1、Designation: E261 10Standard Practice forDetermining Neutron Fluence, Fluence Rate, and Spectra byRadioactivation Techniques1This standard is issued under the fixed designation E261; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision,
2、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 This practice describes procedures for the determinationof neutron fluence rate, fluence, and energy spect
3、ra from theradioactivity that is induced in a detector specimen.1.2 The practice is directed toward the determination ofthese quantities in connection with radiation effects on mate-rials.1.3 For application of these techniques to reactor vesselsurveillance, see also Test Methods E1005.1.4 This stan
4、dard 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.NOTE 1Detailed methods for i
5、ndividual detectors are given in thefollowing ASTM test methods: E262, E263, E264, E265, E266, E343,E393, E481, E523, E526, E704, E705, and E854.2. Referenced Documents2.1 ASTM Standards:2E170 Terminology Relating to Radiation Measurements andDosimetryE181 Test Methods for Detector Calibration andAn
6、alysis ofRadionuclidesE262 Test Method for Determining Thermal Neutron Reac-tion Rates and Thermal Neutron Fluence Rates by Radio-activation TechniquesE263 Test Method for Measuring Fast-Neutron ReactionRates by Radioactivation of IronE264 Test Method for Measuring Fast-Neutron ReactionRates by Radi
7、oactivation of NickelE265 Test Method for Measuring Reaction Rates and Fast-Neutron Fluences by Radioactivation of Sulfur-32E266 Test Method for Measuring Fast-Neutron ReactionRates by Radioactivation of AluminumE343 Test Method for Measuring Reaction Rates by Analy-sis of Molybdenum-99 Radioactivit
8、y From Fission Dosim-eters3E393 Test Method for Measuring Reaction Rates by Analy-sis of Barium-140 From Fission DosimetersE481 Test Method for Measuring Neutron Fluence Rates byRadioactivation of Cobalt and SilverE523 Test Method for Measuring Fast-Neutron ReactionRates by Radioactivation of Copper
9、E526 Test Method for Measuring Fast-Neutron ReactionRates by Radioactivation of TitaniumE693 Practice for Characterizing Neutron Exposures in Ironand LowAlloy Steels in Terms of Displacements PerAtom(DPA), E 706(ID)E704 Test Method for Measuring Reaction Rates by Radio-activation of Uranium-238E705
10、Test Method for Measuring Reaction Rates by Radio-activation of Neptunium-237E722 Practice for Characterizing Neutron Fluence Spectrain Terms of an Equivalent Monoenergetic Neutron Fluencefor Radiation-Hardness Testing of ElectronicsE844 Guide for Sensor Set Design and Irradiation forReactor Surveil
11、lance, E 706(IIC)E854 Test Method for Application and Analysis of SolidState Track Recorder (SSTR) Monitors for Reactor Sur-veillance, E706(IIIB)E944 Guide for Application of Neutron Spectrum Adjust-ment Methods in Reactor Surveillance, E 706 (IIA)E1005 Test Method for Application and Analysis of Ra
12、dio-metric Monitors for Reactor Vessel Surveillance, E706(IIIA)E1018 Guide for Application of ASTM Evaluated CrossSection Data File, Matrix E706 (IIB)E2005 Guide for Benchmark Testing of Reactor Dosimetryin Standard and Reference Neutron Fields2.2 ISO Standard:1This practice is under the jurisdictio
13、n of ASTM Committee E10 on NuclearTechnology and Applications and is the direct responsibility of SubcommitteeE10.05 on Nuclear Radiation Metrology.Current edition approved Jan. 1, 2010. Published May 2010. Originally approvedin 1965 as E261 65 T. Last previous edition approved in 2003 as E261 03. D
14、OI:10.1520/E0261-10.2For referenced ASTM standards, visit the ASTM website, www.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.3Withdrawn. The last approved version of
15、this historical standard is referencedon www.astm.org.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.Guide in the Expression of Uncertainty in Measurement3. Terminology3.1 Descriptions of terms relating to dosimetry are found inTerm
16、inology E170.4. Summary of Practice4.1 A sample containing a known amount of the nuclide tobe activated is placed in the neutron field. The sample isremoved after a measured period of time and the inducedactivity is determined.5. Significance and Use5.1 Transmutation ProcessesThe effect on materials
17、 ofbombardment by neutrons depends on the energy of theneutrons; therefore, it is important that the energy distributionof the neutron fluence, as well as the total fluence, bedetermined.6. Counting Apparatus6.1 A number of instruments are used to determine thedisintegration rate of the radioactive
18、product of the neutron-induced reaction. These include the scintillation counters,ionization chambers, proportional counters, Geiger tubes, andsolid state detectors. Recommendations of counters for particu-lar applications are given in General Methods E181.7. Requirements for Activation-Detector Mat
19、erials7.1 Considerations concerning the suitability of a materialfor use as an activation detector are found in Guide E844.7.2 The amounts of fissionable material needed for fissionthreshold detectors are rather small and the availability of thematerial is limited. Licenses from the U.S. Nuclear Reg
20、ulatoryCommission are required for possession.7.3 A detailed description of procedures for the use offission threshold detectors is given in Test Methods E343,E393, and E854, and Guide E844.8. Irradiation Procedures8.1 The irradiations are carried out in two ways dependingupon whether the instantane
21、ous fluence rate or the fluence isbeing determined. For fluence rate, irradiate the detector for ashort period at sufficiently low power that handling difficultiesand shielding requirements are minimized. Then extrapolatethe resulting fluence rate value to the value anticipated for fullreactor power
22、. This technique is sometimes used for the fluencemapping of reactors (1,2).48.2 The determination of fluence is most often required inexperiments on radiation effects on materials. Irradiate thedetectors for the same duration as the experiment at a positionin the reactor where, as closely as possib
23、le, they will experi-ence the same fluence, or will bracket the fluence of theposition of interest. When feasible, place the detectors in theexperiment capsule. In this case, long-term irradiations areoften required.8.3 It is desirable, but not required, that the neutron detectorbe irradiated during
24、 the entire time period considered and thata measurable part of the activity generated during the initialperiod of irradiation be present in the detector at the end of theirradiation. Therefore, the effective half-life, t81/2= 0.693/l8,ofthe reaction product should not be much less than the totalela
25、psed time from the initial exposure to the final shutdown.8.4 As mentioned in 9.11 and 9.12, the use of cadmium-shielded detectors is convenient in separating contributions tothe measured activity from thermal and epithermal neutrons.Also, cadmium-shielding is helpful in reducing activities dueto im
26、purities and the loss of the activated nuclide by thermal-neutron absorption. The recommended thicknesses of cadmiumis 1 mm. When bare and cadmium-shielded samples are placedin the same vicinity, take care to avoid partial shielding of thebare detectors by the cadmium-shielded ones.9. Calculation9.1
27、 The activity of the sample, A, at the end of the exposureperiod is calculated as follows:A 5lD/1 2 exp 2l tc! exp 2l tw!# (1)where:l = decay constant for the radioactive nuclide,tc= time interval for counting,tw= time elapsed between the end of the irradiation periodand the start of the counting pe
28、riod, andD = number of disintegrations (net number of countscorrected for background, random and true coinci-dence losses, efficiency of the counting system, andfraction of the sample counted) in the interval tc.9.1.1 If, as is often the case, the counting period is shortcompared to the half-life (
29、= 0.693/l) of the radioactive nu-clide, the activity is well approximated as follows:A 5 D/tcexp 2l tw!# (2)9.2 For irradiations at constant fluence rate, the saturationactivity, As, is calculated as follows:As5 A/1 2 exp 2l8ti! (3)where:ti= exposure duration, andl8 = effective decay constant during
30、 the irradiation.NOTE 2The saturation activity corresponds to the number of disinte-grations per foil per unit time for the steady-state condition in which therate of production of the radioactive nuclide is equal to the rate of loss byradioactive decay and transmutation.9.2.1 The effective decay co
31、nstant, which may be a functionof time, is related to the decay constant as follows:l8 5l1*0saE!fE! dE (4)where:sa(E) = neutron absorption cross section for the productnuclide, andf(E) = neutron fluence rate per unit energy.9.2.2 Application of the effective decay constant for irradia-tions under va
32、rying fluence rates is discussed in this sectionand in the detailed methods for individual detectors.9.3 The reaction rate is calculated as follows:4The boldface numbers in parentheses refer to a list of references at the end ofthis standard.E261 102Rs5 Asl8/Nl (5)where:N = number of target nuclei i
33、n the detector at time ofirradiation.9.3.1 The number of target nuclei can often be assumed tobe equal to No, the number prior to irradiation.No5 NAFm/M (6)where:NA= Avogadros number= 6.022 3 1023mole1,F = atom fraction of the target nuclide in the targetelement,m = mass of target element, g, andM =
34、 atomic mass of the target element.9.3.2 Calculations of the isotopic concentration after irra-diation is discussed in 9.6.6 and in the detailed methods forindividual detectors.9.4 The neutron fluence rate, f, is calculated as follows:f5Rs/s (7)where:s = the spectral weighted neutron activation cros
35、s section.9.4.1 Cross sections should be processed from an appropri-ate cross-section library that includes covariance data. GuideE1018 provides information and recommendations on how toselect the cross section library. The International ReactorDosimetry File (IRDF-2002) (3) is one good source for c
36、rosssections. The SNLRML cross section compendium (4) pro-vides a processed fine-group representation of recommendeddosimetry cross sections and covariance matrices.9.4.2 If spectral-averaged cross-section or spectrum data arenot available, one of the alternative procedures discussed in9.10 to 9.13
37、may be used to calculate an approximate neutronfluence rate from the saturation activity.9.5 The neutron fluence, F, is related to the time varyingdifferential neutron fluence rate f(E,t) by the following expres-sion:F5*0*t1t2f E,t! dt dE (8)where:t2t1= duration of the irradiation period.9.5.1 Long
38、irradiations usually involve operation at variouspower levels, and changes in isotopic content of the system;under such conditions f(E, t) can show large variations withtime.9.5.2 It is usual to assume, however, that the neutron fluencerate is directly proportional to reactor power; under thesecondi
39、tions, the fluence can be well approximated by:F5SfPD (i 5 1nPiti(9)where:f/P = average value of the neutron fluence rate, f,atareference power level, P,ti= duration of the ithoperating period during which thereactor operated at approximately constant power,andPi= reactor power level during that ope
40、rating period.9.5.2.1 Alternate methods include measuring the powergeneration rate in a fraction of the reactor volume adjacent tothe volume of interest.9.6 Transmutation Processes:9.6.1 The neutron fluence rate spectrum, f(E), can bedetermined by computer calculations using neutron transportcodes,
41、and adjustment techniques using radioactivation datafrom multiple foil irradiations.9.6.2 The reaction rate is related to the fluence rate by thefollowing equation:Rs5*0sE!f E!dE (10)where:s(E) = activation cross section at energy E, andf(E) = differential neutron fluence rate, that is the fluencepe
42、r unit energy per unit time for neutrons withenergies between E and E +dE.9.6.3 The number of nuclei, Np, of a radioactive productnuclide is related to the reaction rate by the following equation:dNp/dt 5 NRs2 Npl8 (11)9.6.4 Solution of Eq 11, for the case where Rsand N areconstant, yields the follo
43、wing expression for the activity of afoil:A 5 Npl5l/l8! NRs1 2 exp 2l8t! (12)9.6.5 The saturation activity of a foil is defined as theactivity when dNp/dt = 0; thus Eq 11 yields the followingrelationship for the saturation activity:As5 l/l8! NRs(13)9.6.6 The isotopic content of the target nuclide ma
44、y bereduced during the irradiation by more than one transmutationprocess and it may be increased by transmutation of othernuclides so that the rate of change of the number of targetnuclei with time is described by a number of terms:dN/dt 52N Rs1(t 5 1nRi! 1(j 5 1mNjRj(14)where:i = discrete transmuta
45、tion path for removal of the targetisotope, andj = discrete transmutation reaction whereby the target iso-tope is produced from isotope Njand each of the RiandRjterms could be calculated from equations similar toEq 10, using the appropriate cross sections.9.6.6.1 The Rsterm may predominate and, if R
46、sis constant,Eq 14 can be solved as N = Noexp ( Rst). The change in thetarget composition may be negligible and N may, in that case,be approximated by No.9.6.7 During irradiation, the effective decay rate is increasedby transmutations of the product isotope (see Eq 4).9.7 Long Term Irradiations:E261
47、 1039.7.1 Long irradiations for materials testing programs andreactor pressure vessel surveillance are common. Long irradia-tions usually involve operation at various power levels, includ-ing extended zero-power periods; thus, appropriate correctionsmust be made for depletion of the target nuclide,
48、decay andburnout of the radioactive nuclide, and variations in neutronfluence rate. Multiple irradiations and nuclide burnup must alsobe considered in short-irradiation calculations where reaction-product half-lives are relatively short and nuclide cross sec-tions are high.9.7.2 The total irradiatio
49、n period can be divided into acontinuous series of periods during each of which f(E)isessentially constant. Then the activity generated during the ithirradiation period is:Ai5 lNiRs/l8!i#1 2 exp 2l8iti! (15)where:Ni= number of target atoms during the ithperiod, andti= duration of the ithperiod.9.7.2.1 The activity remaining from the ithperiod at the endof the nthperiod can be calculated as the following equation:An!i5 Aiexp 2(j 5 i 1 1nl8jtj! (16)9.7.2.2 The total activity of the foil at the end of theirradiation duration is thus the sum
