1、Designation: E262 08E262 13Standard Test Method forDetermining Thermal Neutron Reaction Rates and ThermalNeutron Fluence Rates by Radioactivation Techniques1This standard is issued under the fixed designation E262; the number immediately following the designation indicates the year oforiginal adopti
2、on or, 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 general procedure for deter
3、mining an unknown thermal-neutron fluence rateby neutron activation techniques. It is not practicable to describe completely a technique applicable to the large number ofexperimental situations that require the measurement of a thermal-neutron fluence rate. Therefore, this method is presented so tha
4、tthe user may adapt to his particular situation the fundamental procedures of the following techniques.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 g
5、old, or gold-aluminum alloy, and1.1.3 Secondary standard comparison techniques using pure indium, indium-aluminum alloy, pure dysprosium, or dysprosium-aluminum alloy.1.2 The techniques presented are limited to measurements at room temperatures. However, special problems when makingthermal-neutron f
6、luence rate measurements in high-temperature environments are discussed in 9.2. For those circumstances wherethe use of cadmium as a thermal shield is undesirable because of potential spectrum perturbations or of temperatures above themelting point of cadmium, the method described in Test Method E48
7、1 can be used in some cases.Alternatively, gadolinium filtersmay be used instead of cadmium. For high temperature applications in which aluminum alloys are unsuitable, other alloys suchas cobalt-nickel or cobalt-vanadium have been used.1.3 This test method may be used to determine the equivalent 220
8、0 m/s fluence rate. The accurate determination of the actualthermal neutron fluence rate requires knowledge of the neutron temperature, and determination of the neutron temperature is notwithin the scope of the standard.1.4 The techniques presented are suitable only for neutron fields having a signi
9、ficant thermal neutron component, in whichmoderating materials are present, and for which the average scattering cross section is large compared to the average absorptioncross section in the thermal neutron energy range.1.5 Table 1 indicates the useful neutron-fluence ranges for each detector materi
10、al.1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibilityof the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatorylimitations prior to use.2. Referenced
11、Documents2.1 ASTM Standards:2E170 Terminology Relating to Radiation Measurements and DosimetryE177 Practice for Use of the Terms Precision and Bias in ASTM Test MethodsE181 Test Methods for Detector Calibration and Analysis of RadionuclidesE261 Practice for Determining Neutron Fluence, Fluence Rate,
12、 and Spectra by Radioactivation TechniquesE481 Test Method for Measuring Neutron Fluence Rates by Radioactivation of Cobalt and Silver1 This method is under the jurisdiction of ASTM Committee E10 on Nuclear Technology and Applicationsand is the direct responsibility of Subcommittee E10.05 onNuclear
13、Radiation Metrology.Current edition approved Nov. 1, 2008Jan. 1, 2013. Published March 2009February 2013. Originally approved in 1965. Last previous edition approved in 20032008 asE262 03.E262-08. DOI: 10.1520/E0262-08.10.1520/E0262-13.2 For referencedASTM standards, visit theASTM website, www.astm.
14、org, or contactASTM Customer Service at serviceastm.org. For Annual Book of ASTM Standardsvolume information, refer to the standards Document Summary page on the ASTM website.This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what chan
15、ges have been made to the previous version. Becauseit 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 versionof the standard as published by ASTM is to be considered the offic
16、ial document.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13. Terminology3.1 cadmium ratiosee Terminology E170.3.2 Calibration Techniques:3.2.1 radiometricthe radiometric technique uses foil properties, decay properties of the activ
17、ation product, the detectorefficiency, and cross section to derive the neutron fluence rate. When beta counting is used, it becomes problematic to determinethe absolute detector efficiency, and calibration is usually performed by exposing the foil to a Standard or Secondary Standard field.3.2.2 stan
18、dard comparisonthe standard comparison technique compares activity from a foil irradiated in a standard ofreference field to the activity from a foil irradiated in the unknown field to derive the neutron fluence rate.3.2.3 secondary standard comparisonthe secondary standard comparison technique is t
19、he same as the standard comparisontechnique, except that the reference field is not a well-calibrated national reference, and is usually local to the facility. This issometimes done because a foil with a short half-life undergoes too much decay in transit from a Standard source.3.2.3.1 DiscussionThe
20、 standard comparison technique is the most accurate. Among the foils discussed in this standard, only gold has a suitablehalf-life for standard counting: long enough to allow transport of the foil from the standards laboratory to the facility for counting,and short enough to allow reuse of the foil.
21、 One might consider moving the radiation detector to the national standard location toaccommodate a short half-life.3.3 equivalent 2200 m/s fluencesee Terminology E170.3.4 foilmaterial whose induced radioactivity is used to help determine the properties of a neutron field. Typical foil shapesare thi
22、n discs or rectangles, but wire segments are another common shape. In this document, all activation materials of every shapewill be called “foils” for the sake of brevity. Foils are also often called “radiometric dosimeters” or “radiometric monitors.”3.5 Maxwell-Boltzmann distributionthe Maxwell-Bol
23、tzman distribution is a probability distribution which describes theenergy or velocity distribution of particles in equilibrium at a given temperature. For neutrons, this is given by:nE!dE5nth 2=piE1/2kT!3/2 e 2E/kTdEornv!dv5nth 4=pi Sm2kTnD32v2e2mv22kTn!dvnv!dv5nth 4=piSm2kT D32v2e2mv22kT!dvwhere:n
24、th = the number of thermal neutrons per volume,m = the neutron mass (931 MeV),k = Boltzmanns constant (8.617 105 ev K1,Tn = the neutron temperature,T = the neutron temperature,v and E = the neutron velocity and energy, respectively.3.6 thermal neutron fluence rate (th)*0 vnv!dvwhere:TABLE 1 Useful N
25、eutron Fluence Ranges of Foil MaterialFoil Material Form Useful Range(neutrons/cm2)Indium pure or alloyed withaluminum103 to 1012Gold pure or alloyed withaluminum107 to 1014Dysprosium pure or alloyed withaluminum103 to 1010Cobalt pure or alloyed withaluminum1014 to 1020E262 132v = the neutron veloci
26、ty and n(v) is the thermal neutron density as a function of velocity.3.7 Thermal neutron fluence rate conventions:3.7.1 Stoughton and Halperin conventionthe neutron spectrum is separated into a thermal part and a 1/E part. The 2200 m/sneutron fluence rate, 0, is the hypothetical neutron fluence rate
27、 in which all the thermal neutrons have a velocity of 2200 m/s.The 1/E part of the spectrum is not included. The Stoughton and Halperin convention is followed in this standard.3.7.2 Westcott convention0 is the hypothetical neutron fluence rate in which all the neutrons have a velocity of 2200 m/s,wh
28、ich gives the same activation as the total neutron fluence incident on a 1/v detector.3.7.2.1 DiscussionSee Theory section and Precision and Bias section for further discussion.3.8 thermal neutronsSee Terminology E170.3.9 neutron temperature, Tnan adjustable parameter used to give the best fit of a
29、calculated or measured thermal neutron speeddistribution to the Maxwell-Boltzmann distribution. Because of increasing absorption for lower energy neutrons, the neutrontemperature is usually higher than the temperature of the moderating materials in the system of interest.3.10 2200 m/s cross sections
30、ee Terminology E170.4. Significance and Use4.1 This test method can be extended to use any material that has the necessary nuclear and activation properties that suit theexperimenters particular situation. No attempt has been made to fully describe the myriad problems of counting techniques,neutron-
31、fluence depression, and thick-foil self-shielding. It is assumed that the experimenter will refer to existing literature on thesesubjects. This test method does offer a referee technique (the standard gold foil irradiation at National Institute of Standards andTechnology (NIST)(NIST) to aid the expe
32、rimenter when he is in doubt of his ability to perform the radiometric technique withsufficient accuracy.4.2 The standard comparison technique uses a set of foils that are as nearly identical as possible in shape and mass. The foilsare fabricated from any material that activates by an (n, ) reaction
33、, preferably having a cross section approximately inverselyproportional to neutron speed in the thermal energy range. Some of the foils are irradiated in a known neutron field (at NIST) orother standards laboratory). The foils are counted in a fixed geometry on a stable radiation-detecting instrumen
34、t. The neutroninduced reaction rate of the foils is computed from the counting data, and the ratio of the known neutron fluence rate to thecomputed reaction rate is determined. For any given foil, neutron energy spectrum, and counting set-up, this ratio is a constant.Other foils from the identical s
35、et can now be exposed to an unknown neutron field. The magnitude of the fluence rate in theunknown field can be obtained by comparing the reaction rates as determined from the counting data from the unknown andreference field, with proper corrections to account for spectral differences between the t
36、wo fields (see Section 5). One importantfeature of this technique is that it eliminates the need for knowing the detector efficiency.4.3 This test method follows the Stoughton and Halperin convention for reporting thermal neutron fluence. Other conventionsare the Wescott convention (followed in Test
37、 Method E481) and the Hogdahl convention. Practice E261 explains the threeconventions and gives conversion formulae relating values determined by the different conventions. Reference (1)3 discusses thethree thermal-neutron conventions in detail.5. Theory5.1 1/v Cross SectionsIt is not possible using
38、 radioactivation techniques to determine the true thermal neutron fluence ratewithout making some assumptions about the spectral shapes of both the thermal and epithermal components of the neutron density.For most purposes, however, the information required is only that needed to make calculations o
39、f activation and other reaction ratesfor various materials exposed to the neutron field. For reactions in which the cross section varies inversely as the neutron speed(1/v cross sections) the reaction rates are proportional to the total neutron density and do not depend on the spectrum shape. Manyra
40、dioactivation detectors have reaction cross sections in the thermal energy range which approximate to 1/v cross sections (1/vdetectors). Departures from the 1/v shape can be accounted for by means of correction factors.5.2 Fluence Rate Conventions:5.2.1 The purpose of a fluence rate convention (form
41、erly called “flux convention”) is to describe a neutron field in terms of afew parameters that can be conveniently used to calculate reaction rates. The best known fluence rate conventions relating tothermal neutron fields are the Westcott convention (12) and the Stoughton and Halperin convention (2
42、3). Both make use of theconcept of an equivalent 2200 m/s fluence rate, that is equal to the product of the neutron density and the standard speed, v0, equalto 2200 m/s which is the most probable speed of Maxwellian thermal neutrons when the characteristic temperature is 293.59K.3 The boldface numbe
43、rs in parentheses refer to the list of references appended to this method.E262 133In the Westcott convention, it is the total neutron density (thermal plus epithermal) which is multiplied by v0 to form the “Westcottflux”, but in the Stoughton and Halperin convention, the conventional fluence rate is
44、 the product of the Maxwellian thermal neutrondensity and v0. The latter convention is the one followed in this method:0 5nthv0 (1)where 0 is the equivalent (or conventional) 2200 m/s thermal fluence rate and nth represents the thermal neutron density, whichis proportional to the reaction rate per a
45、tom in a 1/v detector exposed to thermal neutrons:Rs!0 5nth0v0 500 (2)5.2.2 (Rs)0 represents only that part of the reaction rate that is induced by thermal neutrons, which have the Maxwellianspectrum shape. 0 is the 2200 m/s cross section. For a non-1/v detector Eq 2 needs to be replaced by:Rs!0 5nt
46、hg0v0 5g00 (3)where g is a correction factor that accounts for the departures from the ideal 1/v detector cross section in the thermal energyrange. The same factor appears in the Westcott convention Ref (12), and is usually referred to as the Westcott g factor. g dependson the neutron temperature, T
47、n, and is defined as follows:g 5 1v00*0 4pi1/2 Svv0D3ST0TnD 32 expF2Svv0D2 ST0TnDG v!dv (4)g 5 1v00*0 4pi1/2 Svv0D3ST0T D32 expF2Svv0D2ST0T DGv!dv (4)5.2.3 If the thermal neutron spectrum truly follows the Maxwellian distribution and if the neutron temperature is known, it ispossible to calculate th
48、e true thermal neutron fluence rate by multiplying the conventional (equivalent 2200 m/s) thermal fluencerate by the factor:vv0 5S4TnpiT0D12(5)vv0 5S4TnpiT0D12(5)where v is the Maxwellian mean speed for neutron temperature T, and T0 is the standard temperature of 293.4K.This conversionis most often
49、unnecessary and is usually not made because the temperature T may be unknown. Naturally, it is essential whenreporting results to be absolutely clear whether the true thermal fluence rate or the equivalent 2200 m/s thermal fluence rate or theequivalent 2200 m/s total (Westcott) fluence rate is used. If the true thermal fluence rate is used, then its value must be accompaniedby the associated temperature value.5.3 Epithermal NeutronsIn order to determine the effects of epithermal neutrons, that are invariably pre
