1、Designation: C 1500 02Standard Test Method forNondestructive Assay of Plutonium by Passive NeutronMultiplicity Counting1This standard is issued under the fixed designation C 1500; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the
2、 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 describes the nondestructive assay ofplutonium in forms such as metal, oxide, scrap, residu
3、e, orwaste using passive neutron multiplicity counting. This testmethod provides rapid results that are usually more accuratethan conventional neutron coincidence counting. The methodcan be applied to a large variety of plutonium items in variousgeometries in cans, 208-L drums, or 1900-L Standard Wa
4、steBoxes. It has been used to assay items whose plutoniumcontent ranges from1gto1000s of g.1.2 There are several electronics or mathematical ap-proaches available for multiplicity analysis, including the shiftregister, the Euratom Time Correlation Analyzer, and the ListMode Module, as described brie
5、fly in Ref. (1).21.3 This test method is primarily intended to address theassay of240Pu-effective by moments-based multiplicity analy-sis using shift register electronics (1, 2) and high efficiencyneutron counters specifically designed for multiplicity analysis.This test method requires knowledge of
6、 the relative abun-dances of the plutonium isotopes to determine the totalplutonium mass.1.4 This test method may also be applied to modifiedneutron coincidence counters which were not specificallydesigned as multiplicity counters, with a corresponding degra-dation of results.2. Referenced Documents
7、2.1 ASTM Standards:C 859 Terminology Relating to Nuclear Materials3C 1030 Test Method for Determination of Plutonium Isoto-pic Composition by Gamma-Ray Spectroscopy3C 1207 Test Method for Nondestructive Assay of Plutoniumin Scrap and Waste by Passive Neutron CoincidenceCounting3C 1458 Test Method fo
8、r Nondestructive Assay of Pluto-nium, Tritium, and241Am by Calorimetric Assay33. Terminology3.1 Terms shall be defined in accordance with TerminologyC 859 except for the following:3.2 alpha (a), nthe ratio of the uncorrelated neutronemission rate from (a,n) reactions to the spontaneous neutronemissi
9、on rate from a non-multiplying sample (see Ref. (1) forequation).3.3 coincidence gate length (G), nthe time interval fol-lowing the detection of a neutron during which additionalneutron counts are considered to be in coincidence with theoriginal neutron. In Fig. 1, this is the length of time the (R
10、+ A)and (A) gates are set to accept neutron counts.3.3.1 gate fractions, nthe fraction of the total coincidenceevents that occur within the coincidence gate.3.3.2 doubles gate fraction (fd), nthe fraction of thetheoretical double coincidences that can be detected within thecoincidence gate (see Eq 1
11、).3.3.3 triples gate fraction (ft), nthe fraction of the theo-retical triple coincidences that can be detected within thecoincidence gate (see Eq 2).3.4 die-away time (t), nthe average mean life-time of theneutron population as measured from the time of emission tothe time of detection, escape, or a
12、bsorption. Die-away time isa function of the counter assembly design and the assay item.Fig. 1 illustrates the decreasing probability of detection as afunction of time.3.5 doubles (D), nthe doubles are equivalent to the realsrate and represents the number of double neutroncoincidences/s. The doubles
13、 may be determined from thecoincidence shift register directly or by reduction of themultiplicity (R + A) and (A) histograms (1).3.6 effciency (e), nthis is usually taken to be the absoluteneutron detection efficiency, which is calculated from the ratioof the measured neutron count rate to the decla
14、red neutronemission rate of a non-multiplying reference source.3.7 factorial moment, nthis is a derived quantity repre-senting a summation of the neutron multiplicity distributionweighted by certain factors (see Ref. (1) for equation).3.8 item, nthe entire container being measured and itscontents.3.
15、9 multiplicity distribution, nthis is the distribution ofthe number of neutrons emitted in a fission event. This numbercan vary from 0 to 5 or more.1This test method is under the jurisdiction of ASTM Committee C26 on NuclearFuel Cycle and is the direct responsibility of Subcommittee C26.10 on NonDes
16、tructive Assay.Current edition approved Jan. 10, 2002. Published May 2002.2The boldface numbers in parentheses refer to the list of references at the end ofthis standard.3Annual Book of ASTM Standards, Vol 12.01.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA
17、 19428-2959, United States.3.9.1 spontaneous fission neutron multiplicities (ns1, ns2,ns3),nthe factorial moments of the spontaneous fissionneutron multiplicity distribution. For the multiplicity analysisof Pu materials the spontaneous fission nuclear data for240Pu isused to calculate these moments
18、(3). One commonly used setof moments is ns1= 2.154, ns2= 3.789, ns3= 5.211 (23).3.9.2 induced fission neutron multiplicities (ni1, ni2, ni3),nthe factorial moments of the induced fission neutronmultiplicity distribution. Typically multiplicity analysis willutilize the data from fast neutron-induced
19、fission of239Pu tocalculate these moments (3). One commonly used set ofmoments is ni1= 3.163, ni2= 8.240, ni3= 17.321 (23).3.10 point model, nthe mathematical model used to ana-lyze multiplicity counting data. The model assumes that theneutron detector efficiency and the probability of fission areco
20、nstant across the item, as though it were a point source.3.11 shift-register-based coincidence circuit, nan elec-tronic circuit for determining totals T, reals plus accidentals (R+ A), and accidentals (A) in a selected count time t (4, 5). Theterminology used in this test method refers specifically
21、toshift-register electronics. Fig. 1 shows the probability ofdetecting a neutron as a function of time and illustrates the timeintervals discussed.3.11.1 totals, nthe total number of neutrons detectedduring the count time.3.11.2 reals plus accidentals, (R + A), nthe number ofneutrons detected in the
22、 (R + A) gate period (Fig. 1) followingthe initial detection of each neutron (4). These events are dueto neutrons that are coincident with the given neutron (reals)and to neutrons that are not correlated with the given neutron(accidentals). This is a measured quantity.3.11.3 accidentals (A), nthe nu
23、mber of neutrons detectedin the (A) gate period (Fig. 1) following the initial detection ofeach neutron (4). These neutrons are not correlated with theinitial neutron. They come from many different sources andtheir count rate is assumed to be constant from the item beingassayed. This quantity is mea
24、sured by interrogating the (A)gate time interval window that occurs long after the expectedlifetime of coincident neutrons in the counting chamber. This isa measured quantity.3.11.4 reals (R), nthe number of coincident neutronsdetected in (R + A) gate intervals immediately following thedetection of
25、each neutron during the count time (4). Thisquantity is calculated from the measured (R + A) and (A)quantities.3.11.5 neutron counting multiplicity, nthe number of neu-trons within the coincidence gate for each trigger event in theshift register.3.12 net neutron leakage multiplication (M), nthe rati
26、o ofthe net number of neutrons leaving the item to the numberinitially produced by spontaneous fission and (a,n) reactions(6).3.13 passive mode, ndetermines the total spontaneousfissioning mass in the measured item through the detection ofemitted neutrons rather than neutrons from fissions induced b
27、yexternal interrogation sources.3.14 pre-delay, nthe coincidence circuit has a pre-delayimmediately after a neutron has been detected to allow theamplifiers to recover and prepare to detect subsequent neutrons(4). This principle is shown in Fig. 1.3.15 singles (S), nthe singles are equivalent to the
28、 totals/srepresenting the total neutron detection rate.3.16 triples (T), nThe triple neutron coincidence rate is aderived quantity obtained from the factorial moments of themultiplicity (R + A) and (A) histograms (1). It may bevisualized as the count rate for three neutrons in coincidence.4. Summary
29、 of Test Method4.1 The item is placed in the sample chamber or “well” ofthe multiplicity counter, and the emitted neutrons are detectedby the3He tubes that surround the well.4.2 The detected neutron multiplicity distribution is pro-cessed by the shift register electronics package to obtain thenumber
30、 of neutrons of each multiplicity in the (R + A) and (A)gates.4.3 The first three moments of the (R + A) and (A)FIG. 1(a) Simplified porbability distribution showing the approximately exponential decay, as a function of time, for detecting a secondneutron from a single fission event. The probability
31、 of detecting a random neutron is constant with time. (b) Typical coincidencetiming parameters.C 15002multiplicity distributions are computed to obtain the singles (ortotals), the doubles (or reals), and the triples. Using these threecalculated values, it is possible to solve for 3 unknown itemprope
32、rties, the240Pu-effective mass, the self-multiplication,and the a ratio. Details of the calculations may be found inAnnex A1.4.4 The total plutonium mass is then determined from theknown plutonium isotopic ratios and the240Pu -effective mass.4.5 Corrections are routinely made for neutron background,
33、cosmic ray effects, small changes in detector efficiency withtime, and electronic deadtimes.4.6 Optional algorithms are available to correct for thebiases caused by spatial variations in self-multiplication orchanges in the neutron die-away time.4.7 Multiplicity counters are carefully designed by Mo
34、nteCarlo techniques to minimize variations in detection efficiencycaused by spatial effects and energy spectrum effects. Correc-tions are not routinely made for neutron detection efficiencyvariations across the item, energy spectrum effects on detectionefficiency, or neutron capture in the item.5. S
35、ignificance and Use5.1 This test method is useful for determining the plutoniumcontent of items such as impure Pu oxide, mixed Pu/U oxide,oxidized Pu metal, Pu scrap and waste, Pu process residues,and weapons components.5.2 Measurements made with this test method may besuitable for safeguards or was
36、te characterization requirementssuch as:5.2.1 Nuclear materials accountability,5.2.2 Inventory verification (7),5.2.3 Confirmation of nuclear materials content (8),5.2.4 Resolution of shipper/receiver differences (9),5.2.5 Excess weapons materials inspections (10, 11),5.2.6 Safeguards termination on
37、 waste (12, 13),5.2.7 Determination of fissile equivalent content (14).5.3 A significant feature of neutron multiplicity counting isits ability to capture more information than neutron coinci-dence counting because of the availability of a third measuredparameter, leading to reduced measurement bias
38、 for mostmaterial categories. This feature also makes it possible to assaysome in-plant materials that are not amenable to conventionalcoincidence counting, including moist or impure plutoniumoxide, oxidized metal, and some categories of scrap, waste, andresidues (10).5.4 Calibration for many materi
39、al types does not requirerepresentative standards. Thus, the technique can be used forinventory verification without calibration standards (7), al-though measurement bias may be lower if representativestandards were available.5.4.1 The repeatability of the measurement results due tocounting statisti
40、cs is related to the quantity of nuclear material,the (a,n) reaction rate, and the count time of the measurement(15).5.4.2 For certain materials such as small Pu items of lessthan 1 g, some Pu-bearing waste, or very impure Pu processresidues where the (a,n) reaction rate overwhelms the triplessignal
41、, multiplicity information may not be useful because ofthe poor counting statistics of the triple coincidences withinpractical counting times (12).5.5 For pure Pu metal, pure oxide, or other well-characterized materials, the additional multiplicity informationis not needed, and conventional coincide
42、nce counting willprovide better repeatability because triple coincidences are notused. Conventional coincidence information can be obtainedeither by changing to a coincidence counter, or analyzing themultiplicity data in coincidence mode.5.6 The mathematical analysis of neutron multiplicity datais b
43、ased on several assumptions that are detailed in Annex A1.The most important is the assumption that the item is a point inspace, so that neutron detection efficiency, die-away time, andmultiplication are constant across the entire item (16, 17).5.6.1 Bias in passive neutron multiplicity measurements
44、 isrelated to deviations from the “point model” such as variationsin detection efficiency, matrix composition, or distribution ofnuclear material in the items interior.5.6.2 Heterogeneity in the distribution of nuclear material,neutron moderators, and neutron absorbers may introducebiases that affec
45、t the accuracy of the results. Measurementsmade on items with homogeneous contents will be moreaccurate than those made on items with inhomogeneouscontents.6. Interferences6.1 For measurements of items containing several hundredgrams of plutonium metal or more, multiplication effects arenot adequate
46、ly corrected by this method (18). A variable-multiplication bias correction is required.6.2 For items with high (a,n) reaction rates, the additionaluncorrelated neutrons will significantly increase the accidentalcoincidence rate. The practical application of multiplicitycounting is usually limited t
47、o items where the ratio of (a,n) tospontaneous fission neutrons is about 7 (7).6.3 For measurement of large items with high (a,n) reactionrates, the neutrons from (a,n) reactions can introduce biases iftheir energy spectra are different from the spontaneous fissionenergy spectrum. The ratio of the s
48、ingles in the inner and outerrings can provide a warning flag for this effect (19).6.4 Neutron moderation by low atomic mass materials in theitem affects neutron detection efficiency, neutron multiplicationin the item, and neutron absorption by poisons. For moderatelevels of neutron moderation, the
49、multiplicity analysis willautomatically correct the assay for changes in multiplication. Acorrection for capture in neutron poisons or other absorbers isnot available, so that a bias can result in measurements of suchitems.6.5 It is important to keep neutron background levels fromexternal sources as low and constant as practical for measure-ment of low Pu mass items. High backgrounds may produce abias, depending on the items mass and self-multiplication.6.6 Cosmic rays can produce single, double, and tripleneutrons from spallation events within the detector or
