ASTM C1500-02
(Test Method)Standard Test Method for Nondestructive Assay of Plutonium by Passive Neutron Multiplicity Counting
Standard Test Method for Nondestructive Assay of Plutonium by Passive Neutron Multiplicity Counting
SCOPE
1.1 This test method describes the nondestructive assay of plutonium in forms such as metal, oxide, scrap, residue, or waste using passive neutron multiplicity counting. This test method provides rapid results that are usually more accurate than conventional neutron coincidence counting. The method can be applied to a large variety of plutonium items in various geometries in cans, 208-L drums, or 1900-L Standard Waste Boxes. It has been used to assay items whose plutonium content ranges from 1 g to 1000's of g.
1.2 There are several electronics or mathematical approaches available for multiplicity analysis, including the shift register, the Euratom Time Correlation Analyzer, and the List Mode Module, as described briefly in Ref. (1).
1.3 This test method is primarily intended to address the assay of 240Pu-effective by moments-based multiplicity analysis using shift register electronics (1,2 ) and high efficiency neutron counters specifically designed for multiplicity analysis. This test method requires knowledge of the relative abundances of the plutonium isotopes to determine the total plutonium mass.
1.4 This test method may also be applied to modified neutron coincidence counters which were not specifically designed as multiplicity counters, with a corresponding degradation of results.
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Designation: C 1500 – 02
Standard Test Method for
Nondestructive Assay of Plutonium by Passive Neutron
Multiplicity Counting
This standard is issued under the fixed designation C 1500; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope C 1458 Test Method for Nondestructive Assay of Pluto-
nium, Tritium, and Am by Calorimetric Assay
1.1 This test method describes the nondestructive assay of
plutonium in forms such as metal, oxide, scrap, residue, or
3. Terminology
waste using passive neutron multiplicity counting. This test
3.1 Terms shall be defined in accordance with Terminology
method provides rapid results that are usually more accurate
C 859 except for the following:
than conventional neutron coincidence counting. The method
3.2 alpha (a), n—the ratio of the uncorrelated neutron
can be applied to a large variety of plutonium items in various
emission rate from (a,n) reactions to the spontaneous neutron
geometries in cans, 208-L drums, or 1900-L Standard Waste
emission rate from a non-multiplying sample (see Ref. (1) for
Boxes. It has been used to assay items whose plutonium
equation).
content ranges from1gto 1000’s of g.
3.3 coincidence gate length (G), n—the time interval fol-
1.2 There are several electronics or mathematical ap-
lowing the detection of a neutron during which additional
proaches available for multiplicity analysis, including the shift
neutron counts are considered to be in coincidence with the
register, the Euratom Time Correlation Analyzer, and the List
original neutron. In Fig. 1, this is the length of time the (R + A)
Mode Module, as described briefly in Ref. (1).
and (A) gates are set to accept neutron counts.
1.3 This test method is primarily intended to address the
3.3.1 gate fractions, n—the fraction of the total coincidence
assay of Pu-effective by moments-based multiplicity analy-
events that occur within the coincidence gate.
sis using shift register electronics (1, 2) and high efficiency
3.3.2 doubles gate fraction (f ), n—the fraction of the
d
neutroncountersspecificallydesignedformultiplicityanalysis.
theoretical double coincidences that can be detected within the
This test method requires knowledge of the relative abun-
coincidence gate (see Eq 1).
dances of the plutonium isotopes to determine the total
3.3.3 triples gate fraction (f), n—the fraction of the theo-
t
plutonium mass.
retical triple coincidences that can be detected within the
1.4 This test method may also be applied to modified
coincidence gate (see Eq 2).
neutron coincidence counters which were not specifically
3.4 die-away time (t), n—the average mean life-time of the
designed as multiplicity counters, with a corresponding degra-
neutron population as measured from the time of emission to
dation of results.
the time of detection, escape, or absorption. Die-away time is
2. Referenced Documents a function of the counter assembly design and the assay item.
Fig. 1 illustrates the decreasing probability of detection as a
2.1 ASTM Standards:
function of time.
C 859 Terminology Relating to Nuclear Materials
3.5 doubles (D), n—the doubles are equivalent to the reals
C 1030 Test Method for Determination of Plutonium Isoto-
rate and represents the number of double neutron
pic Composition by Gamma-Ray Spectroscopy
coincidences/s. The doubles may be determined from the
C 1207 TestMethodforNondestructiveAssayofPlutonium
coincidence shift register directly or by reduction of the
in Scrap and Waste by Passive Neutron Coincidence
multiplicity (R + A) and (A) histograms (1).
Counting
3.6 effıciency (e), n—this is usually taken to be the absolute
neutron detection efficiency, which is calculated from the ratio
This test method is under the jurisdiction ofASTM Committee C26 on Nuclear
of the measured neutron count rate to the declared neutron
Fuel Cycle and is the direct responsibility of Subcommittee C26.10 on Non
emission rate of a non-multiplying reference source.
Destructive Assay.
3.7 factorial moment, n—this is a derived quantity repre-
Current edition approved Jan. 10, 2002. Published May 2002.
The boldface numbers in parentheses refer to the list of references at the end of
senting a summation of the neutron multiplicity distribution
this standard.
weighted by certain factors (see Ref. (1) for equation).
Annual Book of ASTM Standards, Vol 12.01.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
C 1500
FIG. 1
(a)Simplifiedporbabilitydistributionshowingtheapproximatelyexponentialdecay,asafunctionoftime,fordetectingasecond
neutron from a single fission event. The probability of detecting a random neutron is constant with time. (b) Typical coincidence
timing parameters.
3.8 item, n—the entire container being measured and its each neutron (4). These neutrons are not correlated with the
contents. initial neutron. They come from many different sources and
3.9 multiplicity distribution, n—this is the distribution of their count rate is assumed to be constant from the item being
the number of neutrons emitted in a fission event.This number assayed. This quantity is measured by interrogating the (A)
can vary from 0 to 5 or more. gate time interval window that occurs long after the expected
3.9.1 spontaneous fission neutron multiplicities (n , n , lifetimeofcoincidentneutronsinthecountingchamber.Thisis
s1 s2
n ),n—the factorial moments of the spontaneous fission a measured quantity.
s3
neutron multiplicity distribution. For the multiplicity analysis 3.11.4 reals (R), n—the number of coincident neutrons
ofPumaterialsthespontaneousfissionnucleardatafor Puis detected in (R + A) gate intervals immediately following the
used to calculate these moments (3). One commonly used set detection of each neutron during the count time (4). This
of moments is n = 2.154, n = 3.789, n = 5.211 (23). quantity is calculated from the measured (R + A) and (A)
s1 s2 s3
3.9.2 induced fission neutron multiplicities (n , n , n ), quantities.
i1 i2 i3
n—the factorial moments of the induced fission neutron 3.11.5 neutron counting multiplicity, n—the number of neu-
multiplicity distribution. Typically multiplicity analysis will trons within the coincidence gate for each trigger event in the
utilize the data from fast neutron-induced fission of Pu to shift register.
calculate these moments (3). One commonly used set of 3.12 net neutron leakage multiplication (M), n—the ratio of
moments is n = 3.163, n = 8.240, n = 17.321 (23). the net number of neutrons leaving the item to the number
i1 i2 i3
3.10 point model, n—the mathematical model used to ana- initially produced by spontaneous fission and (a,n) reactions
lyze multiplicity counting data. The model assumes that the (6).
neutron detector efficiency and the probability of fission are 3.13 passive mode, n—determines the total spontaneous
constant across the item, as though it were a point source. fissioning mass in the measured item through the detection of
3.11 shift-register-based coincidence circuit, n—an elec- emitted neutrons rather than neutrons from fissions induced by
tronic circuit for determining totals T, reals plus accidentals (R external interrogation sources.
+ A), and accidentals (A) in a selected count time t (4, 5). The 3.14 pre-delay, n—the coincidence circuit has a pre-delay
terminology used in this test method refers specifically to immediately after a neutron has been detected to allow the
shift-register electronics. Fig. 1 shows the probability of amplifiers to recover and prepare to detect subsequent neutrons
detectinganeutronasafunctionoftimeandillustratesthetime (4). This principle is shown in Fig. 1.
intervals discussed. 3.15 singles (S), n—the singles are equivalent to the totals/s
3.11.1 totals, n—the total number of neutrons detected representing the total neutron detection rate.
during the count time. 3.16 triples (T), n—The triple neutron coincidence rate is a
3.11.2 reals plus accidentals, (R + A), n—the number of derived quantity obtained from the factorial moments of the
neutrons detected in the (R + A) gate period (Fig. 1) following multiplicity (R + A) and (A) histograms (1). It may be
the initial detection of each neutron (4). These events are due visualized as the count rate for three neutrons in coincidence.
to neutrons that are coincident with the given neutron (reals)
4. Summary of Test Method
and to neutrons that are not correlated with the given neutron
(accidentals). This is a measured quantity. 4.1 The item is placed in the sample chamber or “well” of
3.11.3 accidentals (A), n—the number of neutrons detected the multiplicity counter, and the emitted neutrons are detected
in the (A) gate period (Fig. 1) following the initial detection of by the He tubes that surround the well.
C 1500
4.2 The detected neutron multiplicity distribution is pro- 5.4.2 For certain materials such as small Pu items of less
cessed by the shift register electronics package to obtain the than 1 g, some Pu-bearing waste, or very impure Pu process
number of neutrons of each multiplicity in the (R + A) and (A) residues where the (a,n) reaction rate overwhelms the triples
gates. signal, multiplicity information may not be useful because of
4.3 The first three moments of the (R + A) and (A) the poor counting statistics of the triple coincidences within
multiplicity distributions are computed to obtain the singles (or practical counting times (12).
totals), the doubles (or reals), and the triples. Using these three 5.5 For pure Pu metal, pure oxide, or other well-
calculated values, it is possible to solve for 3 unknown item characterized materials, the additional multiplicity information
properties, the Pu-effective mass, the self-multiplication, is not needed, and conventional coincidence counting will
and the a ratio. Details of the calculations may be found in provide better repeatability because triple coincidences are not
Annex A1. used. Conventional coincidence information can be obtained
4.4 The total plutonium mass is then determined from the either by changing to a coincidence counter, or analyzing the
known plutonium isotopic ratios and the Pu -effective mass. multiplicity data in coincidence mode.
4.5 Corrections are routinely made for neutron background, 5.6 The mathematical analysis of neutron multiplicity data
cosmic ray effects, small changes in detector efficiency with is based on several assumptions that are detailed inAnnexA1.
time, and electronic deadtimes. The most important is the assumption that the item is a point in
4.6 Optional algorithms are available to correct for the space, so that neutron detection efficiency, die-away time, and
biases caused by spatial variations in self-multiplication or multiplication are constant across the entire item (16, 17).
changes in the neutron die-away time. 5.6.1 Bias in passive neutron multiplicity measurements is
4.7 Multiplicity counters are carefully designed by Monte related to deviations from the “point model” such as variations
Carlo techniques to minimize variations in detection efficiency in detection efficiency, matrix composition, or distribution of
caused by spatial effects and energy spectrum effects. Correc- nuclear material in the item’s interior.
tions are not routinely made for neutron detection efficiency 5.6.2 Heterogeneity in the distribution of nuclear material,
variationsacrosstheitem,energyspectrumeffectsondetection neutron moderators, and neutron absorbers may introduce
efficiency, or neutron capture in the item. biases that affect the accuracy of the results. Measurements
made on items with homogeneous contents will be more
5. Significance and Use
accurate than those made on items with inhomogeneous
5.1 Thistestmethodisusefulfordeterminingtheplutonium
contents.
content of items such as impure Pu oxide, mixed Pu/U oxide,
6. Interferences
oxidized Pu metal, Pu scrap and waste, Pu process residues,
and weapons components. 6.1 For measurements of items containing several hundred
5.2 Measurements made with this test method may be grams of plutonium metal or more, multiplication effects are
suitable for safeguards or waste characterization requirements not adequately corrected by this method (18). A variable-
such as: multiplication bias correction is required.
5.2.1 Nuclear materials accountability, 6.2 For items with high (a,n) reaction rates, the additional
5.2.2 Inventory verification (7), uncorrelated neutrons will significantly increase the accidental
5.2.3 Confirmation of nuclear materials content (8), coincidence rate. The practical application of multiplicity
5.2.4 Resolution of shipper/receiver differences (9), counting is usually limited to items where the ratio of (a,n) to
5.2.5 Excess weapons materials inspections (10, 11), spontaneous fission neutrons is about 7 (7).
5.2.6 Safeguards termination on waste (12, 13), 6.3 For measurement of large items with high (a,n) reaction
5.2.7 Determination of fissile equivalent content (14). rates, the neutrons from (a,n) reactions can introduce biases if
5.3 A significant feature of neutron multiplicity counting is their energy spectra are different from the spontaneous fission
its ability to capture more information than neutron coinci- energy spectrum.The ratio of the singles in the inner and outer
dence counting because of the availability of a third measured rings can provide a warning flag for this effect (19).
parameter, leading to reduced measurement bias for most 6.4 Neutronmoderationbylowatomicmassmaterialsinthe
materialcategories.Thisfeaturealsomakesitpossibletoassay itemaffectsneutrondetectionefficiency,neutronmultiplication
some in-plant materials that are not amenable to conventional in the item, and neutron absorption by poisons. For moderate
coincidence counting, including moist or impure plutonium levels of neutron moderation, the multiplicity analysis will
oxide,oxidizedmetal,andsomecategoriesofscrap,waste,and automaticallycorrecttheassayforchangesinmultiplication.A
residues (10). correction for capture in neutron poisons or other absorbers is
5.4 Calibration for many material types does not require not available, so that a bias can result in measurements of such
representative standards. Thus, the technique can be used for items.
inventory verification without calibration standards (7), al- 6.5 It is important to keep neutron background levels from
though measurement bias may be lower if representative external sources as low and constant as pr
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