1、Designation: E262 13E262 17Standard 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 histheir 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 p
5、ure gold, 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-neut
6、ron fluence 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 Metho
7、d E481 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 equivalen
8、t 2200 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
9、significant 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 m
10、aterial.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 safety, health and healthenvironmental practices and determine theapplicability of regulatory limita
11、tions prior to use.1.7 This international standard was developed in accordance with internationally recognized principles on standardizationestablished in the Decision on Principles for the Development of International Standards, Guides and Recommendations issuedby the World Trade Organization Techn
12、ical Barriers to Trade (TBT) Committee.2. Referenced 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 Radionuclides1 This
13、method is under the jurisdiction of ASTM Committee E10 on Nuclear Technology and Applicationsand is the direct responsibility of Subcommittee E10.05 onNuclear Radiation Metrology.Current edition approved Jan. 1, 2013Aug. 1, 2017. Published February 2013September 2017. Originally approved in 1965. La
14、st previous edition approved in 20082013as E262-08.-13. DOI: 10.1520/E0262-13.10.1520/E0262-17.2 For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandardsvolume information, refer to the standards Docume
15、nt 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 changes have been made to the previous version. Becauseit may not be technically possible to adequately depict all changes accurately, ASTM rec
16、ommends 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 official document.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1E261 Practic
17、e for Determining Neutron Fluence, Fluence Rate, and Spectra by Radioactivation TechniquesE481 Test Method for Measuring Neutron Fluence Rates by Radioactivation of Cobalt and Silver3. Terminology3.1 cadmium ratiosee Terminology E170.3.2 Calibration Techniques:3.2.1 radiometricthe radiometric techni
18、que uses foil properties, decay properties of the activation 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 fo
19、il to a Standard or Secondary Standard field.3.2.2 standard comparisonthe standard comparison technique compares activity from a foil irradiated in a standard oforreference field to the activity from a foil irradiated in the unknown field to derive the neutron fluence rate.3.2.3 secondary standard c
20、omparisonthe secondary standard comparison technique is the 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 deca
21、y in transit from a Standardstandard source.3.2.3.1 DiscussionThe 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 f
22、acility for counting,and short enough to allow reuse of the foil. 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 determ
23、ine the properties of a neutron field. Typical foil shapesare thin 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 “radiom
24、etric monitors.”3.5 Maxwell-Boltzmann distributionthe Maxwell-Boltzman 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=piSm
25、2kTD32v2e2mv22kT!dvwhere:nth = the number of thermal neutrons per volume,m = the neutron mass (931 MeV),k = Boltzmanns constant (8.617 105 ev K1,T = the neutron temperature,v and E = the neutron velocity and energy, respectively.3.6 thermal neutron fluence rate (th)*0 vnv!dvwhere:v = the neutron vel
26、ocity and n(v) is the thermal neutron density as a function of velocity.TABLE 1 Useful Neutron 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 withaluminum
27、103 to 1010Cobalt pure or alloyed withaluminum1014 to 1020E262 1723.7 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 in
28、 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,which
29、 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.3.7 Thermal neutron fluence rate conventions:3.7.1 Stoughton and Halperin conventionthe neutron spectrum is separated into a t
30、hermal part and a 1/E part. The 2200 m/sneutron fluence rate, 0, is the hypothetical neutron fluence rate 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 conv
31、ention0 is the hypothetical neutron fluence rate in which all the neutrons have a velocity of 2200 m/s,which 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.3.7.3 Hogdahl conv
32、entionthe Hogdahl convention is similar to the Stoughton and Halperin convention, but separates out thesubcadmium fluence as a separate entitity, sc. See Practice E261 for further discussion.3.8 thermal neutronsSee Terminology E170.3.9 neutron temperature, Tan adjustable parameter used to give the b
33、est fit of a 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 c
34、ross sectionsee 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 techni
35、ques,neutron-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) foil) to
36、 aid the experimenter when he isthey are in doubt of histheir ability to perform the radiometrictechnique with sufficient 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 acti
37、vates by an (n, ) reaction, 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 rad
38、iation-detecting instrument. The neutroninduced neutron-induced reaction rate of the foils is computed from the counting data, and the ratio of the known neutron fluencerate to the computed reaction rate is determined. For any given foil, neutron energy spectrum, and counting set-up, this ratio is a
39、constant. Other foils from the identical set can now be exposed to an unknown neutron field. The magnitude of the fluence ratein the unknown field can be obtained by comparing the reaction rates as determined from the counting data from the unknown andreference field, with proper corrections to acco
40、unt for spectral differences between the two 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 conventionsar
41、e the Wescott convention (followed in Test 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
42、1/v Cross SectionsIt is not possible using 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.3 The boldface numbers in parentheses refer to the li
43、st of references appended to this method.E262 173For most purposes, however, the information required is only that needed to make calculations of activation and other reaction ratesfor various materials exposed to the neutron field. For reactions in which the cross section varies inversely as the ne
44、utron speed(1/v cross sections) the reaction rates are proportional to the total neutron density and do not depend on the spectrum shape. Manyradioactivation detectors have reaction cross sections in the thermal energy range which approximate to 1/v cross sections (1/vdetectors). Departures from the
45、 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 (formerly called “flux convention”) is to describe a neutron field in terms of afew parameters that can be conveniently used to calculate reaction rates. The bes
46、t known fluence rate conventions relating tothermal neutron fields are the Westcott convention (2) and the Stoughton and Halperin convention (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, equalt
47、o 2200 m/s which is the most probable speed of Maxwellian thermal neutrons when the characteristic temperature is 293.59K.In 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 conv
48、ention, the conventional fluence rate is 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
49、 proportional to the reaction rate per atom in a 1/v detector exposed to thermal neutrons:Rs!0 5nth0v0 500 (2)R0 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 5nthg0v0 5g00 (3)R0 5nthg0v0 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.
