1、Designation: E 496 02Standard Test Method forMeasuring Neutron Fluence and Average Energyfrom3H(d,n)4He Neutron Generators by RadioactivationTechniques1This standard is issued under the fixed designation E 496; the number immediately following the designation indicates the year oforiginal adoption o
2、r, in the case of revision, 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 This test method covers a general procedure for themeasurement of the fast-ne
3、utron fluence rate produced byneutron generators utilizing the3H(d,n)4He reaction. Neutronsso produced are usually referred to as 14-MeV neutrons, butrange in energy depending on a number of factors. This testmethod does not adequately cover fusion sources where thevelocity of the plasma may be an i
4、mportant consideration.1.2 This test method uses threshold activation reactions todetermine the average energy of the neutrons and the neutronfluence at that energy. At least three activities, chosen from anappropriate set of dosimetry reactions, are required to charac-terize the average energy and
5、fluence. The required activitiesare typically measured by gamma ray spectroscopy.1.3 This standard 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 de
6、termine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:E 170 Terminology Relating to Radiation Measurementsand Dosimetry2E 181 Test Methods for Detector Calibration and Analysisof Radionuclides2E 261 Practice for Determining Neutron Fluence Rates,
7、Fluence, and Spectra by Radioactivation Techniques2E 265 Test Method for Measuring Reaction Rates andFast-Neutron Fluences by Radioactivation of Sulfur-322E 720 Guide for Selection and Use of Neutron-ActivationFoils for Determining Neutron Spectra Employed inRadiation-Hardness Testing of Electronics
8、22.2 International Commission on Radiation Units andMeasurements (ICRU) Reports:ICRU Report 13Neutron Fluence, Neutron Spectra andKerma3ICRU Report 26Neutron Dosimetry for Biology andMedicine32.3 ISO Standard:Guide to the Expression of Uncertainty in Measurement42.4 NIST Document:Technical Note 1297
9、Guidelines for Evaluating and Ex-pressing the Uncertainty of NIST Measurement Results53. Terminology3.1 DefinitionsRefer to Terminology E 170.4. Summary of Test Method4.1 This test method describes the determination of theaverage neutron energy and fluence by use of three activitiesfrom a select lis
10、t of dosimetry reactions. Three dosimetryreactions are chosen based on a number of factors including theintensity of the neutron field, the reaction half-lives, the slopeof the dosimetry reaction cross section near 14-MeV, and theminimum time between sensor irradiation and the gammacounting. The act
11、ivities from these selected reactions aremeasured. Two of the activities are used, in conjunction withthe nuclear data for the dosimetry reactions, to determine theaverage neutron energy. The third activity is used, along withthe neutron energy and nuclear data for the selected reaction, todetermine
12、 the neutron fluence. The uncertainty of the neutronenergy and the neutron fluence is determined from the activitymeasurement uncertainty and from the nuclear data.1This test method is under the jurisdiction ofASTM Committee E10 on NuclearTechnology and Applications and is the direct responsibility
13、of SubcommitteeE10.07 on Radiation Dosimetry for Radiation Effects on Materials and Devices.Current edition approved June 10, 2002. Published September 2002. Originallypublished as E 496 73. Last previous edition E 496 96.2Annual Book of ASTM Standards, Vol 12.02.3Available from the International Co
14、mmission on Radiation Units, 7910Woodmont Ave., Washington, DC 20014.4Available from American National Standards Institute, 11 W. 42nd St., 13thFloor, New York, NY 10036.5Available from National Institute of Standards and Technology, Gaithersburg,MD 20899.1Copyright ASTM International, 100 Barr Harb
15、or Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.5. Significance and Use5.1 Refer to Practice E 261 for a general discussion of themeasurement of fast-neutron fluence rates with thresholddetectors.5.2 Refer to Test Method E 265 for a general discussion ofthe measurement of fast
16、-neutron fluence rates by radioactiva-tion of sulfur-32.5.3 Reactions used for the activity measurements can bechosen to provide a convenient means for determining theabsolute fluence rates of 14-MeV neutrons obtainedwith3H(d,n)4He neutron generators over a range of irradiationtimes from seconds to
17、approximately 100 days. High puritythreshold sensors referenced in this test method are readilyavailable.5.4 The neutron-energy spectrum must be known in order tomeasure fast-neutron fluence using a single threshold detector.Neutrons produced by bombarding a tritiated target withdeuterons are common
18、ly referred to as 14-MeV neutrons;however, they can have a range of energies depending on: (1)the angle of neutron emission with respect to the deuteronbeam, (2) the kinetic energy of the deuterons, and (3) the targetthickness. In most available neutron generators of theCockroft-Walton type, a thick
19、 target is used to obtain high-neutron yields. As deuterons penetrate through the surface andmove into the bulk of the thick target, they lose energy, andinteractions occurring deeper within the target produce neu-trons with correspondingly lower energy.5.5 Wide variations in neutron energy are not
20、generallyencountered in commercially available neutron generators ofthe Cockroft-Walton type. Figs. 1 and 2 (1)6show the variationof the zero degree3H(d,n)4He neutron production cross sectionwith energy, and clearly indicate that maximum neutron yieldis obtained with deuterons having energies near t
21、he 107 keVresonance. Since most generators are designed for high yield,the deuteron energy is typically about 200 keV, giving a rangeof neutron energies from approximately 14 to 15 MeV. Thedifferential center-of-mass cross section is typically parameter-ized as a summation of Legendre polynomials. F
22、igs. 3 and 4(1,2) show how the neutron yield varies with the emissionangle in the laboratory system. The insert in Fig. 4 shows howthe magnitude,A1, of the P1(u) term, and hence the asymmetryin the differential cross section grows with increasing energy ofthe incident deuteron. The nonrelativistic k
23、inematics (valid forEd3.71 MeV) this reaction is nolonger monoenergetic. Monoenergetic neutron beams withenergies from about 14.8 to 20.4 MeV can be produced by thisreaction at forward laboratory angles (6).5.7 It is recommended that the dosimetry sensors be fieldedin the exact positions that will b
24、e used for the customers of the14-MeV neutron source. There are a number of factors that canaffect the monochromaticity or energy spread of the neutronbeam (6,7). These factors include the energy regulation of theincident deuteron energy, energy loss in retaining windows if agas target is used or en
25、ergy loss within the target if a solidtritiated target is used, the irradiation geometry, and back-ground neutrons from scattering with the walls and floorswithin the irradiation chamber.6. Apparatus6.1 Either a NaI(Tl) or a Ge semiconductor gamma-rayspectrometer, incorporating a multichannel pulse-
26、height ana-lyzer is required. See Test Methods E 181 for a discussion ofspectrometer systems and their use.6.2 If sulfur is used as a sensor, then a beta particle detectoris required. The apparatus required for beta counting of sulfuris described in Test Methods E 181 and E 265.FIG. 3 Energy and Ang
27、le Dependence of the3H(d,n)4HeDifferential Cross Section (1)FIG. 4 Change in Neutron Energy From3H(d,n)4He Reaction withLaboratory Emission Angle (2)E4960236.3 A precision balance for determining foil masses isrequired.7. Materials and Manufacture7.1 High purity threshold foils are available in a la
28、rgevariety of thicknesses. Foils of suitable diameter can bepunched from stock material. Small diameter wire may also beused. Prepunched and weighed high purity foils are alsoavailable commercially. Guide E 720 provides some details ontypical foil masses and purity. Foils of 12.7 and 25.3 mm (0.50an
29、d 1.00 in.) diameter and 0.13 and 0.25 mm (0.005 and 0.010in.) thickness are typical.7.2 SeeTest Method E 265 for details on the availability andpreparation of sulfur sensors.8. Calibration8.1 See Test Methods E 181 for general detector calibrationmethods. Test Methods E 181 addresses both gamma-ray
30、spectrometers and beta counting methods.9. Procedure for Determining the Neutron Energy9.1 Selection of Sensors:9.1.1 Use of an activity ratio method is recommended forthe determination of the neutron energy. The activity ratiomethod has been described in Ref (8). This test method hasbeen validated
31、for ENDF/B-VI cross sections (9) in Ref (10).9.1.2 Sensor selection depends upon the length of theirradiation, the cross section for the relevant sensor reaction,the reaction half-life, and the expected fluence rate. Table 1lists some dosimetry-quality reactions that are useful in the14-MeV energy r
32、egion. The short half-lives of some of thesereaction products, such as27Mg and62Cu, generally limit theuse of these activation products to irradiation times of less thanabout 15 min. Table 2 and Fig. 6 show the recommended crosssections, in the vicinity of 14-MeV, for these reactions. Thecross secti
33、ons recommended in Table 1 are from the ENDF/B-VI and IRDF-90 (15) cross section compilations. TheSNLRML cross section compendium (16) is a single-point-of-reference source for the recommended cross sections anduncertainty data for the reactions mentioned in Table 1. Thereferences for the nuclear da
34、ta in Table 1 are given in the table.9.1.3 Longer high fluence irradiations are recommended forthe determination of the neutron energy. Table 3 and Figs. 7and 8 show the neutron energy-dependent activity ratios forsome commonly used sensor combinations. In general, thesteeper the activity ratio, the
35、 more sensitive the method is tothe neutron energy.9.1.4 Table 4 shows the energy resolutions of some specificsensor combinations for a 14.5 MeV neutron source. The54Fe(n,p)54Mn/58Ni(n,2n)57Ni and27Al(n,a)24Na/58Ni(n,2n)57Ni are recommended sensor combinations due to theirsteep slope and their very
36、accurate dosimetry cross sectionevaluations.9.2 Determine the Sensor MassWeigh each sensor to aprecision of 0.1 %. Nonuniform foil thicknesses can resultfrom the use of dull punches and frequently result in weightvariation of 10 % or more.9.3 Irradiation of SensorsIrradiate the sensors, makingcertai
37、n that both sensors experience exactly the same fluence.The fluence gradients near a 14-MeV source tend to be highand it may be necessary to stack the sensors together or tomount them on a rotating disk during irradiation. Note thelength of the irradiation, ti, and the time the irradiation ended.Som
38、e sensors may have an interference reaction that issensitive to low energy neutrons.The interference reaction maybe associated with the primary sensor element or with acontaminant material in the sensor. Of the reactions listed inTable 1, the use of a Cu sensor is the only case where theprimary sens
39、or element may be responsible for an interferencereaction. In this case the useful65Cu(n,2n)64Cu reaction activitymust be distinguished from the63Cu(n,g)64Cu interferencereaction activity (for example, by using an isotopically puresensor or by experimentally verifying bounds on the maximumpossible l
40、evel of interference). Other examples of interferencereactions from contaminant materials include trace impuritiesof Mn in Fe sensors and Na in Al sensors. Manganese is afrequent contaminant in Fe foils. In this case the55Mn(n,g)56Mn reaction interferes with the desired sensor response fromthe56Fe(n
41、,p)56Mn reaction. Salt from handling Al sensors canresult in the23Na(n,g)24Na contaminant reaction which affectsthe use of the27Al(n,a)24Na dosimetry sensor. If one isuncertain about the importance of an interference reaction thathas a high thermal neutron cross section, it is recommendedthat the se
42、nsor be irradiated with and without a cadmium coverto quantify the importance of this interference term.FIG. 5 Dependence of3H(d,n)4He Neutron Energy on Angle (2)E4960249.4 Determination of Sensor ActivityGuide E 720 pro-vides details on the calculational procedure for determining theactivity of an
43、irradiated sensor. The results of this step shouldbe the activities, corrected to a time corresponding to the endof the irradiation. The activity should be corrected for decayduring the irradiation, as explained in Guide E 720. This decaycorrection is especially important for short half-life reactio
44、ns.The activity should have units of Bq per target atom.9.5 CalculationsSection 11 details the calculations thatuse these two sensor activities to determine the neutron averageenergy.10. Procedure for Determining the Neutron Fluence10.1 Selection of Sensor:10.1.1 To avoid sensitivity to uncertainty
45、in the exactneutron energy, the 14-MeVneutron fluence sensor is generallychosen to have a flat response in the 13 MeVto 15 MeVenergyregion. Fig. 6 and Table 2 show the energy dependence near 14MeV for some frequently used dosimetry sensors. An exami-nation of Fig. 6 and Table 2 clearly indicates a s
46、trongpreference to use the93Nb(n,2n)92mNb reaction. This prefer-ence is based on the flat energy response and the small crosssection uncertainty near 14 MeV. The93Nb(n,2n)92mNb reac-tion has been used as a transfer standard for 14-MeV sourcesby national standards laboratories (17) and in internation
47、alintercomparisons (18). The footnotes in Table 1 list someprecautions about use of some other reactions. Ifthe93Nb(n,2n)92mNb reaction cannot be used in a specific case,the uncertainty of the3H(d,n)4He neutron energy, as determinedfrom Section 9, should be used in conjunction with Table 2 andFig. 6
48、 to determine the best alternative reaction.10.1.2 Paragraph 9.1.2 indicates some other considerationsin the choice of a dosimetry fluence reaction based on theirradiation length and expected strength.10.2 Determine the Sensor MassWeigh the sensor to aprecision of 0.1 %. Nonuniform foil thicknesses
49、can resultfrom the use of dull punches and frequently result in weightvariations of 10 % or more.10.3 Irradiation of SensorParagraph 9.3 provides detailsand precautions on the irradiation of the sensor.10.4 Determination of Sensor ActivityGuide E 720 pro-vides details on the calculational procedure for determining theTABLE 1 Cross Section Parameters for Some Useful ReactionsDosimetryReactionsTarget Nucleus Product NucleusReactionNotesElementalAtomic Weight(12, 12)IsotopicAtomicNumberAbundance, %(12, )Cross Section SourceCross SectionUncertaintyNear14-MeV, %Half-Life(12,Eg,
