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DESIGN AND PERFORMANCE OF A NEW HIGH ACCURACY
COMBINED SMALL SAMPLE NEUTRON/GAMMA DETECTOR*
R. Wellum and B. Brandelise
Commission of the European
Communities,
Institute for Transuranium Elements, ITU
P. 0. Box 2340, D-7500 Karlsruhe, Germany
Howard Menlove,
Los Alamos National Laboratory,
Los Alamos, New Mexico 87545 USA
D. Davidson, J. Verplancke, and P. Vermeulen,
Canberra Industries, Jomar Systems Division,
Los Alamos, New Mexico 87544 USA
K. Mayer
Commission of the European Communities,
Institute for Reference Materials and
Measurements, IRMM
B-2440 Geel, Belgium
H. G. Wagner
Commission of the European Communities,
EURATOM Safeguards Directorate,
L- 2920 Luxembourg
ABSTRACT
This paper describes the design of an optimized combined neutron and gamma detector installed around a measurement well protruding from the floor of a glove box.
The objective of this design was to achieve an overall
accuracy for the plutonium element concentration in gramsized samples of plutonium oxide powder approaching the
-0.1-0.2% accuracies routinely achieved by inspectors'
chemical analysis. The efficiency of the clam-shell neutron detector was increased and the flat response zone
extended in axial and radial directions. The sample holder
introduced from within the glove box was designed to
form the upper reflector, while two graphite half-shells
fitted around the thin neck of the high-resolution LEGe
detector replaced the lower plug. The Institute for
Reference Materials and Measurements (IRMM) in Geel
prepared special plutonium oxide test samples whose
plutonium concentration was determined to better than
0.05%. During a three week initial performance test in
July 1992 at ITU Karlsruhe and in long term tests, it was
established that the target accuracy can be achieved
provided sufficient care is taken to assure the
reproducibility of sample bottling and sample positioning.
The paper presents and discusses the results of all test
measurements.
INTRODUCTION
Nondestructive analytical methods are essential to the
EURATOM Safeguards Directorate's (ESD) concept for
on-site laboratories.1-2 The use of such methods reduces
*Work supported by the Commission of European
Committees and the US Department of Energy, Office of
872
Arms Control and Nonproliferation, International
Safeguards Division.
the quantity of analytical waste, and measurement results
are generally available 1-2 hours after receipt of samples.
Nondestructive methods, however, are generally not as
accurate as classical chemical analysis. ESD therefore
embarked on a special program to push the accuracy of
suitable nondestructive analysis techniques as close as
possible to that of chemical analysis. For the case of plutonium oxide powder samples, the typical operator-inspector differences in plutonium concentration are on the order
of 0.2% provided the uptake of oxygen and humidity is
carefully controlled and corrected.3''* The operator-inspector differences observed for the standard neutron coincidence counter for small samples, the inventory sample
counter (INVS)5-6 range from 0.6% to ~1%.7 The main
factors limiting performance of the INVS are the uncertainties in sample positioning and the effects of packaging
for samples contained in bottles within double polyethylene bags and the insufficient accuracy of the calibration
curves. In 1991 ESD placed a contract with Canberra to
modify the INVS counter to increase the detector efficiency, increase axially and radially the flat response
zones, include a clam-shell design to allow installation
around a measurement well protruding from the floor of a
glove box, and insert a high-purity germanium detector for
simultaneous measurements of high-resolution gamma
spectra. Jointly with Los Alamos National Laboratory and
ESD, Canberra designed a combined neutron/gamma detector, which was delivered to ESD in June 1992. In parallel
the European Commission's Research Center for transuranium elements, ITU Karlsruhe, had designed and built a
special glove box with a measurement finger and a sledge
arrangement to allow easy installation of the combined
detector.
The European Commission's IRMM at Geel prepared
and characterized special test samples of high-purity
plutonium oxide powder. These samples, ranging from 1
to 10 g of oxide powder and weighed into two types of
standard sample bottles, were measured repeatedly during
an initial test exercise of three weeks and in several campaigns since.
DETECTOR
DESIGN
To achieve the target performance and to allow the
detector to be installed below a glove box around a measurement well protruding from the floor of the glove box,
eighteen 3He tubes embedded in high-density polyethylene
were arranged in two concentric rings around the cylindrical counting cavity (Fig. 1). The rings were split asymmetrically into a clam-shell arrangement with external
hinges. The diameter of the counting cavity was set at
5.1 cm and the active length of the neutron detector tubes
was 39.4 cm. The 3 He gas pressure was 6 atm. The
detector efficiency was 40% and a gate setting of 128 (is
was used for the coincidence criteria. To satisfy fire
protection rules, the whole detector body is enclosed in a
stainless steel housing.
The gamma detector is a 500-mm2- upward-looking,
Canberra LEGe detector with a 19-mm-diam. cold finger
extending vertically upwards from the usual full-diameter
horizontal cold finger. Two graphite shells enclose the
cold finger to form the lower end plug of the neutron
detector. The detector capsule fits into a matching cut-out
in the polyethylene shell of the counting cavity (Fig. 2).
Sample bottles are fixed to the bottom of the sample
holder with a spring action clamp inside the glove box.
The sample holder is then lowered into the measurement
well. A hollow aluminum cylinder filled with graphite,
the sample holder forms the upper plug of the neutron
Upper neutron
counter mount
Lower neutron
counter mount
Slider mount (boll—bearing)
Co! d finger
Fig. 1. Diagram of INVS-FV and LEGe detector located underneath the sample handling glove box. The LEGe detector is
positioned near the center of the neutron detector.
873
ings along two rails screwed to the glove box support
frame. Similarly the dewar mount for the germanium
detector slides along the same rails. Docking trolleys
attach to either side of the rails to allow easy removal of
either detector. Mechanical interlocks reduce the danger of
the measurement well being sheared off by movements of
the closed clam-shell detector body.
NUCLEONICS AND DATA EVALUATION
Neutron pulses are collected and pre-processed in three
banks of AMPTEK amplifiers. Coincident neutron counts
are determined in a nuclear instrumentation module (NIM)
JSR 12 shift register. The gamma pulses are amplified and
shaped in a high-resolution spectroscopy amplifier
(Canberra model 2025) and converted in a matching analog
to digital converter (Canberra model 8077). The counters
to be installed in the on-site laboratories will have computer controlled NIM models (Canberra ICB series).
Neutron counts and gamma spectra are transferred to a
VAX computer running GENIE™ spectroscopy software
under VMS 5.3, through an AIM unit and a DECserver
90. They are analyzed by the Los Alamos National
Laboratory neutron coincidence counter code8 and the
Lawrence Livermore MGA algorithm.9-10 Canberra is
developing a special software package for integrated evaluation of neutron coincidence counts and gamma spectra
based on these two evaluation packages.
Fig. 2. Photograph of INVS-FV with the side-door
section open to expose the glove box sample-well and
the LEGe detector covered by a tin cap.
SAMPLES
detector. The mechanical re-positioning accuracy of the
sample bottles was better than 0.5 mm both axially and
radially with this arrangement
The measurement well is a flanged stainless steel
cylinder with a welded bottom plate. It is fixed to a matching flange on the glove box floor and the glove box is
sealed by an O-ring between the flanges. A rim within the
glove box and a thin aluminum liner protect the well from
contamination.
The detector is not shielded against neutron and
gamma background radiation beyond its own self-shielding. To minimize the effect of Compton scattering, both
the well and the gamma detector capsule are enclosed by
1—mm-thick tin caps.
The neutron detector body is fixed to the glove box
floor by a mounting ring and additionally supported by an
aluminum table. For easy positioning and removal of the
neutron detector, the aluminum table slides on ball bear874
Samples for calibrating and testing the on-site laboratory counter in the range from 1-10 g of Pu02 were prepared by IRMM from special reference material (EC-NRM
210). Before dispensing, the PuO2 powder was calcined at
1250 °C for 4 hours and then allowed to cool down over
night. Samples were weighed into specially prepared and
pre-weighed bottles: one series made of polycarbonate, the
other of stainless steel. Special precautions were taken
during weighing to avoid errors caused by buoyancy effects and electrostatic effects. Element content was certified to 0.05% relative (la). Additionally, the isotopic
abundances were determined through isotopic ratio measurements of purified aliquots of dissolved PuC*2 powder
on a Finnigan MAT 261 mass spectrometer. This instrument was calibrated directly with IRMM primary isotopic
reference materials. Sample weights and isotopic abundances are listed in Tables I and II.
N E U T R O N MEASUREMENTS
Problems with the gamma system caused most of the
test measurements to be neutron counts only. Samples
For one sample, we studied the effects of powder filling (tapped, untapped, deliberately loose powder), of
defined positioning errors (sample moved by 1 cm from
correct measurement position), and of adding up to 0.1
wt% of water to the sample.
TABLE I. Samples and Masses
Weight PuO2
Vial type
Vial No.
(R)
1
P
1.0050
P
2
2.0012
P
3
2.9903
4
P
4.0473
P
4.9907
5
P
6.0392
6
P
7
7.0203
P
8.0211
8
P
9.0056'
9
P
10
10.0215
11
S
1.0233
12
S
2.9976
S
13
5.0138
14
S
7.0200
15
S
8.9905
P: polycarbonate, S: Stainless-steel
Relative weighing errors (Is) were 10"4 for
all samples except numbers 1 and 1 1
which were 2- 1(H
The neutron counts of one sample were observed over
a long period from July 1992 to March 1993 to study the
long-term stability of the system.
GAMMA-RAY MEASUREMENTS
The few gamma measurements that could be carried
out at design resolution produced clean gamma spectra
with reasonable MGA fits. The resulting plutonium isotopic abundance vector however, showed biases of up to
1.3% depending on the exact measurement conditions. For
measurements outside the neutron detector well with a
lower counting rate, larger distance between sample and
detector, and less gamma scattering, the bias in 240Pu-eff
fell to 0.17%. The model to be installed in the OSL will
have a smaller LEGe crystal (200mm2) and the gamma
radiation from the sample will be collimated.
The detailed results from the gamma measurements
will be reported at a later date.
TABLE II. Isotopic Compositions of
Plutonium Samples (valid for 30 June 1992)
Isotope
Mass %
Accuracy
(2o)
238
0.0115
0.0002
239
93.446
0.0022
240
6.3168
0.0019
241
0.1667
0.0006
242
0.0390
0.0003
244
< 0.0001
RESULTS AND DISCUSSION
The results of the long neutron measurements on the
samples in the plastic vials are given in Table III. For
three of the samples, a second value is given, consisting
of the average of three short runs. The coincidence backgrounds were measured and subtracted from the data. These
background values were small: between 0.1 and 0.4 real
counts per second.
were introduced into the glove box in groups of approximately 15 g total weight and kept in borated, high-density
polyethylene containers with walls at least 2 cm thick.
Each sample was inserted for long (-12 hour) counts and
for short (~2 hour) counts repeatedly. The measurements
should thus reflect any sample re-positioning errors.
To detect and reject short counting rate excursions
caused by cosmic radiation or electric counts spikes, we
accumulated data as a series of 30-second counts and averaged them subsequently to "reals" and "totals" counts. If
the reals count deviated from the average by more than 2.5
standard deviations, that count was ejected from the data
set. 11
875
The reals counts of the 16 sample runs in Table III
were fitted using the Deming code to a second-order polynomial against the declared 240Pu-eff, and the results are
shown in Fig. 3.
The average absolute mass residual for the 16-point
data set was 0.183%, compared to the average statistical
uncertainty in the counting of 0.147%. We can conclude
that the error contributions from factors such as electronic
stability, positioning, sample geometry, and density are
less than 0.1%.
The short runs, repeated on six of the samples, gave
an average mass residual of 0.442%, comparing well with
the individual counting statistical errors of between 0.3%
and 0.5% (Table IV).
TABLE III. Long-Run Measurement Results on Polyethylene Vial Samples
Run
No.
Sample
ID
1
1
2
2
3
3
4
4
5
5
6
5
7
6
8
9
6
7
10
11
7
8
12
8
13
14
9
15
10
16
10
9
24
°Pu-eff
(9)
0.05669
0.11288
0.16868
0.22830
0.28149
0.28149
0.34066
0.34066
0.39600
0.39600
0.45245
0.45245
0.50799
0.50799
0.56529
0.56529
Netf
(counts/s)
Net ft
(counts/s)
6.253
12.899
19.565
26.892
33.603
33.450
40.970
40.900
48.148
48.010
55.482
55.27
62.232
62.750
70.108
70.220
40
80
121
163
201
(201 )b
239
(239)
288
(288)
329
327
368
369
410
(410)
Statistical
o
(counts/s)
0.015
0.023
0.039
0.035
0.040
0.070
0.044
0.077
0.051
0.104
0.074
0.056
0.060
0.060
0.067
0.093
|Avj =
o
(%)
0.24
0.18
0.20
0.13
0.12
0.21
0.11
0.19
0.11
0.22
0.11
0.10
0.10
0.10
0.09
0.14
0.147
Meas. Diff.a
(%)
+ 0.18
-0.11
-0.35
-0.12
+0.23
-0.20
+0.03
-0.13
+0.27
+0.00
+0.31
-0.05
-0.50
+0.28
-0.08
+0.07
0.183
a
The measured differences are the mass residuals from the calibration curve (quadratic
polynomial) compared to the tag mass.
'-'The (totals) are duplicates of the first run because of uncertainties in the totals background
from the glove box.
80
y E 15.28« + 116.18X • 0.3941
AV. ABS. MASS RESIDUALS = 0.183%
60
c
3
o
U
40
Fig. 3. Reals rate versus 240Pu-eff (g) calibration
curve for the INVS-IV.
UJ
20
0.10
0.20
240
0.30
Pu-eff
0.40
0.50
0.60
(g)
876
TABLE IV. Short-Time Measurement Results
No.
Sample
1
2
5
5
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
5
6
6
6
7
7
7
8
8
8
9
9
10
10
10
Netfl
(counts/s)
33.48
33.33
33.55
40.76
41.08
40.86
47.94
48.34
47.76
55.32
55.67
55.97
63.17
63.25
69.92
70.25
70.48
Gate. 240Pu
Calc. PuO2
Tag PuO2
(g)
(g)
(g)
0.2812
0.2800
0.2817
0.3391
0.3415
0.3399
0.3955
0.3986
0.3940
0.4526
0.4553
0.4576
0.5126
0.5132
0.5635
0.5659
0.5676
Diff.
(%)
4.985
4.990
4.964
4.990
4.994
4.990
6.012
6.039
6.051
6.039
6.026
6.039
7.011
7.020
7.066
7.020
6.985
7.020
8.024
8.021
8.072
8.021
8.112
8.021
9.087
9.006
9.098
9.006
10.022
9.990
10.032
10.022
10.062
10.022
Av absolute residual = 0.442
-0.102
-0.531
+0.076
-0.460
+0.276
-0.223
-0.127
+0.656
-0.508
+0.032
+0.629
+1.137
+0.908
+1.010
-0.318
+0.107
+0.408
130
For typical high-burnup samples, a neutron multiplication of less than 2% is expected, which can conveniently be included in the calibration curve. The samples
in this experiment however, have high ^Pu concentra0)
tions and fissile density and the multiplication is rela- o
120
tively high. Thus the 10 g PuO2 sample has a leakage CM
multiplication M of about 1.02 and a neutron coincidence
rate enhancement of -12%. A summary of the multiplication-corrected results for the 16 long runs is given in
110 o
Table V. R^. is the multiplication-corrected reals rate and
12
M is the leakage multiplication.
The value of Rmdg is almost constant for the entire
mass range. However the scatter of 1.3% is considerably
higher than the 0.183% for R values before multiplication
corrections (see Fig. 4). This arises from uncertainties in
the background counts from samples waiting to be
counted in the glove-box and other plutonium samples in
the laboratory. The background was uncertain to approximately ±2%. A plot of M as a function of sample mass is
shown in Fig. 5. It can be seen that the value of M increases rapidly for low masses as the sample shape progresses to a right-cylinder at about 5 g. For the tall samples, with heights greater than diameters, M is almost
constant.
877
° UNCORRECTED REALS
O MULTIPLICATION CORR.
+ 2%
O
-2%
100
0.0
0.1
0.2
0.3
Pu-240eff
0.4
0.5
0.6
(9)
Fig. 4. Reals rate per gram versus grams of 240Pu-eff
before and after the multiplication correction.
both
Both plastic (polycarbonate) and stainless-steel vials
were used in this experiment. Differences are expected
between the two types from the change in material end in
geometry. The steel vials were slightly narrower and
longer than the plastic vials. The masses of the samples
in stainless-steel vials were calculated from the neutron
samples. The neutron counting showed that the errors
from positioning, density, and electronic variation sources
are less than 0.1% and that slight variations in positioning and moisture content do not have an appreciable effect
on the results. The accuracy obtained under normal circumstances depends on the count time. Short counts
showed typical counting statistics of between 0.3% and
0.5%, depending on the sample weight; long counts (1012 hours) had average counting statistics of 0.147%.
1.03
Ox D
1.02
1.01
FILL HEIGHT=DIAM.
1.00O
0.99
0
1
2
3
4
5
6
7
8
9
1
0
1
1
Pu02 (g)
Fig. 5. Leakage multiplication M versus PuO2 mass showing
the saturation ofM at a mass of about 6 g.
counts using the fit to the plastic vials. The relative difference for the two types of vials is small (-0.84%) but it
is clear that separate calibration curves will be needed for
different containers.
One of the plastic containers, containing 4 g of plutonium has been regularly counted to determine the longterm stability of the counter. The same container was also
used to determine the effect on the reals counts from the
positioning and addition of water.
The addition of 0.1% water by weight, the displacement of the sample by 4, 6, and 8 mm, and the shaking of
the powder for density reduction did not perturb the measurements outside the statistical limits of -0.4% (Icr).
For the period from July 1992 to March 1993, there
was excellent long-term stability (0.44%,la) that was
consistent with the counting statistics.
For a subset of the total data set, a series of 10 consecutive counts of at least 10 hours counting time, taken
over a period of one month gave a standard deviation of
0.13%, entirely consistent with the calculated statistical
value of 0.126% for single measurements.
CONCLUSIONS
The combined neutron-coincidence/gamma counter,
designed for the on-site laboratories in Sellafield and La
Hague has been tested for the determination of plutonium
element concentration with a special set of standard PuO2
878
The neutron multiplication factor was relatively high
for these samples. For the samples to be expected at
Sellafield and La Hague, with higher burnups than the
present material, the multiplication factor will not be so
high. Individual, burnup-dependent calibration curves may
be called for. Further experiments will be carried out on
plutonium material with different isotopic composition.
This material will then be used to optimize the gamma
counting.
The gamma-ray isotopic results inside the INVS-IV
were degraded because of gamma scattering in the CH2
detector and the high-counting rates. Future gamma measurements will use a smaller LEGe detector combined
with additional shielding and collimation to reduce the
scattering and count rate problems.
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879