
the consistency of all details of the technique, such as neutron
2. Referenced Documents
source,collimation,geometry,film,etc.,ismaintainedthrough
2.1 ASTM Standards:
the practices. These practices are limited to the use of photo-
E94Guide for Radiographic Examination
graphic or radiographic film in combination with conversion
E543Specification forAgencies Performing Nondestructive
screens for image recording; other imaging systems are avail-
Testing
able. Emphasis is placed on the use of nuclear reactor neutron
E545Test Method for Determining Image Quality in Direct
sources.
Thermal Neutron Radiographic Examination
1.3 Interpretation and Acceptance Standards—
E803TestMethodforDeterminingthe L/DRatioofNeutron
Interpretation and acceptance standards are not covered by
Radiography Beams
these practices. Designation of accept-reject standards is rec-
E1316Terminology for Nondestructive Examinations
ognized to be within the cognizance of product specifications.
E1496Test Method for Neutron Radiographic Dimensional
Measurements (Withdrawn 2012)
1.4 Safety Practices—General practices for personnel pro-
tection against neutron and associated radiation peculiar to the
2.2 ASNT Standard:
neutron radiologic process are discussed in Section 17. For
Recommended Practice SNT-TC-1Afor Personnel Qualifi-
further information on this important aspect of neutron
cation and Certification
radiology, refer to current documents of the National Commit-
1 3
These practices are under the jurisdiction of ASTM Committee E07 on For referenced ASTM standards, visit the ASTM website, www.astm.org, or
NondestructiveTestingandarethedirectresponsibilityofSubcommitteeE07.05on contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Radiology (Neutron) Method. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved July 1, 2008. Published September 2008. Originally the ASTM website.
approved in 1980. Last previous edition approved in 2002 as E748–02. DOI: The last approved version of this historical standard is referenced on
10.1520/E0748-02R08. www.astm.org.
2 5
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof Available from the American Society for Nondestructive Testing, 1711 Arlin-
these practices. gate Lane, P.O. Box 28518, Columbus, OH 43228-0518.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E748 − 02 (2008)
2.3 ANSI Standard: object details. X rays or gamma rays are replaced by neutrons
ANSI/ASNT-CP-189Standard for Qualification and Certifi- as the penetrating radiation in a through-transmission exami-
cation of Nondestructive Testing Personnel nation.SincetheabsorptioncharacteristicsofmatterforXrays
2.4 AIA Document: and neutrons differ drastically, the two techniques in general
serve to complement one another.
NAS-410Nondestructive Testing Personnel Qualification
and Certification
6.2 Facilities—The basic neutron radiography facility con-
sists of a source of fast neutrons, a moderator, a gamma filter,
3. Terminology
a collimator, a conversion screen, a film image recorder or
3.1 Definitions—For definitions of terms used in these
other imaging system, a cassette, and adequate biological
practices, see Terminology E1316, Section H.
shielding and interlock systems. A schematic diagram of a
representative neutron radiography facility is illustrated in Fig.
4. Significance and Use
1.
4.1 These practices include types of materials to be
6.3 Thermalization—Theprocessofslowingdownneutrons
examined, neutron radiographic examination techniques, neu-
bypermittingtheneutronstocometothermalequilibriumwith
tron production and collimation methods, radiographic film,
their surroundings; see definition of thermal neutrons in
and converter screen selection. Within the present state of the
Terminology E1316, Section H.
neutron radiologic art, these practices are generally applicable
to specific material combinations, processes, and techniques.
7. Neutron Sources
7.1 General—The thermal neutron beam may be obtained
5. Basis of Application
from a nuclear reactor, a subcritical assembly, a radioactive
5.1 Personnel Qualification—Nondestructive testing (NDT)
neutron source, or an accelerator. Neutron radiography has
personnel shall be qualified in accordance with a nationally
been achieved successfully with all four sources. In all cases
recognized NDT personnel qualification practice or standard
the initial neutrons generated possess high energies and must
such as ANSI/ASNT-CP-189, SNT-TC-1A, NAS-410, or a
be reduced in energy (moderated) to be useful for thermal
similar document. The practice or standard used and its
neutron radiography.This may be achieved by surrounding the
applicable revision shall be specified in the contractual agree-
source with light materials such as water, oil, plastic, paraffin,
ment between the using parties.
beryllium, or graphite.The preferred moderator will be depen-
5.2 Qualification of Nondestructive Agencies—If specified
dent on the constraints dictated by the energy of the primary
in the contractual agreement, NDT agencies shall be qualified
neutrons, which will in turn be dictated by neutron beam
and evaluated as described in Practice E543. The applicable
parameters such as thermal neutron yield requirements, cad-
edition of Practice E543 shall be specified in the contractual
mium ratio, and beam gamma ray contamination. The charac-
agreement.
teristics of a particular system for a given application are left
for the seller and the buyer of the service to decide. Charac-
5.3 Procedures and Techniques—The procedures and tech-
teristics and capabilities of each type of source are referenced
niquestobeusedshallbeasdescribedinthesepracticesunless
in the References section. A general comparison of sources is
otherwisespecified.Specifictechniquesmaybespecifiedinthe
shown in Table 1.
contractual agreement.
7.2 Nuclear Reactors—Nuclear reactors are the preferred
5.4 Extent of Examination—Theextentofexaminationshall
thermalneutronsourceingeneral,sincehighneutronfluxesare
be in accordance with Section 16 unless otherwise specified.
available and exposures can be made in a relatively short time
5.5 Reporting Criteria/Acceptance Criteria—Reporting cri-
span.The high neutron intensity makes it possible to provide a
teriafortheexaminationresultsshallbeinaccordancewith1.3
tightlycollimatedbeam;therefore,high-resolutionradiographs
unless otherwise specified. Acceptance criteria (for example,
can be produced.
for reference radiographs) shall be specified in the contractual
agreement.
5.6 Reexamination of Repaired/Reworked Items—
Reexamination of repaired/reworked items is not addressed in
these practices and, if required, shall be specified in the
contractual agreement.
6. Neutron Radiography
6.1 The Method—Neutron radiography is basically similar
to X radiography in that both techniques employ radiation
beam intensity modulation by an object to image macroscopic
Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
4th Floor, New York, NY 10036.
Available from Aerospace Industries Association of America, Inc., 1250 Eye FIG. 1 Typical Neutron Radiography Facility with Divergent Colli-
St., NW, Washington, DC 20005. mator
E748 − 02 (2008)
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
7.3 Subcritical Assembly—A subcritical assembly is 8.2 Direct Method—Inthedirectmethod,afilmisplacedon
achieved by the addition of sufficient fissionable material
the source side of the conversion screen (front film) and
surrounding a moderated source of neutrons, usually a radio- exposed to the neutron beam together with the conversion
isotope source. Although the total thermal neutron yield is
screen. Electron emission upon neutron capture is the mecha-
smaller than that of a nuclear reactor, such a system offers the nism by which the film is exposed in the case of gadolinium
attractions of adequate image quality in a reasonable exposure
conversion screens. The screen is generally one of the follow-
time,relativeeaseoflicensing,adequateneutronyieldformost ing types: (1) a free-standing gadolinium metal screen acces-
industrial applications, and the possibility of transportable
sible to film on both sides; (2) a sapphire-coated, vapor-
operation.
deposited gadolinium screen on a substrate such as aluminum;
or (3) a light-emitting fluorescent screen such as gadolinium
7.4 Accelerator Sources—Accelerators used for thermal
oxysulfide or LiF/ZnS. Exposure of an additional film (with-
neutron radiography have generally been of the low-voltage
3 4
out object) is often useful to resolve artifacts that may appear
type which utilize the H(d,n) He reaction, high-energy X-ray
in radiographs. Such artifacts could result from screen marks,
machines in which the (x,n) reaction is applied and Van de
excess pressure, light leaks, development, or nonuniform film.
Graaff and other high-energy accelerators which employ reac-
9 10
In the case of light-emitting conversion screens, it is recom-
tions such as Be(d,n) B. In all cases, the targets are
mended that the spectral response of the light emission be
surrounded by a moderator to reduce the neutrons to thermal
matched as closely as possible to that of the film used for
energies. The total neutron yields of such machines can be on
12 −1
optimum results. The direct method should be employed
theorderof10 ·n·s ;thethermalneutronfluxofsuchsources
9 −2 −1
whenever high-resolution radiographs are required, and high
before collimation can be on the order of 10 n·cm ·s , for
beam contamination of low-energy gamma rays or highly
example, the yield from a Van de Graaff accelerator.
radioactive objects do not preclude its use.
7.5 Isotopic Sources—Many isotopic sources have been
employed for neutron radiologic applications. Those that have 8.3 Indirect Method—Thismethodmakesuseofconversion
been most widely utilized are outlined in Table 2. Radioactive
screens that can be made temporarily radioactive by neutron
sources offer the best possibility for portable operation. capture.Theconversionscreenisexposedalonetotheneutron-
However, because of the relatively low neutron yield, the
imaging beam; the film is not present. Candidate conversion
exposure times are usually long for a given image quality.The materials include rhodium, gold, indium, and dysprosium.
isotopic source Cf offers a number of advantages for
Indium and dysprosium are recommended with dysprosium
thermal neutron radiology, namely, low neutron energy and yielding the greater speed and emitting less energetic gamma
small physical size, both of which lead to efficient neutron
radiation. It is recommended that the conversion screens be
moderation, and the possibility for high total neutron yields.
activated in the neutron beam for a maximum of three
half-lives.Furtherneutronirradiationwillresultinanegligible
8. Imaging Methods and Conversion Screens
amount of additional induced activity. After irradiation, the
8.1 General—Neutronsarenonionizingparticulateradiation conversion screens should be placed in intimate contact with a
that have little direct effect on radiographic film. To obtain a radiographic film in a vacuum cassette, or other light-tight
neutron radiographic image on film, a conversion screen is assemblyinwhichgoodcontactcanbemaintainedbetweenthe
normally employed; upon neutron capture, screens emit radiographic film and radioactive screen. X-ray intensification
prompt and delayed decay products in the form of nuclear screens may be used to increase the speed of the autoradio-
radiation or light. In all cases the screen should be placed in graphic process if desired. For the indirect type of exposure,
intimate contact with the radiographic film in order to obtain thematerialfromwhichthecassetteisfabricatedisimmaterial
sharp images. astherearenoneutronstobescatteredintheexposureprocess.
TABLE 2 Radioactive
...
This document is not anASTM standard and is intended only to provide the user of anASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
An American National Standard Designation: E 748 – 02 (Reapproved 2008)
Designation:E748–95
Standard Practices 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.Anumber 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 (radiology, radiographic, and
related terms are defined in Terminology E1316). 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—Acceptancestandardshavenotbeenestablishedforanymaterialorproductionprocess(seeSection5onBasis
of Application). Adherence to the practices will, however, produce reproducible results that could serve as standards. Neutron
radiography,whetherperformedbymeansofareactor,anaccelerator,subcriticalassembly,orradioactivesource,willbeconsistent
in sensitivity and resolution only if the consistency of all details of the technique, such as neutron source, collimation, geometry,
film, etc., is maintained through the practices. These practices are 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 these practices.
Designation of accept-reject standards is recognized to be within the cognizance of product specifications.
1.4 Safety Practices—Generalpracticesforpersonnelprotectionagainstneutronandassociatedradiationpeculiartotheneutron
radiologic process are discussed in Section 17. For further information on this important aspect of neutron radiology, refer to
currentdocumentsoftheNationalCommitteeonRadiationProtectionandMeasurement,theCodeofFederalRegulations,theU.S.
Nuclear Regulatory Commission, the U.S. Department of Energy, the National Institute of Standards and Technology, and to
applicable state and local codes.
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. (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 and health practices and determine the applicability of regulatory
limitations prior to use. (For more specific safety information see 1.4.)
2. Referenced Documents
2.1 ASTM Standards:
E94 Guide for Radiographic Testing Examination
E543Practice for Evaluating Agencies that Perform Nondestructive Testing 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
E1496 Test Method for Neutron Radiographic Dimensional Measurements
These practices are under the jurisdiction of ASTM Committee E-7 on Nondestructive Testing and are the direct responsibility of Subcommittee E07.05 on Neutron
Radiography.
Current edition approved Dec. 10, 1995. Published February 1996. Originally published as E748–80. Last previous edition E748–90.
These practices are under the jurisdiction ofASTM Committee E07 on Nondestructive Testing and are the direct responsibility of Subcommittee E07.05 on Radiology
(Neutron) Method.
Current edition approved July 1, 2008. Published September 2008. Originally approved in 1980. Last previous edition approved in 2002 as E748–02.
The boldface numbers in parentheses refer to the list of references at the end of these practices.
ForreferencedASTMstandards,visittheASTMwebsite,www.astm.org,orcontactASTMCustomerServiceatservice@astm.org.For Annual Book of ASTM Standards
, Vol 03.03.volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E748–02 (2008)
2.2 ASNT Standard:
SNT-TC-1ARecommended Practice for Personnel Qualification and CertificationRecommended 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 Military Standard: AIA Document:
MIL-STD-410NAS-410 Nondestructive Testing Personnel Qualification and Certification
3. Terminology
3.1 Definitions—For definitions of terms used in these practices, see Terminology E1316, Section H.
4. Significance and Use
4.1 Thesepracticesincludetypesofmaterialstobeexamined,neutronradiographicexaminationtechniques,neutronproduction
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—Nondestructive testing (NDT) personnel shall be qualified in accordance with a nationally
recognizedNDTpersonnelqualificationpracticeorstandardsuchasANSI/ASNT-CP-189,SNT-TC-1A,MIL-STD-410,NAS-410,
or a similar document. The practice or standard used and its applicable revision shall be specified 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 Practice E543. The applicable edition of Practice 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 Extent of Examination—The extent of examination shall be in accordance with Section 16 unless otherwise specified.
5.5 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.
5.6 Reexamination of Repaired/Reworked Items—Reexaminationofrepaired/reworkeditemsisnotaddressedinthesepractices
and, if required, shall be specified in the contractual agreement.
6. Neutron Radiography
6.1 The Method—Neutron radiography is basically similar to X 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
penetratingradiationinathrough-transmissionexamination.SincetheabsorptioncharacteristicsofmatterforXraysandneutrons
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.
6.3 Thermalization—The process of slowing down neutrons by permitting the neutrons to come to thermal equilibrium with
their surroundings; see definition of thermal neutrons in Terminology E1316, Section H.
7. Neutron Sources
7.1 General—The thermal neutron beam may be obtained from a nuclear reactor, a subcritical assembly, a radioactive neutron
source,oranaccelerator.Neutronradiographyhasbeenachievedsuccessfullywithallfoursources.Inallcasestheinitialneutrons
generatedpossesshighenergiesandmustbereducedinenergy(moderated)tobeusefulforthermalneutronradiography.Thismay
beachievedbysurroundingthesourcewithlightmaterialssuchaswater,oil,plastic,paraffin,beryllium,orgraphite.Thepreferred
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
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.Ageneral comparison of sources
is shown in Table 1.
Available from the American Society for Nondestructive Testing, 1711 Arlingate Lane, P.O. Box 28518, Columbus, OH 43228-0518.
Available from American National Standards Institute, 11 West 42nd St., 13th Floor, New York, NY 10036.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036.
Available from Standardization Documents Order Desk, Bldg. 4 Section D, 700 Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS.
Available from Aerospace Industries Association of America, Inc., 1250 Eye St., NW, Washington, DC 20005.
E748–02 (2008)
FIG. 1 Typical Neutron Radiography Facility with Divergent
Collimator
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
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—Acceleratorsusedforthermalneutronradiographyhavegenerallybeenofthelow-voltagetypewhich
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 aVan de Graaff
accelerator.
7.5 Isotopic Sources—Many isotopic sources have been employed for neutron radiologic applications. Those that have been
mostwidelyutilizedareoutlinedinTable2.Radioactivesourcesofferthebestpossibilityforportableoperation.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 nonionizing 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
TABLE 2 Radioactive Sources Employed for Thermal Neutron Radiography
A
Source Type Half-Life Comments
Sb-Be (g,n) 60 days short half-life and high g-background, low neutron energy is advantage for
moderation, high yield source
Po-Be (a,n) 138 days short half-life, low g-background
Am-Be (a,n) 458 years long half-life, easily shielded g-background
241 242
Am- Cm-Be (a,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.
E748–02 (2008)
filmisexposedinthecaseofgadoliniumconversionscreens.Thescreenisgenerallyoneofthefollowingtypes:(1)afree-standing
gadolinium metal screen accessible to film on both sides; (2) a sapphire-coated, vapor-deposited gadolinium screen on a substrate
suchasaluminum;or(3)alight-emittingfluorescentscreensuchasgadoliniumoxysulfideor LiF/ZnS.Exposureofanadditional
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 nonuniform film. In the case of light-emitting c
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Questions, Comments and Discussion
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