1、Designation: E 262 08Standard Test Method forDetermining Thermal Neutron Reaction Rates and ThermalNeutron Fluence Rates by Radioactivation Techniques1This standard is issued under the fixed designation E 262; the number immediately following the designation indicates the year oforiginal adoption or
2、, in the case of revision, the year of last revision. 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 The purpose of this test method is to define a generalprocedure for determining
3、 an unknown thermal-neutron flu-ence rate by neutron activation techniques. It is not practicableto describe completely a technique applicable to the largenumber of experimental situations that require the measure-ment of a thermal-neutron fluence rate. Therefore, this methodis presented so that the
4、 user may adapt to his particularsituation the fundamental procedures of the following tech-niques.1.1.1 Radiometric counting technique using pure cobalt,pure gold, pure indium, cobalt-aluminum, alloy, gold-aluminum alloy, or indium-aluminum alloy.1.1.2 Standard comparison technique using pure gold,
5、 orgold-aluminum alloy, and1.1.3 Secondary standard comparison techniques using pureindium, indium-aluminum alloy, pure dysprosium, ordysprosium-aluminum alloy.1.2 The techniques presented are limited to measurements atroom temperatures. However, special problems when makingthermal-neutron fluence r
6、ate measurements in high-temperature environments are discussed in 9.2. For thosecircumstances where the use of cadmium as a thermal shield isundesirable because of potential spectrum perturbations or oftemperatures above the melting point of cadmium, the methoddescribed in Test Method E 481 can be
7、used in some cases.Alternatively, gadolinium filters may be used instead of cad-mium. For high temperature applications in which aluminumalloys are unsuitable, other alloys such as cobalt-nickel orcobalt-vanadium have been used.1.3 This test method may be used to determine the equiva-lent 2200 m/s f
8、luence rate. The accurate determination of theactual thermal neutron fluence rate requires knowledge of theneutron temperature, and determination of the neutron tem-perature is not within the scope of the standard.1.4 The techniques presented are suitable only for neutronfields having a significant
9、thermal neutron component, inwhich moderating materials are present, and for which theaverage scattering cross section is large compared to theaverage absorption cross section in the thermal neutron energyrange.1.5 Table 1 indicates the useful neutron-fluence ranges foreach detector material.1.6 Thi
10、s standard does not purport 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.2. Referenced Document
11、s2.1 ASTM Standards:2E 170 Terminology Relating to Radiation Measurementsand DosimetryE 177 Practice for Use of the Terms Precision and Bias inASTM Test MethodsE 181 Test Methods for Detector Calibration and Analysisof RadionuclidesE 261 Practice for Determining Neutron Fluence, FluenceRate, and Spe
12、ctra by Radioactivation TechniquesE 481 Test Method for Measuring Neutron Fluence Ratesby Radioactivation of Cobalt and Silver3. Terminology3.1 cadium ratiosee Terminology E 170.3.2 Calibration Techniques3.2.1 radiometricthe radiometric technique uses foilproperties, decay properties of the activati
13、on product, thedetector efficiency, and cross section to derive the neutronfluence rate. When beta counting is used, it becomes problem-atic to determine the absolute detector efficiency, and calibra-tion is usually performed by exposing the foil to a Standard orSecondary Standard field.3.2.2 standa
14、rd comparisonthe standard comparison tech-nique compares activity from a foil irradiated in a standard ofreference field to the activity from a foil irradiated in theunknown field to derive the neutron fluence rate.1This method is under the jurisdiction of ASTM Committee E10 on NuclearTechnology and
15、 Applications and is the direct responsibility of SubcommitteeE10.05 on Nuclear Radiation Metrology.Current edition approved Nov. 1, 2008. Published March 2009. Originallyapproved in 1965. Last previous edition approved in 2003 as E 262 03.2For referenced ASTM standards, visit the ASTM website, www.
16、astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.2.3
17、secondary standard comparisonthe secondary stan-dard comparison technique is the same as the standard com-parison technique, except that the reference field is not awell-calibrated national reference, and is usually local to thefacility. This is sometimes done because a foil with a shorthalf-life un
18、dergoes too much decay in transit from a Standardsource.3.2.4 DiscussionThe standard comparison technique isthe most accurate. Among the foils discussed in this standard,only gold has a suitable half-life for standard counting: longenough to allow transport of the foil from the standardslaboratory t
19、o the facility for counting, and short enough toallow reuse of the foil. One might consider moving theradiation detector to the national standard location to accom-modate a short half-life.3.3 equivalent 2200 m/s fluencesee Terminology E 170.3.4 foilmaterial whose induced radioactivity is used tohel
20、p determine the properties of a neutron field. Typical foilshapes are thin discs or rectangles, but wire segments areanother common shape. In this document, all activation mate-rials of every shape will be called “foils” for the sake ofbrevity. Foils are also often called “radiometric dosimeters” or
21、“radiometric monitors.”3.5 Maxwell-Boltzmann distributionthe Maxwell-Boltzman distribution is a probability distribution which de-scribes the energy or velocity distribution of particles inequilibrium at a given temperature. For neutrons, this is givenby:nE!dE 5 nth2=pE1/2kTn!3/2e2E/kTndEornv!dv 5 n
22、th4=pSm2kTnD3/2v2e2mv22kTn!dvwhere:nth= the number of thermal neutrons per volume,m = the neutron mass (931 MeV),k = Boltzmanns constant (8.617 3 105ev K1,Tn= the neutron temperature,v and E = the neutron velocity and energy, respectively.3.6 thermal neutron fluence rate (Fth)*0v nv!dvwhere:v = the
23、neutron velocity and n(v) is the thermal neutrondensity as a function of velocity.3.7 Thermal neutron fluence rate conventions:3.7.1 Stoughton and Halperin conventionthe neutronspectrum is separated into a thermal part and a 1/E part. The2200 m/s neutron fluence rate, F0, is the hypothetical neutron
24、fluence rate in which all the thermal neutrons have a velocityof 2200 m/s. The 1/E part of the spectrum is not included. TheStoughton and Halperin convention is followed in this stan-dard.3.7.2 Westcott conventionF0is the hypothetical neutronfluence rate in which all the neutrons have a velocity of
25、2200m/s, which gives the same activation as the total neutronfluence incident on a 1/v detector.3.7.3 DiscussionSee Theory section and Precision andBias section for further discussion.3.8 thermal neutronssee Terminology E 170.3.9 Tnan adjustable parameter used to give the best fit ofa calculated or
26、measured thermal neutron speed distribution tothe Maxwell-Boltzmann distribution. Because of increasingabsorption for lower energy neutrons, the neutron temperatureis usually higher than the temperature of the moderatingmaterials in the system of interest.3.10 2200 m/s cross sectionsee Terminology E
27、 170.4. Significance and Use4.1 This test method can be extended to use any materialthat has the necessary nuclear and activation properties that suitthe experimenters particular situation. No attempt has beenmade to fully describe the myriad problems of countingtechniques, neutron-fluence depressio
28、n, and thick-foil self-shielding. It is assumed that the experimenter will refer toexisting literature on these subjects. This test method does offera referee technique (the standard gold foil irradiation atNational Institute of Standards and Technology (NIST) to aidthe experimenter when he is in do
29、ubt of his ability to performthe radiometric technique with sufficient accuracy.4.2 The standard comparison technique uses a set of foilsthat are as nearly identical as possible in shape and mass. Thefoils are fabricated from any material that activates by an (n, g)reaction, preferably having a cros
30、s section approximatelyinversely proportional to neutron speed in the thermal energyrange. Some of the foils are irradiated in a known neutron field(at NIST) or other standards laboratory). The foils are countedin a fixed geometry on a stable radiation-detecting instrument.The neutron induced reacti
31、on rate of the foils is computed fromthe counting data, and the ratio of the known neutron fluencerate to the computed reaction rate is determined. For any givenfoil, neutron energy spectrum, and counting set-up, this ratio isa constant. Other foils from the identical set can now beexposed to an unk
32、nown neutron field. The magnitude of thefluence rate in the unknown field can be obtained by comparingthe reaction rates as determined from the counting data fromthe unknown and reference field, with proper corrections toaccount for spectral differences between the two fields (seeSection 5). One imp
33、ortant feature of this technique is that iteliminates the need for knowing the detector efficiency.TABLE 1 Useful Neutron Fluence Ranges of Foil MaterialFoil Material Form Useful Range(neutrons/cm2)Indium pure or alloyed withaluminum103to 1012Gold pure or alloyed withaluminum107to 1014Dysprosium pur
34、e or alloyed withaluminum103to 1010Cobalt pure or alloyed withaluminum1014to 1020E2620825. Theory5.1 1/v Cross SectionsIt is not possible using radioactiva-tion techniques to determine the true thermal neutron fluencerate without making some assumptions about the spectralshapes of both the thermal a
35、nd epithermal components of theneutron density. For most purposes, however, the informationrequired is only that needed to make calculations of activationand other reaction rates for various materials exposed to theneutron field. For reactions in which the cross section variesinversely as the neutro
36、n speed (1/v cross sections) the reactionrates are proportional to the total neutron density and do notdepend on the spectrum shape. Many radioactivation detectorshave reaction cross sections in the thermal energy range whichapproximate to 1/v cross sections (1/v detectors). Departuresfrom the 1/v s
37、hape can be accounted for by means of correctionfactors.5.2 Fluence Rate Conventions:5.2.1 The purpose of a fluence rate convention (formerlycalled “flux convention”) is to describe a neutron field in termsof a few parameters that can be conveniently used to calculatereaction rates. The best known f
38、luence rate conventions relat-ing to thermal neutron fields are the Westcott convention (1)3and the Stoughton and Halperin convention (2). Both make useof the concept of an equivalent 2200 m/s fluence rate, that isequal to the product of the neutron density and the standardspeed, v0, equal to 2200 m
39、/s which is the most probable speedof Maxwellian thermal neutrons when the characteristic tem-perature is 293.59K. In the Westcott convention, it is the totalneutron density (thermal plus epithermal) which is multipliedby v0to form the “Westcott flux”, but in the Stoughton andHalperin convention, th
40、e conventional fluence rate is theproduct of the Maxwellian thermal neutron density and v0. Thelatter convention is the one followed in this method:f05 nthv0(1)where f0is the equivalent 2200 m/s thermal fluence rate andnthrepresents the thermal neutron density, which is propor-tional to the reaction
41、 rate per atom in a 1/v detector exposed tothermal neutrons:Rs!05 nths0v05s0f0(2)5.2.2 (Rs)0represents only that part of the reaction rate thatis induced by thermal neutrons, which have the Maxwellianspectrum shape. s0is the 2200 m/s cross section. For a non-1/vdetector Eq 2 needs to be replaced by:
42、Rs!05 nthgs0v05 gs0f0(3)where g is a correction factor that accounts for the departuresfrom the ideal 1/v detector cross section in the thermal energyrange. The same factor appears in the Westcott convention Ref(1), and is usually referred to as the Westcott g factor. gdepends on the neutron tempera
43、ture, Tn, and is defined asfollows:g 51v0s0*0 4p1/2Svv0D3ST0TnD3/2 expFSvv0D2ST0TnDGsv!dv(4)5.2.3 If the thermal neutron spectrum truly follows theMaxwellian distribution and if the neutron temperature isknown, it is possible to calculate the true thermal neutronfluence rate by multiplying the conve
44、ntional (equivalent 2200m/s) thermal fluence rate by the factor:vv05S4TnpT0D1/2(5)where v is the Maxwellian mean speed for neutron tempera-ture T, and T0is the standard temperature of 293.4K. Thisconversion is most often unnecessary and is usually not madebecause the temperature T may be unknown. Na
45、turally, it isessential when reporting results to be absolutely clear whetherthe true thermal fluence rate or the equivalent 2200 m/s thermalfluence rate or the equivalent 2200 m/s total (Westcott) fluencerate is used. If the true thermal fluence rate is used, then itsvalue must be accompanied by th
46、e associated temperaturevalue.5.3 Epithermal NeutronsIn order to determine the effectsof epithermal neutrons, that are invariably present togetherwith thermal neutrons, cadmium covered foil irradiations aremade. It is important to realize that some epithermal neutronscan have energies below the effe
47、ctive cadmium cut-off energy,Ecd. The lowest energy of epithermal neutrons is usually takento be equal to 5kTn(where k is Boltzmanns constant) that isequal to 0.13 eV for room temperature (293K) neutrons (1),though 4 kTnhas been recommended for some reactors (3).Inorder to correct for these, it is n
48、ecessary to make someassumption about the epithermal neutron spectrum shape, andthe assumption made in Refs 1 and 2 is that the epithermalneutron fluence rate per unit energy is proportional to 1/E:feE! 5fe/E,E$5kTn(6)where feis an epithermal fluence parameter equal to thefluence rate per unit energ
49、y, fe(E), at 1 eV. This assumption isusually adequate for the purpose of correcting thermal neutronfluence rate measurements for epithermal neutrons at energiesbelow the cadmium cut-off. To represent the epithermal fluencemore correctly, however, many authors have shown that the useof a 1/E(1+a)spectrum shape is preferable, where a is anempirical parameter. Refs (4-10).5.4 Resonance Integral:5.4.1 The resonance integral for an ideal dilute detector isdefined as follows:I05*EcdsE!dEE(7)5.4.2 The cadmium cut-off energy is taken to be 0.55 eV
