1、Designation: E 481 03Standard Test Method forMeasuring Neutron Fluence Rates by Radioactivation ofCobalt and Silver1This standard is issued under the fixed designation E 481; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year
2、 of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) 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 modera
3、tednuclear reactor environments where the use of cadmium, as athermal neutron shield as described in Method E 262, 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 flue
4、nce rate (Note 1) by activation of cobalt-and silver-foil monitors (See Terminology E 170). Thereaction59Co(n,g)60Co results in a well-defined gamma emitterhaving a half-life of 1925.5 days (1).2Thereaction109Ag(n,g)110mAg results in a nuclide with a complexdecay scheme which is well known and havin
5、g a half-life of249.76 days (14). Both cobalt and silver are available either invery pure form or alloyed with other metals such as aluminum.A reference source of cobalt in aluminum alloy to serve as aneutron fluence rate monitor wire standard is available from theNational Institute of Standards and
6、 Technology (NIST) asStandard Reference Material 953.3The competing activitiesfrom neutron activation of other isotopes are eliminated, for themost part, by waiting for the short-lived products to die outbefore counting. With suitable techniques, thermal neutronfluence rate in the range from 109cm2s
7、1to 3 3 1015cm2s1can be measured. For this method to be applicable, thereactor must be well moderated and be well represented by aMaxwellian low-energy distribution and an (1/E) epithermaldistribution. These conditions are usually met in positionssurrounded by hydrogenous moderator without nearby st
8、ronglyabsorbing materials. Otherwise the true spectrum must becalculated to obtain effective activation cross sections over allenergies.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 the
9、safety 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:E 170 Terminology Relating to
10、 Radiation Measurementsand Dosimetry4E 177 Practice for Use of the Terms Precision and Bias inASTM Test Methods5E 181 Test Methods for Detector Calibration and Analysisof Radionuclides4E 262 Test Method for Determining Thermal Neutron Re-action and Fluence Rates by Radioactivation Techniques43. Sign
11、ificance 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 thermal cross
12、 section. Theequations are based on the Westcott formalism (3, 4) anddetermine a Westcott 2200 m/s neutron fluence rate nv0and theWestcott epithermal index parameter r=T/T0. References 5,6, and 7 contain a general discussion of the two-reaction testmethod. In this test method, the absolute activitie
13、s 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 of the field
14、by the cadmium; (2) the inexactcadmium cut-off energy; (3) the low melting temperature ofcadmium. In addition, the reactivity changes accompanying therapid insertion and removal of cadmium may prohibit the useof the cadmium-ratio method. However, the self-shielding1This test method is under the juri
15、sdiction of ASTM Committee E10 on NuclearTechnology and Applications and is the direct responsibility of SubcommitteeE10.05 on Nuclear Radiation Metrology.Current edition approved Feb. 10, 2003. Published March 2003. Originallyapproved in 1973 T. Last previous edition approved in 1997 as E 481 97.2T
16、he boldface numbers in parentheses refer to references listed at the end of thistest method.3Standard Reference Material 953 is available from National Institute ofStandards and Technology, U.S. Dept. of Commerce, Washington, DC 20234.4Annual Book of ASTM Standards, Vol 12.02.5Annual Book of ASTM St
17、andards, Vol 14.02.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.corrections remain important unless the concentrations ofcobalt and silver are small. Studies indicate that the accuracy ofthe two-reaction method is comparable to th
18、e cadmium-ratiomethod.3.3 The long half-lives of the two monitors 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 E 181.4.2 Precisi
19、on Balance.4.3 Digital Computer.5. Materials and Manufacture5.1 The two monitors required for this test method arecobalt and silver. Although these two materials are availablecommercially in very pure form, they have been used (8)alloyed with aluminum (#1 % cobalt and #1 % silver) tominimize the sel
20、f-shielding effect and to permit insertion intoa high thermal-neutron fluence rate (1015cm2s1) facility (7,9). 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 test
21、methods. These might include chemical and activation analy-sis, 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-tio
22、n depends upon the characteristics of the facility in whichthe measurements 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. Howe
23、ver, the low mechanical strength of the monitorsrequires in many 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 pre
24、ferable. Perturbation due to the presence of the containermust be 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 r
25、emovedremotely.6. Procedure6.1 Decide on the size and shape of the 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 leve
26、l. To facilitate the convergenceof the two activity equations for 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.6.2 Weigh the samples to a precision of 61.0 % (1S %) asdefined
27、 in Practice E 177.6.3 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 irradiationfacility.6.4 A waiting period is nece
28、ssary between termination ofthe exposure and start of counting when using Co-Al andAg-Al monitors. This allows the 0.62356 days (1) half-life24NaTABLE 1 Recommended ConstantsSymbol ParameterCobalt (60Co) Silver (110mAg)ValueAReference ValueAReferencet1/2Half-life 1925.5 (5) days (1) 249.76 (4) days
29、(14)A Abundance of parent isotope 100 % (59Co) (14) 48.161 (8) % (109Ag) (14)D Mass excess of residual isotope (scaled toD 12C=0)(1 amu = 931.494MeV)B61.644 MeV (14) 87.340 MeV (14)saAbsorption 2200 m/s cross section for target590Co and109Ag37.233 b 6 0.16 %C,D91.0 b 6 1% (16)s02200 m/s cross sectio
30、n for formation of60Co and110mAg 37.233 b 6 0.16 %C,D4.7 b 6 4% (16)S0Correction factor which describes the departure of thecross section from the 1/v law in the epithermalregion1.6959Co(n,g)60Co(7) 18.53109Ag(n,g)110mAg17.10109Ag(n,g)110m+110gAg(7)I0Resonance Integral 75.421 b 6 0.77 %59Co(n,g)60Co
31、(15),E66 b109Ag(n,g)110mAg(16)s2Effective absorption cross section for product nuclide(reactor spectrum)2b (11) 82 b (13)GthThermal neutron self-shielding factor Table 3 (12) 1 4/3 R(B(5)G8resResonance neutron self-shielding factor Table 3 (12) Fig. 1F(5)g Correction factor which describes the depar
32、ture of thecross section from 1/v law in thermal region1.0 (3) See Table 4 (3)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 calculated as A + 931.49432/D, where
33、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 Reference (15).DCross section uncertainty data is taken from Reference (16), the cross section comes from the other reference.ECross section uncertainty comes from convariance data
34、provided in the cross section source. The other reference indicates the source of the cross section.FIn Fig. 1, Q =4ErkT/AG2= 0.2 corresponds to the value for109Ag.E481032which is formed by fast-neutron reactions on27Al or bythermal-neutron captures by23Na impurities to decay belowlevels at which it
35、s 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 spectrometer, analyze the sil
36、versample 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 E 181). See Table 2 for gammaradiations of110mAg.7. Calculation7.1 Calculate the activities of110
37、mAg and60Co in disintegra-tions 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 N0l2BFGs1fwti(1)where:A = measured activity at the
38、end of the exposuretime, disintegrations/s,N0= number of target atoms of59Co or109Ag atstart of irradiation,l2= 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-shielding factor (see
39、 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 time.The self-absorpt
40、ion factor, if not known for the gamma raysbeing measured, can be approximated by the following equa-tion:B . 1 2 4/3!aR! (2)where:a= linear absorption coefficient in monitor, cm1, andR = radius of monitor wire, cm.The burnup and decay correction factor is given by:F 5exp2safwti! 2 exp2l21s2fw!ti#$l
41、2ti/fwti! 1s2# 2s2# 2sa%fwti(3)where:sa= Westcotts effective absorption cross section for targetnuclide, cm2, ands2= Westcotts effective absorption cross section for theproduct nuclide, cm2.The self-shielding factor is given by:G 5gGth1 r=T/T0!S0G8resg 1 r=T/T0!S0(4)where:g = correction factor which
42、 describes the departure ofthe cross section from the 1/v law in the thermalregion (see Table 4 for silver “g” factors),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
43、temperature, K,T0= 293.6 K, andS0= correction factor which describes the departure ofthe cross section from the 1/v law in the epithermalregion.Although the S0values in Table 1 are measured values, S0can be calculated by the following equation:S052=pI90s052=pSI0s02 2gE0ECdD (5)where:I90= resonance i
44、ntegral excess over the 1/v cross sectionvalue, cm2,s0= 2200 m/s cross-section value, cm2,I0= resonance integral,E0= 0.0253 eV, andECd= 0.55 eV.Substituting the measured activities of the cobalt and thesilver monitors into Eq 1 yields two nonlinear equations in theTABLE 2 Gamma Radiations of110mAg (
45、2)Energy of GammaA(keV) RelativeB,AEmission Probability (%)1. 657.7622 (21) 110.0 (4)2. 884.685 (3) 76.8 (3)3. 937.493 (4) 36.31 (12)4. 1384.300 (4) 25.66 (8)5. 763.944 (3) 23.55 (9)6. 706.682 (3) 17.37 (10)7. 1505.040 (5) 13.78 (5)8. 667.6227 (24) 10.94 (8)9. 818.031 (4) 7.76 (4)10. 687.015 (3) 6.8
46、0 (6)11. 744.277 (3) 5.00 (3)12. 1562.302 (5) 1.087 (7)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 multiple emission probabilities by 0.940.TABLE 3 Self-Shi
47、elding 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) 10
48、0 0.38 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 0.03E481033two unknown parameters r=T/T0and f0ti. PC softwareproducts such as MathCad and Mathematica can be pro-grammed to solve these two nonlinear equations with a varietyof iterative solvers. A F
49、ORTRAN IV computer program,COAG2 (10), was written to solve these equations. Theprogram iterates until the epithermal index and the fluencevalues give calculated activities that are within 0.1 % of theirmeasured values. The constants, cross sections, and othermeasured values used in the program should be set equal tothose listed in Table 1.7.3 The Westcott convention is designed primarily forcalculations involving reactions rather than those involvingscattering or diffusion. It states that the reaction rate per atompresent, R, is
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