1、Designation: E 748 02Standard 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 revision. A number in p
2、arentheses indicates the year of last reapproval. Asuperscript epsilon (e) 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 herein. They are
3、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).Statements concerning pref
4、erred 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 of Application).
5、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 consistency of all
6、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 avail-able. Emphas
7、is 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.1.4 Safety Pract
8、icesGeneral 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 onRadiation Prot
9、ection 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 important aspects of
10、neutron radiography such astechnique, files, viewing of radiographs, storage of radio-graphs, film processing, and record keeping, refer to GuideE 94. (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 purport to addres
11、s 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 1.4.)2. Reference
12、d Documents2.1 ASTM Standards:E 94 Guide for Radiographic Examination3E 543 Practice for Agencies Performing NondestructiveTesting3E 545 Test Method for Determining Image Quality in DirectThermal Neutron Radiographic Examination3E 803 Test Method for Determining the L/D Ratio ofNeutron Radiography B
13、eams3E 1316 Terminology for Nondestructive Examinations3E 1496 Test Method for Neutron Radiographic DimensionalMeasurements32.2 ASNT Standard:Recommended Practice SNT-TC-1A for Personnel Qualifi-cation and Certification42.3 ANSI Standard:ANSI/ASNT-CP-189 Standard for Qualification and Certi-fication
14、 of Nondestructive Testing Personnel52.4 AIA Document:NAS-410 Nondestructive Testing Personnel Qualificationand Certification61These practices are under the jurisdiction of ASTM Committee E07 onNondestructive Testing and are the direct responsibility of Subcommittee E07.05 onRadiology (Neutron) Meth
15、od.Current edition approved July 10, 2002. Published September 2002. Originallypublished as E 748 80. Last previous edition E 748 95.2The boldface numbers in parentheses refer to the list of references at the end ofthese practices.3Annual Book of ASTM Standards, Vol 03.03.4Available from the America
16、n Society for Nondestructive Testing, 1711 Arlin-gate Lane, P.O. Box 28518, Columbus, OH 43228-0518.5Available from American National Standards Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY 10036.6Available from Aerospace Industries Association of America, Inc., 1250 EyeSt., NW, Washingto
17、n, DC 200051Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3. Terminology3.1 DefinitionsFor definitions of terms used in thesepractices, see Terminology E 1316, Section H.4. Significance and Use4.1 These practices include types of ma
18、terials to be exam-ined, neutron radiographic examination techniques, neutronproduction and collimation methods, radiographic film, andconverter screen selection. Within the present state of theneutron radiologic art, these practices are generally applicableto specific material combinations, process
19、es, and techniques.5. Basis of Application5.1 Personnel QualificationNondestructive testing (NDT)personnel shall be qualified in accordance with a nationallyrecognized NDT personnel qualification practice or standardsuch as ANSI/ASNT-CP-189, SNT-TC-1A, NAS-410, or asimilar document. The practice or
20、standard used and itsapplicable revision shall be specified in the contractual agree-ment between the using parties.5.2 Qualification of Nondestructive AgenciesIf specifiedin the contractual agreement, NDT agencies shall be qualifiedand evaluated as described in Practice E 543. The applicableedition
21、 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 practices unlessotherwise specified. Specific techniques may be specified in thecontractual agreement.5.4 Extent of ExaminationThe e
22、xtent of examination shallbe in accordance with Section 16 unless otherwise specified.5.5 Reporting Criteria/Acceptance CriteriaReporting cri-teria for the examination results shall be in accordance with 1.3unless otherwise specified. Acceptance criteria (for example,for reference radiographs) shall
23、 be specified in the contractualagreement.5.6 Reexamination of Repaired/Reworked ItemsReexamination of repaired/reworked items is not addressed inthese practices and, if required, shall be specified in thecontractual agreement.6. Neutron Radiography6.1 The MethodNeutron radiography is basically simi
24、larto 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 neutronsas the penetrating radiation in a through-transmission exami-nation. Since the absorption characteristics of matter for
25、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 of fast neutrons, a moderator, a gamma filter,a collimator, a conversion screen, a film image recorder orother imaging system
26、, a cassette, and adequate biologicalshielding and interlock systems. A schematic diagram of arepresentative neutron radiography facility is illustrated in Fig.1.6.3 ThermalizationThe process of slowing down neutronsby permitting the neutrons to come to thermal equilibrium withtheir surroundings; se
27、e 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 assembly, a radioactiveneutron source, or an accelerator. Neutron radiography hasbeen achieved successfully with all four source
28、s. 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 by surrounding thesource with light materials such as water, oil, plastic, paraffin,beryllium, or graphite. The preferred mod
29、erator 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 yield requirements, cad-mium ratio, and beam gamma ray contamination. The charac-teristics of a particular system for a given a
30、pplication 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 general comparison of sources isshown in Table 1.7.2 Nuclear ReactorsNuclear reactors are the preferredthermal neutron source
31、in general, since high neutron fluxes areavailable and exposures can be made in a relatively short timeFIG. 1 Typical Neutron Radiography Facility with DivergentCollimatorTABLE 1 Comparison of Thermal Neutron SourcesType of Source Typical Radiographic Flux, n/cm2s Radiographic Resolution Characteris
32、ticsNuclear 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 possibleE748022span. The high n
33、eutron intensity makes it possible to provide atightly collimated 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-isotop
34、e source. Although the total thermal neutron yield issmaller than that of a nuclear reactor, such a system offers theattractions of adequate image quality in a reasonable exposuretime, relative ease of licensing, adequate neutron yield for mostindustrial applications, and the possibility of transpor
35、tableoperation.7.4 Accelerator SourcesAccelerators used for thermalneutron radiography have generally been of the low-voltagetype which utilize the3H(d,n)4He reaction, high-energy X-raymachines in which the (x,n) reaction is applied and Van deGraaff and other high-energy accelerators which employ re
36、ac-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 can be onthe order of 1012ns1; the thermal neutron flux of such sourcesbefore collimation can be on the order of 109ncm2s1, forexam
37、ple, the yield from a Van de Graaff accelerator.7.5 Isotopic SourcesMany isotopic sources have beenemployed for neutron radiologic applications. Those that havebeen most widely utilized are outlined in Table 2. Radioactivesources offer the best possibility for portable operation. How-ever, because o
38、f the relatively low neutron yield, the exposuretimes are usually long for a given image quality. The isotopicsource252Cf offers a number of advantages for thermalneutron radiology, namely, low neutron energy and smallphysical size, both of which lead to efficient neutron modera-tion, and the possib
39、ility for high total neutron yields.8. Imaging Methods and Conversion Screens8.1 GeneralNeutrons are nonionizing particulate radia-tion that have little direct effect on radiographic film. To obtaina neutron radiographic image on film, a conversion screen isnormally employed; upon neutron capture, s
40、creens 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 film in order to obtainsharp images.8.2 Direct MethodIn the direct method, a film is placed onthe source side of the conversion scr
41、een (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 in the case of gadoliniumconversion screens. The screen is generally one of the follow-ing types: (1) a free-standing gadolinium m
42、etal 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 fluorescent screen such as gadoliniumoxysulfide or6LiF/ZnS. Exposure of an additional film (with-out object) is often useful to resolve arti
43、facts that may appearin radiographs. Such artifacts could result from screen marks,excess pressure, light leaks, development, or nonuniform film.In the case of light-emitting conversion screens, it is recom-mended that the spectral response of the light emission bematched as closely as possible to t
44、hat of the film used foroptimum results. The direct method should be employedwhenever high-resolution radiographs are required, and highbeam contamination of low-energy gamma rays or highlyradioactive objects do not preclude its use.8.3 Indirect MethodThis method makes use of conversionscreens that
45、can be made temporarily radioactive by neutroncapture. The conversion screen is exposed alone to the neutron-imaging beam; the film is not present. Candidate conversionmaterials include rhodium, gold, indium, and dysprosium.Indium and dysprosium are recommended with dysprosiumyielding the greater sp
46、eed and emitting less energetic gammaradiation. It is recommended that the conversion screens beactivated in the neutron beam for a maximum of threehalf-lives. Further neutron irradiation will result in a negligibleamount of additional induced activity. After irradiation, theconversion screens shoul
47、d 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 film and radioactive screen. X-ray intensificationscreens may be used to increase the speed of the autoradio-graphic process if d
48、esired. 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.In this case, as in the activation process, there is little to begained for conversion screen-film exposures extending beyondthre
49、e 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 reducedcontrast sensitivity, or when highly radioactive objects are tobe radiographed. In short, this method is beam gamma-insensitive.TABLE 2 Radioactive Sources Employed for Thermal Neutron RadiographySource Type Half-Life CommentsA124Sb-Be (g,n) 60 days short half-life and high g-background, low neutron energy is advantage formoderation, high yield source210Po-Be (a,n) 138 days short half-l
