1、Designation: E 262 03Standard Test Method forDetermining Thermal Neutron Reaction and Fluence Ratesby Radioactivation Techniques1This standard is issued under the fixed designation E 262; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revis
2、ion, the year 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 The purpose of this method is to define a generalprocedure for determining an unknown thermal neutro
3、n-fluence rate by neutron activation techniques. It is not practi-cable to describe completely a technique applicable to the largenumber of experimental situations that require the measure-ment of a thermal-neutron fluence rate. Therefore, this methodis presented so that the user may adapt to his pa
4、rticularsituation the fundamental procedures of the following tech-niques.1.1.1 Absolute counting technique using pure cobalt, puregold, or cobalt-aluminum or gold-aluminum alloy.1.1.2 Standard foil technique using pure gold, or gold-aluminum alloy, and1.1.3 Secondary standard foil techniques using
5、pure indium,indium-aluminum alloy, and dysprosium-aluminum alloy.1.2 The techniques presented are limited to measurements atroom temperatures. However, special problems when makingthermal-neutron fluence rate measurements in high-temperature environments are discussed in 8.2. For thosecircumstances
6、where the use of cadmium as a thermal shield isundesirable because of potential spectrum perturbations or oftemperatures above the melting point of cadmium, the methoddescribed in Test Method E 481 can be used in some cases.Alternatively, gadolinium filters may be used instead of cad-mium. For high
7、temperature applications in which aluminumalloys are unsuitable, other alloys such as cobalt-nickel orcobalt-vanadium have been used.1.3 Table 1 indicates the useful neutron-fluence ranges foreach detector material.1.4 This standard does not purport to address all of thesafety concerns, if any, asso
8、ciated 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 177 Practice for Use of the Terms Precision and Bias inA
9、STM Test Methods2E 181 Test Methods for Detector Calibration and Analysisof Radionuclides3E 261 Practice for Determining Neutron Fluence Rate, Flu-ence, and Spectra by Radioactivation Techniques3E 481 Test Method for Measuring Neutron Fluence Rate byRadioactivation of Cobalt and Silver33. Significan
10、ce and Use3.1 This method can be extended to use any material thathas the necessary nuclear and activation properties that suit theexperimenters particular situation. No attempt has been madeto fully describe the myriad problems of absolute countingtechniques, neutron-fluence depression, and thick-f
11、oil self-shielding. It is assumed that the experimenter will refer toexisting literature on these subjects. This method does offer areferee method (the standard gold foil irradiation at NationalInstitute of Standards and Technology (NIST) to aid theexperimenter when he is in doubt of his ability to
12、measure anabsolute thermal fluence rate.3.2 The standard foil technique uses a set of foils that are asnearly identical as possible in shape and mass. The foils arefabricated from any material that activates by an (n, g)reaction, preferably having a cross section approximatelyinversely proportional
13、to neutron speed in the thermal energyrange. Some of the foils are irradiated in a known neutron field(at NIST) or other standards laboratory). The foils are countedin a fixed geometry on a stable radiation-detecting instrument.The neutron induced reaction rate of the foils is computed fromthe count
14、ing data, and the ratio of the known neutron fluencerate to the computed reaction rate is determined. For any givenfoil, neutron energy spectrum, and counting set-up, this ratio isa constant. Other foils from the identical set can now beexposed to an unknown neutron field. The magnitude of thefluenc
15、e rate in the unknown field can be obtained by comparing1This method is under the jurisdiction 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 200
16、3. Originallyapproved in 1965 T. Last previous edition approved as E 262 97.2Annual Book of ASTM Standards, Vol 14.02.3Annual Book of ASTM Standards, Vol 12.02.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.the reaction rates as det
17、ermined from the counting data fromthe unknown and reference field, with proper corrections toaccount for spectral differences between the two fields (seeSection 4). One important feature of this technique is that iteliminates the need for absolute counting.4. Theory4.1 1/v Cross SectionsIt is not p
18、ossible using radioactiva-tion techniques to determine the true thermal neutron fluencerate without making some assumptions about the spectralshapes of both the thermal and epithermal components of theneutron density. For most purposes, however, the informationrequired is only that needed to make ca
19、lculations of activationand other reaction rates for various materials exposed to theneutron field. For reactions in which the cross section variesinversely as the neutron speed (1/v cross sections) the reactionrates are proportional to the total neutron density and do notdepend on the spectrum shap
20、e. Many radioactivation detectorshave reaction cross sections in the thermal energy range whichapproximate to 1/v cross sections (1/v detectors). Departuresfrom the 1/v shape can be accounted for by means of correctionfactors.4.2 Fluence Conventions:4.2.1 The purpose of a fluence convention (formerl
21、y called“flux convention”) is to describe a neutron field in terms of afew parameters that can be conveniently used to calculatereaction rates. The best known fluence conventions relating tothermal neutron fields are the Westcott convention (1)4and theStoughton and Halperin convention (2). Both make
22、 use of theconcept of an equivalent 2200 m/s fluence rate, that is equal tothe product of the neutron density and the standard speed, v0,equal to 2200 m/s which is the most probable speed ofMaxwellian thermal neutrons when the characteristic tempera-ture is 293.4K. In the Westcott convention, it is
23、the totalneutron density (thermal plus epithermal) which is multipliedby v0to form the “Westcott flux”, but in the Stoughton andHalperin convention, the conventional fluence rate is theproduct of the Maxwellian thermal neutron density and v0. Thelatter convention is the one followed in this method:f
24、05 nthv0(1)where f0is the equivalent 2200 m/s thermal fluence rate andnthrepresents the thermal neutron density, which is propor-tional to the reaction rate per atom in a 1/v detector exposed tothermal neutrons:Rs!05 nths0v05s0f0(2)4.2.2 (Rs)0represents only that part of the reaction rate thatis ind
25、uced by thermal neutrons, which have the Maxwellianspectrum shape. s0is the 2200 m/s cross section. For a non-1/vdetector Eq 2 needs to be replaced by:Rs!05 nthgs0v05 gs0f0(3)where g is a correction factor that accounts for the departuresfrom the ideal 1/v detector cross section in the thermal energ
26、yrange. The same factor appears in the Westcott convention Ref(1), and is usually referred to as the Westcott g factor. gdepends on the neutron temperature, T, and is defined asfollows:g 51v0s0*0 4p1/2Svv0D3ST0TD3/2 expFSvv0D2ST0TDGsv!dv(4)4.2.3 If the thermal neutron spectrum truly follows theMaxwe
27、llian distribution and if the neutron temperature isknown, it is possible to calculate the true thermal neutronfluence rate by multiplying the conventional (equivalent 2200m/s) thermal fluence rate by the factorvv05S4TpT0D1/2(5)where v is the Maxwellian mean speed for neutron tempera-ture T, and T0i
28、s the standard temperature of 293.4K. Thisconversion is most often unnecessary and is usually not madebecause the temperature T may be unknown. Naturally, it isessential when reporting results to be absolutely clear whetherthe true thermal fluence rate or the equivalent 2200 m/s thermalfluence rate
29、or the equivalent 2200 m/s total (Westcott) fluencerate is used. If the true thermal fluence rate is used, then itsvalue must be accompanied by the associated temperaturevalue.4.3 Epithermal NeutronsIn order to determine the effectsof epithermal neutrons, that are invariably present togetherwith the
30、rmal neutrons, cadmium covered foil irradiations aremade. It is important to realize that some epithermal neutronscan have energies below the effective cadmium cut-off energy,Ecd. The lowest energy of epithermal neutrons is usually takento be equal to 5kT (where k is Boltzmanns constant) that isequa
31、l to 0.13eV for room temperature (293K) neutrons (1),though 4 kT has been recommended for some reactors (3).Inorder to correct for these, it is necessary to make someassumption about the epithermal neutron spectrum shape, andthe assumption made in Refs 1 and 2 is that the epithermalneutron fluence r
32、ate per unit energy is proportional to 1/E:feE! 5fe/E,E$ 5kT (6)where feis an epithermal fluence parameter equal to thefluence rate per unit energy, fe(E), at 1 eV. This assumption isusually adequate for the purpose of correcting thermal neutronfluence rate measurements for epithermal neutrons at en
33、ergiesbelow the cadmium cut-off. To represent the epithermal fluencemore correctly, however, many authors have shown that the useof a 1/E(1+a)spectrum shape is preferable, where a is anempirical parameter. Refs (4-10).4.4 Resonance Integral:4The boldface numbers in parentheses refer to the list of r
34、eferences appended tothis method.TABLE 1 Useful Neutron Fluence Ranges of Foil MaterialFoil Material Form Useful Range(neutrons/cm2)Indium pure or alloyed withaluminum103to 1012Gold pure or alloyed withaluminum107to 1014Dysprosium pure or alloyed withaluminum103to 1010Cobalt pure or alloyed withalum
35、inum1014to 1020E2620324.4.1 The resonance integral for an ideal dilute detector isdefined as follows:I05*EcdsE!dEE(7)4.4.2 The cadmium cut-off energy is taken to be 0.55eV fora cylindrical cadmium box of wall thickness 1 mm. (11). Thedata needed to correct for epithermal neutron reactions in themeth
36、ods described are the values of I0/gs0for each reaction(see Table 2). These values, taken from Refs (26-28), are basedon integral measurements.4.5 Reaction Rate:4.5.1 The reaction rate per atom, for an isotope exposed toa mixed thermal and epithermal neutron field is given by:Rs5f0gs01fegs0f11 w8 /g
37、 1 I0/gs0# (8)f1is a function that describes the epithermal activation of a1/v detector in the energy range 5kT to Ecd:f15*5kTEcdSkT0ED1/2dEE(9)4.5.2 For Ecdequal to 0.55eV and T0equal to 293.4K, f1=0.468. w8 in Eq 8 is a function which accounts for departure ofthe cross section from the 1/v law in
38、the energy range 5kT toEcd:w8 51s0*5kTEcdFsE! gs0 SkTED1/2GdEE(10)Some values of w8 for T equal 293.4K are given in Table 2.4.5.3 For a cadmium covered foil, the reaction rate is givenas:Rs,Cd5feI0(11)4.5.4 This can be used to eliminate the unknown epithermalfluence rate parameter, fe, from Eq 8. Af
39、ter rearrangement, oneobtains an expression for the saturation activity due to thermalneutrons only:f0gs05 Rs!05 Rs Rs,Cd S1 1gs0I0f11s0w8I0D(12)4.6 Neutron Self-Shielding:4.6.1 Unless extremely thin or dilute alloy materials areused, all of the measurement methods are subject to the effectsof neutr
40、on self-shielding. The modified version of Eq 12 whichtakes into account both a thermal self-shielding factor Gth, andan epithermal self shielding factor Gresis:f0gs05Rs!0Gth(13)51GthFRs Rs,Cd S1 1gs0GresI0f11s0w8GresI0DG4.6.2 Values of the self-shielding factors Gthand Gresforgold and cobalt foils
41、and wires and for indium foils are givenin Tables 3-7. In the literature, values for the resonanceself-shielding factor are given in two ways, and those must notbe confused. Gresused here, is a factor by which multiplies theresonance integral as defined in Eq 7. G8resis a self-shieldingfactor that m
42、ultiplies the reduced resonance integral fromwhich the 1/v part of the cross section has been subtracted. Thenecessary conversion factor that has been applied whereneeded in Tables 3-7 is:Gres5 G8res1 1G8res! 0.429gs0I0(14)4.7 Fluence Depression FactorsThermal fluence depres-sion is an additional pe
43、rturbation that occurs when an absorberis surrounded by a moderator. Because the effects are sensitiveto the details of individual situations, it is not possible toprovide correction factors here. References (12-20) describethese effects. The problem is avoided when foils are exposed incavities of v
44、ary large volume compared to the detector volume.In other cases, a rough guide is that the external perturbationeffect is usually less than the thermal self-shielding effect, andmuch less when the hydrogenous moderator is absent.5. Apparatus5.1 Radiation-Detection Instruments:5.1.1 The radiation det
45、ectors that may be used in neutronactivation techniques are described in the Standard Methods,E 181. In addition, or as an alternative, a calibration high-pressure ionization chamber may be used. Details for itsconstruction and calibration may be found in Ref (21).5.2 Precision Punch:5.2.1 A precisi
46、on punch is required to fabricate a set ofidentical foils for the standard foil technique. The punch mustcut foils that have smooth edges. Since finding such a punchcommercially available is difficult, it is recommended that thepunch be custom made. It is possible to have several dies madeto fit one
47、 punch so that a variety of foil sizes can be obtained.Normally, foil diameters are 12.7 mm (0.500 in.) or less. Theprecision punch is one of the most important items in thestandard foil technique particularly if the counting techniqueincludes b or soft-photon events.5.3 Aluminum and Cadmium Boxes:5
48、.3.1 One set of foils must be irradiated in cadmium boxesor covers to determine that part of the neutron activationresulting from absorption of epicadmium neutrons. The cad-mium box must be constructed so that the entire foil issurrounded by 1 mm (0.040 in.) of cadmium. This can beaccomplished by us
49、ing a circular cup-shaped design as shownTABLE 2 Nuclear Data from References (23), (26-28)Reaction s0barnsg (T =293 K)I0gs0w859Co(n,g)60Co 37.233 6 0.16 % 1.0 1.98 6 .034 0197Au(n,g)198Au 98.69 6 0.14 % 1.0051 15.7 6 0.3 .0500115ln(n,g)116ln 166.413 6 0.6 % 1.0194 15.8 6 0.5 .2953163Dy(n,g)164Dy 2650 6 3.8 % 0.975 0.23 6 0.04 0TABLE 3 Resonance Self-Shielding Data for Cobalt Foils(Reference (30)Foil ThicknessG8res(132 eV)Gres(in.) (cm)0.0004 0.001018 0.8264 0.8640.0010 0.02254 0.7000 0.7650.0025 0.00635 0.5470 0.6450.0050 0.0127 0.4395 0.5610.0075 0.0190
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