1、Designation: E481 10Standard Test Method forMeasuring Neutron Fluence Rates by Radioactivation ofCobalt and Silver1This standard is issued under the fixed designation E481; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year o
2、f 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 test method covers a suitable means of obtainingthe thermal neutron fluence rate, or fluence, in well moderated
3、nuclear reactor environments where the use of cadmium, as athermal neutron shield as described in Method E262,isundesirable because of potential spectrum perturbations or oftemperatures above the melting point of cadmium.1.2 This test method describes a means of measuring aWestcott neutron fluence r
4、ate (Note 1) by activation of cobalt-and silver-foil monitors (See Terminology E170). Thereaction59Co(n,g)60Co results in a well-defined gammaemitter having a half-life of 1925.28 days (1).2Thereaction109Ag(n,g)110mAg results in a nuclide with a com-plex decay scheme which is well known and having a
5、 half-lifeof 249.76 days (1). Both cobalt and silver are available eitherin very pure form or alloyed with other metals such asaluminum. A reference source of cobalt in aluminum alloy toserve as a neutron fluence rate monitor wire standard isavailable from the National Institute of Standards and Tec
6、h-nology (NIST) as Standard Reference Material 953.3Thecompeting activities from neutron activation of other isotopesare eliminated, for the most part, by waiting for the short-livedproducts to die out before counting. With suitable techniques,thermal neutron fluence rate in the range from 109cm2s1t
7、o3 3 1015cm2s1can be measured. For this method to beapplicable, the reactor must be well moderated and be wellrepresented by a Maxwellian low-energy distribution and an(1/E) epithermal distribution. These conditions are usually metin positions surrounded by hydrogenous moderator withoutnearby strong
8、ly absorbing materials. Otherwise the true spec-trum must be calculated to obtain effective activation crosssections over all energies.NOTE 1Westcott fluence rate = v0*0nv!dv.1.3 The values stated in SI units are to be regarded as thestandard.1.4 This standard does not purport to address all of thes
9、afety 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 Documents2.1 ASTM Standards:4E170 Terminology Relating to
10、Radiation Measurements andDosimetryE177 Practice for Use of the Terms Precision and Bias inASTM Test MethodsE181 Test Methods for Detector Calibration andAnalysis ofRadionuclidesE262 Test Method for Determining Thermal Neutron Reac-tion Rates and Thermal Neutron Fluence Rates by Radio-activation Tec
11、hniques3. Significance and Use3.1 The pertinent data for these two reactions are given inTable 1. This test method uses one monitor (cobalt) with anearly 1/v absorption cross-section curve and a second monitor(silver) with a large resonance peak so that its resonanceintegral is large compared to the
12、 thermal cross section. Theequations are based on the Westcott formalism (2, 3) anddetermine a Westcott 2200 m/s neutron fluence rate nv0and theWestcott epithermal index parameter r=T/T0. References 4,5, and 6 contain a general discussion of the two-reaction testmethod. In this test method, the abso
13、lute activities of bothcobalt and silver monitors are determined. This differs from thetest method in the references wherein only one absoluteactivity is determined.3.2 The advantages of this test method are the elimination ofthree difficulties associated with the use of cadmium: (1) theperturbation
14、 of the field by the cadmium; (2) the inexactcadmium cut-off energy; (3) the low melting temperature of1This test method is under the jurisdiction ofASTM Committee E10 on NuclearTechnology and Applications and is the direct responsibility of SubcommitteeE10.05 on Nuclear Radiation Metrology.Current
15、edition approved Jan. 1, 2010. Published May 2010. Originally approvedin 1973. Last previous edition approved in 2003 as E481 03. DOI: 10.1520/E0481-10.2The boldface numbers in parentheses refer to references listed at the end of thistest method.3Standard Reference Material 953 is available from Nat
16、ional Institute ofStandards and Technology, U.S. Dept. of Commerce, Washington, DC 20234.4For 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 Summ
17、ary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.cadmium. In addition, the reactivity changes accompanying therapid insertion and removal of cadmium may prohibit the useof the cadmium-ratio method. However,
18、 the self-shieldingcorrections remain important unless the concentrations ofcobalt and silver are small. Studies indicate that the accuracy ofthe two-reaction method for determination of thermal neutronfluence is comparable to the cadmium-ratio method (14).3.3 The long half-lives of the two monitors
19、 permit thedetermination of fluence for long-term monitoring.4. Apparatus4.1 NaI(Tl) or Germanium Gamma-Ray Spectrometer (us-ing a multichannel analyzer)For the NaI(Tl) technique andthe germanium technique, see Method E181.4.2 Precision Balance.4.3 Digital Computer.5. Materials and Manufacture5.1 Th
20、e two monitors required for this test method arecobalt and silver. Although these two materials are availablecommercially in very pure form, they have been used (15)alloyed with aluminum (#1 % cobalt and #1 % silver) tominimize the self-shielding effect and to permit insertion intoa high thermal-neu
21、tron fluence rate (1015cm2s1) facility (6,16). Typical alloys contain 0.1 % silver or cobalt in aluminum)see 6.1 and 8.1).5.2 The uncertainties and nonuniformity of alloy concentra-tions must be established by one or more different testmethods. These might include chemical and activation analy-sis,
22、or spectrometry. The purity of the aluminum matrix shouldalso be established.5.3 Whenever possible, the alloys should be tested forinterfering impurities by neutron activation.5.4 The method of encapsulating the monitors for irradia-tion depends upon the characteristics of the facility in whichthe m
23、easurements are to be made. The monitors have essen-tially the same chemical characteristics as pure aluminum;therefore, an environment in which aluminum would not beadversely affected would be generally satisfactory for thealloys. However, the low mechanical strength of the monitorsrequires in many
24、 instances that it be encapsulated or shieldedfrom physical disturbances by some type of container. Alumi-num cans or tubing are satisfactory for many cases of interest,but for hostile environments, stainless steel or vanadium maybe preferable. Perturbation due to the presence of the containermust b
25、e accounted for, especially in the case of stainless steel.The container should be constructed in such a manner that itwill not create a significant flux perturbation and that it may beopened easily, especially if the monitors must be removedremotely.6. Procedure6.1 Decide on the size and shape of t
26、he monitors to beirradiated, taking into consideration the size and shape of theirradiation space. The mass and exposure time are parameterswhich can be varied to obtain a desired disintegration rate fora given neutron fluence rate level. To facilitate the convergenceof the two activity equations fo
27、r the fluence rate and theepithermal index in Section 7, the concentration of the alloysshould be chosen so that the ratio of the disintegration rates ison the order of one.TABLE 1 Recommended ConstantsSymbol ParameterCobalt (60Co) Silver (110mAg)ValueAReference ValueAReferencet1/2Half-life 1925.28
28、(14) days (1) 249.76 (4) days (1)A Abundance of parent isotope 100 % (59Co) (1) 48.161 (8) % (109Ag) (1)D Mass excess of residual isotope (scaled toD 12C = 0)(1 amu = 931.494MeV)B61.64904 MeV (1) 87.3424 MeV (1)saAbsorption 2200 m/s cross section for target590Co and109Ag37.233 b 6 0.16 %C,D91.0 b 6
29、1% (7)s02200 m/s cross section for formation of60Co and110mAg 37.233 b 6 0.16 %C,D4.12 b 6 2.54 % (8)S0Correction factor which describes the departure of thecross section from the 1/v law in the epithermalregion1.8059Co(n,g)60CoE18.13 6 4%109Ag(n,g)110mAg17.76109Ag(n,g)110m+110gAg(8)I0Resonance Inte
30、gral 75.421 b 6 0.77 %59Co(n,g)60Co(9)F67.9 b 6 4.5 %109Ag(n,g)110mAg(8)s2Effective absorption cross section for product nuclide(reactor spectrum)2b (10) 82 b (11)GthThermal neutron self-shielding factor Table 3 (12) 14/3R(a(4)G8resResonance neutron self-shielding factor Table 3 (12) Fig. 1Gg Correc
31、tion factor which describes the departure of thecross section from 1/v law in thermal region1.0 (2) See Table 4 (2)AThe numbers in parenthesis following given values is the uncertainty in the last digit(s) of the value; 0.729 (8) means 0.729 6 0.008, 70.8(1) means 70.8 6 0.1.BIsotopic masses may be
32、calculated as A + 931.49432/D, where A is the atomic mass number.CA 2200 m/s cross section (E = 0.0253 eV, T = 20C) was taken from the sources indicated in Ref (9).DCross section uncertainty data is taken from Ref (7), the cross section comes from the other reference.ECalculated using Eq 5.FCross se
33、ction uncertainty comes from convariance data provided in the cross section source. The other reference indicates the source of the cross section.GIn Fig. 1, Q =4ErkT/AG2= 0.2 corresponds to the value for109Ag for T = 293 K, (r=N0sr, max sr, max= 29999 barn at 5.19 eV (13) .E481 1026.2 Weigh the sam
34、ples to a precision of 61.0 % (1S %) asdefined in Practice E177.6.3 Irradiate the samples for the predetermined time period.Record the power level and any changes in power during theirradiation, the time at the beginning and end of the irradiation,and the relative position of the monitors in the irr
35、adiationfacility.6.4 A waiting period is necessary between termination ofthe exposure and start of counting when using Co-Al andAg-Al monitors. This allows the 0.62356 days (17) half-life24Na which is formed by fast-neutron reactions on27Al orby thermal-neutron captures by23Na impurities to decay be
36、lowlevels at which its radiations may cause interferences. It issometimes advisable to count the samples periodically andfollow the decay of the portions of the activities due to the24Na.The length of the waiting period can be reduced by the use ofa germanium detector.6.5 With the gamma-ray spectrom
37、eter, analyze the silversample for110mAg and the cobalt sample for60Co. Obtain thenet count rate in each full-energy gamma-ray peak of interest,that is, 657.7623 keV or 884.684 keV for110mAg: 1332.501keV for60Co (see Method E181). See Table 2 for gammaradiations of110mAg.7. Calculation7.1 Calculate
38、the activities of110mAg and60Co in disinte-grations per second.7.2 A Westcott 2200 m/s neutron fluence rate, nv0,orfwand the Westcott epithermal index parameter, r=T/T0arerelated to the measured activities of the silver and cobaltmonitors by the following equation:A 5 N0lBFGs1fwti(1)where:A = measur
39、ed activity at the end of the exposuretime, disintegrations/s,N0= number of target atoms of59Co or109Ag atstart of irradiation,l = disintegration constant of product nuclide,s1,B = Self-absorption factor of the decay gammaray in the monitor material,F = burnup and decay correction factor,G = self-sh
40、ielding factor (see Eq 4, Table 3 andFig. 1).s1= Westcotts effective absorption cross sectionfor production of the product nuclide, cm2,fw(or nv0) = a 2200 m/s neutron fluence rate in which nis the neutron density (including both ther-mal and epithermal neutrons) and tiis 2200m/s, andti= exposure ti
41、me.7.3 The self-absorption factor, if not known for the gammarays being measured, can be approximated by the followingequation:B . 1 2 4/3!aR! (2)where:a= linear absorption coefficient in monitor, cm1, andR = radius of monitor wire, cm.7.4 The burnup and decay correction factor is given by:F 5exp2sa
42、fwti! 2 exp2l1s2fw!ti!l1s2fw2safw!ti(3)where:sa= Westcotts effective absorption cross section for targetnuclide, cm2, ands2= Westcotts effective absorption cross section for theproduct nuclide, cm2.7.5 The self-shielding factor is given by:TABLE 2 Gamma Radiations of110mAg (17,18)Energy of GammaA(ke
43、V) RelativeB,AEmission Probability (%)1. 657.7600 (11) 1002. 884.6781 (13) 77.1 (3)3. 937.485 (3) 36.3 (6)4. 1384.2931 (20) 26.4 (8)5. 763.9424 (17) 23.98 (21)6. 706.6760 (15) 17.31 (5)7. 1505.0280 (20) 14.42 (19)8. 677.6217 (12) 11.20 (2)9. 818.0244 (18) 7.78 (8)10. 687.0091 (18) 6.83 (5)11. 744.27
44、53 (18) 5.06 (9)12. 1562.294 (18) 1.319 (17)AThe number of parentheses following some given values is the uncertainty inthe last digit(s) of the value: 0.729 (8) means 0.729 6 0.008, 80.8 (1) means 70.86 0.1.BFor absolute intensity multiply emission probabilities by 0.943 6 0.004.TABLE 3 Self-Shield
45、ing Factors for Cobalt Wires (12)WireDiameterin. (mm)CobaltContent,(mass %)G8res(132 eV) Gth0.050 (1.27) 0.104 1.00 1.000.050 (1.27) 0.976 0.95 6 0.04 0.99 6 0.010.001 (0.03) 100 0.81 6 0.03 0.99 6 0.020.005 (0.13) 100 0.52 6 0.02 0.97 6 0.010.010 (0.25) 100 0.42 6 0.02 0.94 6 0.010.015 (0.38) 100 0
46、.38 6 0.01 0.92 6 0.020.020 (0.51) 100 0.34 6 0.01 0.90 6 0.020.025 (0.64) 100 0.32 6 0.01 0.88 6 0.03E481 103G 5gGth1 r=T/T0!S0G8resg 1 r=T/T0!S0(4)where:g = correction factor which describes the departure ofthe cross section from the 1/v law in the thermalregion (see Table 4 for silver “g” factors
47、),Gth= thermal neutron self-shielding factor,G8res= resonance neutron self-shielding factor,r = a measure of the proportion of epithermal neutronsin the reactor spectrum,T = neutron temperature, K,T0= 293.6 K, andS0= correction factor which describes the departure ofthe cross section from the 1/v la
48、w in the epithermalregion.7.6 Although the Ag109(n,l)Ag110mS0value in Table 1 is ameasured value, S0can be calculated by the following equa-tion:S052=pI90s052=pSI0s02 2gE0ECdD (5)where:I90= resonance integral excess over the 1/v cross sectionvalue, cm2,s0= 2200 m/s cross-section value, cm2,I0= reson
49、ance integral,E0= 0.0253 eV, andECd= 0.55 eV.7.7 Substituting the measured activities of the cobalt and thesilver monitors into Eq 1 yields two nonlinear equations in thetwo unknown parameters r=T/T0and fw.7.8 PC software products such as MathCad and Math-ematica can be programmed to solve these two nonlinearequations with a variety of iterative solvers. A FORTRAN IVcomputer program, COAG2 (19), was written to solve theseequations. The program iterates until the epithermal index andthe fluence values give calculated activities
