Post on 17-May-2023
Initial Evaluation for a Combined Neutron and Gamma1
Ray Multiplicity Counter2
Andreas Enqvista,∗, Sara A. Pozzib, Marek Flaskab, Imre Pazsita3
aDepartment of Nuclear Engineering, Chalmers University of Technology,4
SE-412 96 Goteborg, Sweden5
bDepartment of Nuclear Engineering and Radiological Sciences, University of Michigan,6
Ann Arbor, MI 48109, USA7
Abstract8
Multiplicity counters for neutron assay have been extensively used in ma-9
terials control and accountability for nonproliferation and nuclear safeguards.10
Typically, neutron coincidence counters are utilized in these fields. In this pa-11
per we present a measurement system that makes use not only of neutron (n)12
multiplicity counting but also of gamma ray (γ) multiplicity counting and the13
combined higher-order multiplets containing both neutrons and gamma rays.14
The benefit of this approach is in using both particle types available from15
the sample, leading to a reduction in measurement time when using more16
measurables. We present measurement results of n, γ, nn, nγ, γγ, nnn, nnγ,17
nγγ and γγγ multiplets emitted by a 252Cf source and a 239Pu-Be source. A18
dual radiation measuring system proposed here could use extra measurables19
either to improve the statistics when compared to a neutron-only system,20
or alternatively be used for extended analysis and interpretation of sample21
parameters.22
The study presented here provides an answer to the questions regarding23
the ability to measure both neutron and gamma ray coincidences at once.24
The results show that the measurements system proposed in this study has a25
potential to be valuable in the area of nuclear nonproliferation and homeland26
security.27
Key words: nuclear nonproliferation, pulse shape discrimination, liquid28
scintillator detectors, multiplicity counting, coincidences.29
∗Corresponding author.Email addresses: andreas@nephy.chalmers.se (Andreas Enqvist),
pozzisa@umich.edu (Sara A. Pozzi), mflaska@umich.edu (Marek Flaska),imre@chalmers.se (Imre Pazsit)Preprint submitted to Nuclear Instr. & Meth. A May 11, 2010
1. Introduction30
Neutron multiplicity counters have been used for several decades, mostly31
as thermal multiplicity counters based on 3He detectors [1] or as counters32
with slightly wider neutron energy ranges [2]. These systems have very good33
detection efficiency; however, the time resolution suffers from the required34
moderation of the neutrons. A number of new measurement options have35
recently become available with the advancements in applying organic liquid36
scintillation detectors within the areas of nuclear nonproliferation and mate-37
rials control and accountability and with the developments in fast digitizer38
technology.39
Organic scintillation detectors can provide some energy spectroscopy [3]40
as well as excellent die-away properties, thus reducing the number of ac-41
cidentals in measurements. Furthermore, the pulse shape can be used to42
discriminate between different types of incoming radiation. For example43
neutron pulses can be separated out from the gamma ray pulses. This means44
that data analysis techniques such as neutron spectrum unfolding from the45
measured pulse height distributions (PHDs) [4, 5] can be applied, even in46
the presence of gamma ray radiation.47
Recently, liquid scintillation detectors have been investigated for use in48
neutron multiplicity counters [6]. The main benefit of such a design is that it49
is much less sensitive to accidental coincidences than the traditional approach50
based on 3He detectors. Thus, the liquid scintillators are suitable for use51
even on samples with high single-neutron background, such as (α, n) rates52
exhibited in some samples containing the isotopes of plutonium. However,53
Frame et al. [6] estimated that such a system will have problems with neutron54
classification at energies below 2 MeV, as well as having a low total detection55
efficiency. Such problems might be corrected with alternative designs of the56
detector. It can be noted that all gamma ray pulses would be ignored in this57
type of design.58
Fission multiplicity counters [7] use systems of plastic scintillation detec-59
tors sensitive to both fast neutrons and gamma rays. However, these coun-60
ters offer no possibility of discriminating the gamma rays from the neutrons,61
thus only the total fission multiplicity is measured, rather than a neutron62
or gamma ray multiplicity. More recent developments, especially those using63
the Nuclear Materials Identification System [8], allowed temporal discrimina-64
tion of gamma rays from neutrons to further enhance the analysis of nuclear65
materials.66
2
We suggest a novel approach for finding sample characteristics based on67
the use of multiple liquid scintillators, a fast digitizing system, and advanced68
analysis algorithms. The proposed system uses pulse shape discrimination69
(PSD) [9] to determine what particle caused the detection pulse. The excel-70
lent timing properties of liquid scintillators are used to find out what pulses71
are correlated in time and the particle types that created them. This ap-72
proach has the benefit of providing more measurables compared to the case73
of pure neutron multiplicity counting, while having a few extra parameters -74
but not more than the amount of added measurables - which could be used75
in a number of ways to determine the sample characteristics [10, 11, 12].76
This paper describes the measurement system used for performing multi-77
plicity measurements using neutron and gamma ray sources such as 252Cf and78
239Pu-Be. The approach builds upon previously described cross-correlation79
measurements [13, 14]. The PSD will be described in detail as one of the80
key parameters for the investigation of the proposed approach. The results81
of initial assessment will then be discussed and future possible improvements82
along with potential drawbacks of the proposed system.83
2. Description of Experimental Setup84
The measurements were performed at the University of Michigan, Depart-85
ment of Nuclear Engineering and Radiological Sciences - where the measure-86
ment system was developed - in July 2009. The measurement system consists87
of four EJ-309 cylindrical scintillation detectors with a height of 13.3 cm and88
a diameter of 13 cm. This liquid has a high flash point (144 ◦C), and low89
chemical toxicity.90
In the measurements (Fig. 1), the detectors were placed at 90 degrees91
with the source-detector distance kept constant. The data acquisition is92
performed with a 250-MHz, 12-bit, 8-channel CAEN V1720 digitizer. Four93
channels were used, each being able to trigger independently on the incoming94
detector pulses. The data from all channels are stored and then subsequently95
analyzed to determine if any additional detectors, in addition to the trigger96
detector, contain time-correlated pulses. The detectors were calibrated using97
a 137Cs source.98
A threshold of approximately 70 keVee corresponding to a neutron energy99
deposited of approximately 450 keV, was applied to achieve excellent PSD100
performance. The 70-keVee threshold was chosen based on previous investi-101
gations focused on the accuracy of particle discrimination. At this threshold,102
3
Figure 1: Measurement setup.
a gamma rejection of approximately 1/1000 is expected. The 252Cf source103
was placed at different detector distances to determine the effect of changing104
absolute detector efficiency, while the Pu-Be source was only used at a larger105
distance with 5 cm lead shielding to suppress high gamma ray production.106
The count rates when using all four detectors varied from approximately107
4.5 kHz to 15 kHz, with the background being approximately 1 kHz. The108
measurement setups were as follows:109
•252Cf, 15 cm source–detector distance,110
•252Cf, 20 cm source–detector distance,111
•252Cf, 30 cm source–detector distance,112
• Pu-Be shielded with 5 cm of lead, 50 cm source–detector distance.113
The measurement times varied between 10 and 30 minutes resulting in ap-114
proximately 10 million recorded events per setup.115
3. Pulse Shape Discrimination and Pulse Timing116
PSD and pulse timing are performed with an offline algorithm. Tradi-117
tionally, PSD can be performed based on charge integration or differentiation118
[15, 16]. In previous work, we have used charge integration and analyzed the119
tail-to-total integral ratio versus pulse height [14]. In this work, the total120
4
integral is calculated from the start of the pulse to 220 ns past the pulse121
maximum, while the tail integral is calculated from 20 ns to 220 ns past the122
pulse maximum. These values were previously determined by analyzing the123
figure of merit for the PSD as a function of the time ranges for the pulse124
integration.125
A linear discrimination boundary is used to discriminate neutrons from126
gamma rays (Fig. 2). This method show high accuracy, but can unfor-127
tunately not be quantified with simple figures of merit (FOM) due to the128
nonlinear discrimination curve chosen. The FOM is defined as follows: FOM129
= distance between peaks of neutron and gamma distributions / (FWHMγ130
+ FWHMneutron)131
Pulse timing is performed on the digitized pulse data by simulating a132
constant fraction discriminator where the the start of the pulse is defined as133
20% of the peak value. The timing is then less sensitive to the pulse am-134
plitude. By using interpolation between the data values allows for increased135
time resolution from the initial 4 ns to approximately 1 ns. The sharp rise136
of the pulse edge means linear interpolation can be used without large losses137
of accuracy. The gamma ray–gamma ray peak in the cross-correlation mea-138
surements was used to test the timing accuracy and a FWHM of less than139
1.5 ns was observed.140
4. Neutron and Gamma Ray Multiplicities141
The benefit of the proposed system is its speed and robustness which is142
due to the fact that several quantities are measured (nine multiplets up to143
the third order). Further, the system operates on short time scales (300 ns144
is the width of a typical pulse) due to the removal of neutron moderation145
when compared to a 3He neutron multiplicity counter. While a 3He counter146
design usually encompasses the entire sample to maximize the neutron mod-147
eration and consequently the detection efficiency, a scintillation system could148
be made with a sparse layout and thus be a candidate for a portable system.149
Such a design requires that detection rates of multiplets are sufficiently high150
even with relatively low absolute detector efficiency. For allowing the unfold-151
ing of sample parameters such as proposed in [11], successful measurement152
systems for all the multiplets need to be devised.153
5
0 2 4 6 8 100
0.5
1
1.5
2
2.5
Total integral
Tai
l int
egra
l
(a) Full range.
0 0.2 0.4 0.6 0.8 1 1.20
0.1
0.2
0.3
0.4
0.5
Total integral
Tai
l int
egra
l
(b) Zoomed in.
Figure 2: Tail vs. total integrals for a 252Cf measurement. The data shown are for asingle EJ-309 detector, and the discrimination line indicates which pulses are created byneutrons (above the line) and by gamma rays (below the line).
6
0 100 200 300 400 500 600 700−0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Time (ns)
Am
plitu
de (
Vol
ts)
Pulse 1Pulse 2Pulse 3
Figure 3: Sample pulses from the data-acquisition system.
4.1. Measurement Results and Discussion154
The measurements were performed with a 252Cf source producing 19500155
fissions/second [17] and a shielded 1-Ci Pu-Be source [18]. All pulses that156
seem to be a real coincidence, considering time of flight for the neutrons and157
the total size of the system (up to approximately 50-ns difference), are fully158
registered with the digitizer in a window of 400 ns (100 sampling points).159
Table 1 shows the measured rates of multiplets for various source–detector160
distances from the 252Cf source. The results indicate that for measurement161
times in the order of a few minutes good accuracy for the singlet and doublet162
rates can be achieved.163
4.2. Measurements with 252Cf164
The rates of the different multiplets for the 252Cf source are shown in Fig.165
4. It is evident that the source–detector distance has a large impact on the166
higher-order multiplets, which increase by more than an order of magnitude167
when the distance is decreased from 30 cm to 15 cm. With reduced distance,168
the rate of the higher-order multiplets increases more than the total count169
rate at the largest distance. Optimal rates are considered those where the170
measurement uncertainty for the high-order multiplets is low (acceptable)171
for reasonably short measurement times. With detection rates for gamma172
rays being higher, this might indicate that the minimum detectable activity173
could be reduced when also accounting for gamma multiplets compared to174
pure neutron multiplicity.175
7
Count rates (s−1) and statistical errors:30cm ±1σ 20cm ±1σ 15cm ±1σ
Rn 690.4 0.63 1338.6 0.95 1947.4 1.36Rγ 3119.9 1.34 5335.6 1.90 7256.6 2.62Rnn 13.3 0.09 51.5 0.19 114.3 0.33Rnγ 52.3 0.17 197.3 0.37 409.3 0.62Rγγ 63.4 0.19 227.7 0.39 455.0 0.66Rnnn 0.1 0.01 0.8 0.02 3.0 0.05Rnnγ 0.6 0.02 5.5 0.06 18.7 0.13Rnγγ 1.5 0.03 12.0 0.09 38.7 0.19Rγγγ 1.2 0.03 8.8 0.08 27.0 0.16
Measurement time (s):T 1735.8 1475.3 1054.6
Table 1: 252Cf count rates for different source–detector distances and all possible neutron,gamma ray and mixed multiplets up to the third order. The statistical error is calculatedas the square root of the total counts divided by the total measurement time.
Ratios:30cm 20cm 15cm
Rn/Rnn 52.1 26.0 17.0Rn/Rnnn 6809.3 1669.3 658.2Rnn/Rnnn 130.7 64.2 38.6Rγ/Rγγ 49.2 23.4 15.9Rγ/Rγγγ 2646.8 606.1 268.4Rγγ/Rγγγ 53.7 25.9 16.8Rn/Rγ 0.22 0.25 0.27Rnn/Rγγ 0.21 0.23 0.25Rnnn/Rγγγ 0.09 0.09 0.11
Table 2: Different 252Cf ratios of the count rates could be used to obtain geometricalinformation.
8
n γ nn nγ γ γ nnn nnγ nγ γ γ γ γ10
−1
100
101
102
103
104
Multiplet
Rat
es
15cm20cm30cm
Figure 4: Detection rates of different multiplets at different distances for the 252Cf source.
Table 2 reports a number of the ratios of rates that could be of interest,176
such as ratios between different multiplets of neutrons, which show the strong177
dependence on absolute detector efficiency with varying distance. More intri-178
cate ratios such as those between the same multiplet of neutrons and gamma179
rays could be used for possible PSD error analysis (see below). In the mea-180
surements it has been noted that the neutron doublets are clearly depending181
on the orientation of the detectors, with opposing detectors recording more182
than 50% more doublets compared to adjacent detectors. This corresponds183
well with the findings in Ref. [19].184
The values of the ratios Rn/Rγ , Rnn/Rγγ , Rnnn/Rγγγ should be also em-185
phasized, because they are almost fully independent of the source–detector186
distance. However, they do change depending on if they are the singlet, dou-187
blet, or triplet neutron to gamma ray ratio, since the number of gamma-rays188
and neutrons produced per single fission significantly varies. Specifically, up189
to 20 gamma rays, 10 neutrons can be emitted from a single fission, which190
can significantly change the above mentioned ratios. With the spontaneous-191
fission distribution of different isotopes being different, this set of ratio values192
could be used for isotopic determination. The singlet ratios also depend on193
the existence of (α, n) events, which needs to be taken into account for the194
potential identification of the isotopes present in the measured sample.195
One particular character of the multiplets is the behaviour of their mag-196
9
nitude when neutrons are successively replaced by gamma photons in the197
same order of multiplet. This can be easily seen in Table 1 and especially198
in Fig. 4, since the values are arranged just in this order. For the doublets,199
starting with the pure neutron doublet, replacing a neutron with a gamma200
always leads to a higher value, although the increase is smaller in the second201
step than in the first. For the triplets, the change of the magnitude is not202
monotonic; the magnitude of the pure gamma triplet rate is smaller than203
that of the nγγ triplet rate. This shows the special value of the use of the204
mixed rates, as it was also mentioned in [11, 12].205
4.3. Measurements with Pu-Be206
Measurements were performed with a strong Pu-Be source, which emits207
one neutron per (α, n) reaction accompanied with one or more gamma rays.208
The source also emits a large number of uncorrelated gamma rays, which was209
the reason for placing 5 cm of lead shielding around the source. Measurement210
results are given in Table 3: as can be noted, very few multiplet above the first211
order are detected. Even nγ doublets which should have high frequency in212
connection to the (α, n) events, are severely suppressed by the Pb shielding.213
Table 3 shows that the singlets of neutrons and gamma rays are very dom-214
inating, while triplets are essentially non-existent due to the low number of215
correlated events. As expected, measured singlet-to-doublet and singlet-to-216
triplet ratios are much lower for the Pu-Be source than for the 252Cf source,217
which is caused by lack of time-correlated detection events. These quanti-218
ties could be used as a quick estimate of sample types, such as fissile versus219
non-fissile. In the case of the fissile 252Cf source, the triplet rates are approx-220
imately a factor 1000 lower than the singlet rates (see Rn/Rnnn and Rγ/Rγγγ221
in Table 2), while for the Pu-Be source, high-order correlated events are222
almost nonexistent, and occur a factor 1,000,000 times more rarely than a223
single event. In addition, other source properties such as ratios of gamma224
rays to neutrons for different radiation shields that would indicate the source225
type could be obtained. The multiplicity system discussed here could be226
‘calibrated’ to allow identification of various neutron and gamma ray sources227
and nuclear materials. The additional benefit of measuring also gamma rays228
could depend on the assayed material, making the approach more suitable229
to some types of measurements.230
10
PuBu source:Rate ±1σ
Rn 8160.3 3.2494Rγ 3283.3 2.0611Rnn 7.0647 0.0956Rnγ 10.571 0.1170Rγγ 3.9179 0.0712Rnnn 0.0013 0.0013Rnnγ 0.0116 0.0039Rnγγ 0.0078 0.0032Rγγγ 0.0103 0.0037
Table 3: Measured multiplet rates for the Pu-Be source. The measurement time was 772seconds.
n γ nn nγ γγ nnn nnγ nγγ γ γ γ10
−4
10−2
100
102
104
Multiplet
Rat
es
Pu−Be, 50cm
Figure 5: Detection rates for different multiplets for the Pu-Be source.
4.4. Analysis of PSD error231
Assume that the count rates are related as Rnnn = 0.1Rnnγ = 0.1Rnγγ =232
0.1Rγγγ , which is only a rough estimate of what was observed in measure-233
ments. It means that if 1% of all pulses are misclassified the value of Rγγγ234
will be underestimated by 2%, while the Rnnn value will be overestimated by235
7%. Similarly, if the PSD error is 5% then the value of Rγγγ could be off by236
9.5%, while the Rnnn value could be off by 33%. The values are related as237
11
follows:238
Rnnn = R′
nnn(1 − ǫ)3 + R′
nnγǫ(1 − ǫ)2 + R′
nγγǫ2(1 − ǫ) + R′
γγγǫ3, (1)
239
Rγγγ = R′
γγγ(1 − ǫ)3 + R′
nγγǫ(1 − ǫ)2 + R′
nnγǫ2(1 − ǫ) + R′
nnnǫ3, (2)
where ǫ is the PSD error and R′
xxx are the real rates and Rxxx are the mea-240
sured rates.241
When considering a pure neutron multiplicity assay, the resulting errors242
on the measured rates would be high if the PSD error was very high, but243
with the additional information available, such as the additional count rates244
and ratios from the proposed method, it is possible to calculate correction245
factors for the Rn, Rnn, Rnnn values. Designing calibration measurements246
that analyze the PSD error would be beneficial for further development of247
the applicability and accuracy of the proposed system.248
5. Summary and Conclusions249
The measurement system presented in this paper uses techniques that,250
when combined together, create a novel way of characterizing materials using251
both neutron and gamma ray multiplets. A study was performed to investi-252
gate the feasibility of the method, and to gain some insight on whether it is253
an applicable approach to materials accounting and identification.254
The measurement system also shows potential in accuracy of correctly255
differentiating the neutron pulses from the gamma ray pulses. This capa-256
bility is vital for the correct classification of mixed multiplets. The initial257
tests have shown that the system can acquire rates of n, γ, nn, nγ, γγ, nnn,258
nnγ, nγγ and γγγ multiplets in a relatively short measurement time for a259
252Cf source with 19500 fissions/s and a 1-Ci Pu-Be source. In the future,260
several aspects will be further investigated, including the number and con-261
figuration of detectors, and the possible utilization of shielding to avoid cross262
talk between detectors.263
It has been shown that a number of characteristic values can be obtained264
and ratios of the measured values can be used to assess the type of reactions265
causing the particle emission such as fission versus (α, n) reaction. The266
measured rates obtained in the measurements described in this paper are of267
satisfactory level for the simple design of the current system. The analysis268
shows that the ratios between rates could also be utilized as an indicator of269
source type and possibly also source isotope composition, once the analysis270
is extended further in connection to multiplicity theory.271
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6. Acknowledgments272
The work of Andreas Enqvist was supported by the Swedish Radiation273
Safety Authority.274
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