1、Designation: E 748 02 (Reapproved 2008)Standard Practices forThermal Neutron Radiography of Materials1This standard is issued under the fixed designation E 748; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revis
2、ion. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 PurposePractices to be employed for the radiographicexamination of materials and components with thermal neu-trons are outlined
3、herein. They are intended as a guide for theproduction of neutron radiographs that possess consistentquality characteristics, as well as aiding the user to consider theapplicability of thermal neutron radiology (radiology, radio-graphic, and related terms are defined in Terminology E 1316).Statement
4、s concerning preferred practice are provided withouta discussion of the technical background for the preference.The necessary technical background can be found in Refs(1-16).21.2 LimitationsAcceptance standards have not been es-tablished for any material or production process (see Section 5on Basis
5、of Application). Adherence to the practices will,however, produce reproducible results that could serve asstandards. Neutron radiography, whether performed by meansof a reactor, an accelerator, subcritical assembly, or radioactivesource, will be consistent in sensitivity and resolution only ifthe co
6、nsistency of all details of the technique, such as neutronsource, collimation, geometry, film, etc., is maintained throughthe practices. These practices are limited to the use of photo-graphic or radiographic film in combination with conversionscreens for image recording; other imaging systems are a
7、vail-able. Emphasis is placed on the use of nuclear reactor neutronsources.1.3 Interpretation and Acceptance StandardsInterpretation and acceptance standards are not covered bythese practices. Designation of accept-reject standards is rec-ognized to be within the cognizance of product specifications
8、.1.4 Safety PracticesGeneral practices for personnel pro-tection against neutron and associated radiation peculiar to theneutron radiologic process are discussed in Section 17. Forfurther information on this important aspect of neutron radiol-ogy, refer to current documents of the National Committee
9、 onRadiation Protection and Measurement, the Code of FederalRegulations, the U.S. Nuclear Regulatory Commission, theU.S. Department of Energy, the National Institute of Standardsand Technology, and to applicable state and local codes.1.5 Other Aspects of the Neutron Radiographic ProcessFor many impo
10、rtant aspects of neutron radiography such astechnique, files, viewing of radiographs, storage of radio-graphs, film processing, and record keeping, refer to GuideE94. (See Section 2.)1.6 The values stated in either SI or inch-pound units are tobe regarded as the standard.1.7 This standard does not p
11、urport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use. (For more specificsafety information see 1
12、.4.)2. Referenced Documents2.1 ASTM Standards:3E94 Guide for Radiographic ExaminationE 543 Specification for Agencies Performing Nondestruc-tive TestingE 545 Test Method for Determining Image Quality in DirectThermal Neutron Radiographic ExaminationE 803 Test Method for Determining the L/D Ratio of
13、Neu-tron Radiography BeamsE 1316 Terminology for Nondestructive ExaminationsE 1496 Test Method for Neutron Radiographic DimensionalMeasurements2.2 ASNT Standard:Recommended Practice SNT-TC-1A for Personnel Qualifi-cation and Certification42.3 ANSI Standard:ANSI/ASNT-CP-189 Standard for Qualification
14、 and Certi-fication of Nondestructive Testing Personnel51These practices are under the jurisdiction of ASTM Committee E07 onNondestructive Testing and are the direct responsibility of Subcommittee E07.05 onRadiology (Neutron) Method.Current edition approved July 1, 2008. Published September 2008. Or
15、iginallyapproved in 1980. Last previous edition approved in 2002 as E 748 02.2The boldface numbers in parentheses refer to the list of references at the end ofthese practices.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For
16、 Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.4Available from the American Society for Nondestructive Testing, 1711 Arlin-gate Lane, P.O. Box 28518, Columbus, OH 43228-0518.5Available from American National Standards Institute (ANSI
17、), 25 W. 43rd St.,4th Floor, New York, NY 10036.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.2.4 AIA Document:NAS-410 Nondestructive Testing Personnel Qualificationand Certification63. Terminology3.1 DefinitionsFor definitions of
18、terms used in thesepractices, see Terminology E 1316, Section H.4. Significance and Use4.1 These practices include types of materials to be exam-ined, neutron radiographic examination techniques, neutronproduction and collimation methods, radiographic film, andconverter screen selection. Within the
19、present state of theneutron radiologic art, these practices are generally applicableto specific material combinations, processes, and techniques.5. Basis of Application5.1 Personnel QualificationNondestructive testing (NDT)personnel shall be qualified in accordance with a nationallyrecognized NDT pe
20、rsonnel qualification practice or standardsuch as ANSI/ASNT-CP-189, SNT-TC-1A, NAS-410,orasimilar document. The practice or standard used and itsapplicable revision shall be specified in the contractual agree-ment between the using parties.5.2 Qualification of Nondestructive AgenciesIf specifiedin t
21、he contractual agreement, NDT agencies shall be qualifiedand evaluated as described in Practice E 543. The applicableedition of Practice E 543 shall be specified in the contractualagreement.5.3 Procedures and TechniquesThe procedures and tech-niques to be used shall be as described in these practice
22、s unlessotherwise specified. Specific techniques may be specified in thecontractual agreement.5.4 Extent of ExaminationThe extent of examination shallbe in accordance with Section 16 unless otherwise specified.5.5 Reporting Criteria/Acceptance CriteriaReporting cri-teria for the examination results
23、shall be in accordance with 1.3unless otherwise specified. Acceptance criteria (for example,for reference radiographs) shall be specified in the contractualagreement.5.6 Reexamination of Repaired/Reworked ItemsReexamination of repaired/reworked items is not addressed inthese practices and, if requir
24、ed, shall be specified in thecontractual agreement.6. Neutron Radiography6.1 The MethodNeutron radiography is basically similarto X radiography in that both techniques employ radiationbeam intensity modulation by an object to image macroscopicobject details. X rays or gamma rays are replaced by neut
25、ronsas the penetrating radiation in a through-transmission exami-nation. Since the absorption characteristics of matter for X raysand neutrons differ drastically, the two techniques in generalserve to complement one another.6.2 FacilitiesThe basic neutron radiography facility con-sists of a source o
26、f fast neutrons, a moderator, a gamma filter,a collimator, a conversion screen, a film image recorder orother imaging system, a cassette, and adequate biologicalshielding and interlock systems. A schematic diagram of arepresentative neutron radiography facility is illustrated in Fig.1.6.3 Thermaliza
27、tionThe process of slowing down neutronsby permitting the neutrons to come to thermal equilibrium withtheir surroundings; see definition of thermal neutrons inTerminology E 1316, Section H.7. Neutron Sources7.1 GeneralThe thermal neutron beam may be obtainedfrom a nuclear reactor, a subcritical asse
28、mbly, a radioactiveneutron source, or an accelerator. Neutron radiography hasbeen achieved successfully with all four sources. In all casesthe initial neutrons generated possess high energies and mustbe reduced in energy (moderated) to be useful for thermalneutron radiography. This may be achieved b
29、y surrounding thesource with light materials such as water, oil, plastic, paraffin,beryllium, or graphite. The preferred moderator will be depen-dent on the constraints dictated by the energy of the primaryneutrons, which will in turn be dictated by neutron beamparameters such as thermal neutron yie
30、ld requirements, cad-mium ratio, and beam gamma ray contamination. The charac-teristics of a particular system for a given application are leftfor the seller and the buyer of the service to decide. Charac-teristics and capabilities of each type of source are referencedin the References section. A ge
31、neral comparison of sources isshown in Table 1.7.2 Nuclear ReactorsNuclear reactors are the preferredthermal neutron source in general, since high neutron fluxes areavailable and exposures can be made in a relatively short timespan. The high neutron intensity makes it possible to provide atightly co
32、llimated beam; therefore, high-resolution radiographscan be produced.7.3 Subcritical AssemblyA subcritical assembly isachieved by the addition of sufficient fissionable materialsurrounding a moderated source of neutrons, usually a radio-isotope source. Although the total thermal neutron yield issmal
33、ler than that of a nuclear reactor, such a system offers theattractions of adequate image quality in a reasonable exposure6Available from Aerospace Industries Association of America, Inc., 1250 EyeSt., NW, Washington, DC 20005FIG. 1 Typical Neutron Radiography Facility with DivergentCollimatorE 748
34、02 (2008)2time, relative ease of licensing, adequate neutron yield for mostindustrial applications, and the possibility of transportableoperation.7.4 Accelerator SourcesAccelerators used for thermalneutron radiography have generally been of the low-voltagetype which utilize the3H(d,n)4He reaction, h
35、igh-energy X-raymachines in which the (x,n) reaction is applied and Van deGraaff and other high-energy accelerators which employ reac-tions such as9Be(d,n)10B. In all cases, the targets aresurrounded by a moderator to reduce the neutrons to thermalenergies. The total neutron yields of such machines
36、can be onthe order of 1012ns1; the thermal neutron flux of such sourcesbefore collimation can be on the order of 109ncm2s1, forexample, the yield from a Van de Graaff accelerator.7.5 Isotopic SourcesMany isotopic sources have beenemployed for neutron radiologic applications. Those that havebeen most
37、 widely utilized are outlined in Table 2. Radioactivesources offer the best possibility for portable operation. How-ever, because of the relatively low neutron yield, the exposuretimes are usually long for a given image quality. The isotopicsource252Cf offers a number of advantages for thermalneutro
38、n radiology, namely, low neutron energy and smallphysical size, both of which lead to efficient neutron modera-tion, and the possibility for high total neutron yields.8. Imaging Methods and Conversion Screens8.1 GeneralNeutrons are nonionizing particulate radia-tion that have little direct effect on
39、 radiographic film. To obtaina neutron radiographic image on film, a conversion screen isnormally employed; upon neutron capture, screens emitprompt and delayed decay products in the form of nuclearradiation or light. In all cases the screen should be placed inintimate contact with the radiographic
40、film in order to obtainsharp images.8.2 Direct MethodIn the direct method, a film is placed onthe source side of the conversion screen (front film) andexposed to the neutron beam together with the conversionscreen. Electron emission upon neutron capture is the mecha-nism by which the film is exposed
41、 in the case of gadoliniumconversion screens. The screen is generally one of the follow-ing types: (1) a free-standing gadolinium metal screen acces-sible to film on both sides; (2) a sapphire-coated, vapor-deposited gadolinium screen on a substrate such as aluminum;or (3) a light-emitting fluoresce
42、nt screen such as gadoliniumoxysulfide or6LiF/ZnS. Exposure of an additional film (with-out object) is often useful to resolve artifacts that may appearin radiographs. Such artifacts could result from screen marks,excess pressure, light leaks, development, or nonuniform film.In the case of light-emi
43、tting conversion screens, it is recom-mended that the spectral response of the light emission bematched as closely as possible to that of the film used foroptimum results. The direct method should be employedwhenever high-resolution radiographs are required, and highbeam contamination of low-energy
44、gamma rays or highlyradioactive objects do not preclude its use.8.3 Indirect MethodThis method makes use of conversionscreens that can be made temporarily radioactive by neutroncapture.The conversion screen is exposed alone to the neutron-imaging beam; the film is not present. Candidate conversionma
45、terials include rhodium, gold, indium, and dysprosium.Indium and dysprosium are recommended with dysprosiumyielding the greater speed and emitting less energetic gammaradiation. It is recommended that the conversion screens beactivated in the neutron beam for a maximum of threehalf-lives. Further ne
46、utron irradiation will result in a negligibleamount of additional induced activity. After irradiation, theconversion screens should be placed in intimate contact with aradiographic film in a vacuum cassette, or other light-tightassembly in which good contact can be maintained between theradiographic
47、 film and radioactive screen. X-ray intensificationscreens may be used to increase the speed of the autoradio-graphic process if desired. For the indirect type of exposure,the material from which the cassette is fabricated is immaterialas there are no neutrons to be scattered in the exposure process
48、.In this case, as in the activation process, there is little to begained for conversion screen-film exposures extending beyondthree half-lives. It is recommended that this method be em-ployed whenever the neutron beam is highly contaminated withgamma rays, which in turn cause film fogging and reduce
49、dcontrast sensitivity, or when highly radioactive objects are tobe radiographed. In short, this method is beam gamma-insensitive.TABLE 1 Comparison of Thermal Neutron SourcesType of Source Typical Radiographic Flux, n/cm2s Radiographic Resolution CharacteristicsNuclear reactor 105to 108excellent stable operation, not portableSubcritical assembly 104to 106good stable operation, portability difficultAccelerator 103to 106medium on-off operation, transportableRadioisotope 101to 104poor to medium stable operation, portability possibleTABLE 2 Radioactive Sou
