LA-UR-02-2530
FRAM Isotopic Analysis of Uranium in Thick-Walled Containers Using
High Energy Gamma Rays and Planar HPGe Detectors
Thomas E. Sampson, Phillip A. Hypes, and Duc T. Vo
Safeguards Science and Technology, Group NIS-5
Nonproliferation and International Security Division
Los Alamos National Laboratory
Los Alamos, New Mexico 87545 USA
Abstract
We describe the use of the Los Alamos FRAM isotopic analysis software to make the first reported
measurements on thick-walled UF6 cylinders using small planar HPGe detectors of the type in common use at
the IAEA. Heretofore, planar detector isotopic analysis measurements on uranium have used the 100-keV region
and can be defeated by 10 mm of steel absorber. The analysis of planar detector measurements through 13-16
mm of steel shows that FRAM can successfully carry out these measurements and analysis in the 120-1024 keV
energy range, a range previously thought to be the sole province of more efficient coaxial detectors. This paper
describes the measurement conditions and results and also compares the results to other FRAM measurements
with coaxial HPGe detectors.
Introduction
The technique of gamma-ray isotopic analysis of arbitrary samples is desirable for measuring the
isotopic composition of uranium in UF6 cylinders because it does not require calibration with standards or
knowledge of the cylinder wall thickness.
The International Atomic Energy Agency (IAEA) uses the MGAU (Multi Group Analysis Uranium)
uranium isotopic analysis software [1] with planar high purity germanium (HPGe) detectors to measure the
isotopic composition of uranium. Measurements on UF6 cylinders with 13-16-mm thick steel walls are usually
unsuccessful because of the strong absorption of the 89-100 keV gamma rays and x-rays that MGAU requires for
the measurement.
This paper describes the use of the Los Alamos FRAM isotopic analysis software [2-6] to make these
measurements on UF6 cylinders. Uranium measurements with FRAM typically cover the energy range from
120-1001 keV and can easily be made through the walls of UF6 cylinders. While these measurements are
usually performed with efficient coaxial HPGe detectors, this paper reports the first successful measurements
using small planar HPGe detectors of the type in common use at the IAEA.
Background
Gamma ray isotopic analysis of arbitrary samples of uranium was first demonstrated at Los Alamos
over 12 years ago [7]. The technique used the same FRAM analysis software that had been developed for
plutonium isotopic analysis and measured uranium gamma rays in the energy range from 143 keV to 1001 keV.
The Los Alamos uranium measurements using FRAM were typically performed using a modest-sized
coaxial HPGe detector of about 50-mm diam by 50-mm thick with an efficiency of 25-30% in the usual
definition. This type of detector has adequate high-energy efficiency and still provides good resolution (< 750
eV at 122 keV) at lower energies.
MGAU measurements used by the IAEA are typically performed with planar HPGe detectors of the
same type used for its plutonium sister code, MGA [8]. These detectors can be various sizes but a common size
of 25-mm diam by 15-mm thick is widely used by the IAEA.
Requested Task
The task requested by the IAEA was to characterize the performance of FRAM for measuring the
isotopic composition of uranium under three constraints
1.
Must work through 13-16 mm of steel.
2.
Must use planar HPGe detectors in common use at IAEA (25-mm diam by 15-mm thick) and
possibly also 60-mm diam by 20-mm thick detectors.
3.
Must work with multichannel analyzers with a maximum of 4096 channels of data storage.
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LA-UR-02-2530
Previous FRAM Experience
Coaxial Detector
Los Alamos had previously demonstrated isotopic analysis of uranium through thick steel with coaxial
detectors and 8192 data channels. Figure 1 below shows the geometry of these laboratory measurements. The
samples were the EC-171/NBS-969 uranium reference materials certified for 235U isotopic abundance. The
detector was a Canberra coaxial detector with dimensions 60 mm diam by 42 mm long with 28.4% relative
efficiency, in the usual definition.
EC-171/NBS-969
071 (Natural U)
28% rel eff Coax HPGe
Fig. 1. Measurement
geometry with detector,
steel absorber, and sample
simulating the measurement
of a UF6 cylinder.
17 mm Steel
We made 15 replicate measurements on each standard in the set. The relative standard deviation of a
single measurement obtained from the distribution of the replicates is plotted in Fig. 2.
Fig. 2. Single measurement
precision of the 235U fraction
calculated from 15 replicates.
The five standards were
measured in 8192 channels with
a 28% relative efficiency
coaxial HPGe detector and
analyzed with the FRAM
software.
Large Diameter Semi-Planar Detector
We repeated these measurements using a detector that was 70 mm diam by 30 mm thick to see if the
larger detector diameter would improve the peak-to-background ratio at 258 keV.
We obtained the same results as for the coaxial detector. This is thought to occur because much of the
continuum underneath the 258-keV peak arises from bremsstrahlung, not Compton scattering.
Coaxial Detector Measurements on 30B UF6 Cylinders
Los Alamos has made measurements, analyzed with FRAM, on type 30B UF6 cylinders at the ABB
Atom fuel fabrication facility in Vasteras, Sweden [9]. A type 30B UF6 cylinder has 13-mm thick steel walls.
These measurements were made with a 26% relative efficiency coaxial detector in 8192 channels. The
measurements on 30B cylinders showed a measurement precision or repeatability of 6-8 % (1 RSD) for 20-30 min
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LA-UR-02-2530
measurements covering enrichments from natural to 4.7%. This is consistent with the results in Fig. 2 and even
better than Fig. 2 for natural uranium.
Considerations for Uranium Measurements with FRAM [9-10]
Coincidence Summing
One of the principal considerations is the sample-detector distance sensitivity of the measurement that
arises from coincidence summing. Vo and Sampson [5] have completed a solution for this problem that is
implemented in version 4 of FRAM. The planar detector data to be presented were analyzed with a version of
FRAM that incorporated these corrections.
Relative Efficiency Curve
Another potential difficulty for uranium isotopic analysis with FRAM occurs because of the large “gap”
in the relative efficiency curve between the 235U peaks at low energies and the 234mPa (238U daughter) peaks at
high energy. The coaxial detector data discussed above have all been successfully analyzed with the standard
empirical relative efficiency curve that has worked well in all FRAM versions to date. However, for weak
uranium spectra we feel that more robust relative efficiency formalism is appropriate. We have developed an
improved, physics-based relative efficiency curve [10] that greatly aids in spanning this gap and preserving the
correct analysis for the ratio of 235U/238U. This formalism has been used in the analysis of the planar HPGe data
discussed later.
The 258-keV Peak from the 234mPa Daughter of 238U
This peak plays an important role in FRAM for defining the relative efficiency curve for uraniumbearing materials. Data points from 235U at energies of 143-, 163-, 186-, and 205 keV usually define the lowenergy portion of the relative efficiency curve. The high-energy portion of the curve is usually defined by a series
of gamma rays from 234mPa beginning at about 700 keV with only the single point at 258 keV to make the
normalization between the high- and low-energy portions of the curve. The importance of the 258-keV peak in
establishing the relative efficiency curve is illustrated in Fig. 3.
Relative Efficiency
Fig. 3. The relative efficiency
curve for a depleted uranium
sample (0.31% 235U)
measured with a coaxial
HPGe detector. There was
no steel “shielding” present
for this measurement. This
relative efficiency curve uses
the improved physics-based
relative efficiency formalism
now incorporated in FRAM.
Energy (keV)
Also complicating the measurement is the very weak intensity of the 258-keV peak that also rides on an
intense background continuum. This peak is strongly attenuated by the steel walls of a UF6 cylinder and also
becomes vanishingly small and cannot be used for highly enriched uranium samples. Figures. 4–5 display the
relation of the 258-keV peak to the remainder of the uranium gamma ray spectrum. As an example Table I
displays (Net Peak/Background) data where Net Peak is the height of the 258-keV peak above background. The
attenuation at 258 keV caused by the steel is fairly consistent with that calculated from attenuation coefficients.
Table I. Net Peak/Background at 258 keV for Natural Uranium, Nominal
25% Relative Efficiency Coaxial HPGe detector @ 0.125 keV/ch
Conditions
Net Peak/Background
200 g U3O8, NBS 071, bare
1330 g U3O8, 16-mm steel
1330 g U3O8, bare
30B UF6 cyl, 13-mm steel
0.67
0.20
0.63
0.13
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LA-UR-02-2530
Fig. 4. A gamma ray
spectrum from a 200-g
sample of natural
uranium (235U = 0.71%)
as U3O8 with the 258keV peak designated.
Fig. 5. The 258-keV
gamma ray for several
different 235U
enrichments. The plots
are offset and the Y axis
is logarithmic. The
258-keV peak cannot be
used in the analysis of
highly enriched
uranium.
Measurements with a Planar HPGe Detector
Samples
We chose a set of uranium samples from NIS-5’s extensive inventory of well-characterized uranium
oxide samples [11, 12] with 235U enrichments from 0.3% to 10%. Their characteristics are given in Table II.
Table II. Characteristics of U3O8 Uranium Reference Materials
Sample
U3O8 Mass (g)
EC-171-031
A1-408-1
EC-171-194
EC-171-446
A1-324-1
200
1339
200
200
1170
wt%
234
U
wt%
235
U
wt%
236
U
wt%
238
U
0.0020
0.0049
0.0171
0.0359
0.0512
0.3166
0.7135
1.9420
4.4623
10.0863
0.0146
0.0010
0.0003
0.0068
0.0903
99.6668
99.2805
98.0406
95.4950
89.7722
The EC171 set of standards is also available under the issue NBL-CRM-969.
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Detector
The detector characteristics are given in Table III.
Table III. Planar HPGe Detector Characteristics
Manufacturer
Model
Serial No.
Bias Voltage
Diameter
Thickness
Resolution
122 keV, 3 µs, 1 kHz
122 keV, 1µs, 50 kHz
ORTEC
SH-SGD-25520-S
39-N21194D
2000 V Negative
25.4 mm
16.2 mm
509 eV
549 eV
Data Acquisition
Data acquisition and storage were provided by an ORTEC DSPEC Plus digital spectroscopy system.
The system was operated with a rise time = 4.0 µs, cusp = 0.6 µs, and flat top = 0.8. The gross counting rates
ranged from ~500 counts/s for the depleted U sample with 16-mm steel to ~1100 counts/s for the 10% enriched
sample with 13-mm steel.
We made 24 measurements on each sample at counting times (true time) of 60 min, 30 min, and 15 min.
Each sample-count time sequence was carried out with absorbers of 16 mm and 13 mm of steel. The measurement
geometry was identical to that shown in Fig. 1 except for the use of a planar detector. The measured results are the
average of the 24 individual measurements and the % RSD is the relative standard deviation from the distribution of
the 24 individual measurements. Tables IV-VI shows the results for the measurements at the three measurement
times with the two absorbers.
Table IV. Twenty-Four 60 Minute Measurements
16 mm Steel Absorber
Sample
EC-171-031
A1-408-1
EC-171-194
EC-171-446
A1-324-1
13 mm Steel Absorber
Accepted Measured
wt% 235U wt% 235U Meas./Accept. % RSD
0.3166
0.7135
1.9420
4.4623
10.086
0.2960
0.7009
1.9516
4.5219
10.271
0.9349
0.9823
1.0049
1.0134
1.0183
Average
0.9908
15.6
8.8
9.7
8.5
9.0
Table V. Twenty-Four 30 Minute Measurements
16 mm Steel Absorber
Sample
EC-171-031
A1-408-1
EC-171-194
EC-171-446
A1-324-1
Accepted
wt% 235U
0.3166
0.7135
1.9420
4.4623
10.086
1.1317
1.0153
0.9792
1.0388
1.0325
Average
1.0395
48.2
15.0
13.0
10.6
12.1
5
Meas./Accept.
% RSD
0.2955
0.7411
1.9533
4.5284
10.101
0.9333
1.0387
1.0058
1.0148
1.0015
13.8
9.5
6.6
6.6
7.7
Average
0.9988
13 mm Steel Absorber
Measured
wt% 235U Meas./Accept. % RSD
0.3583
0.7244
1.9016
4.6354
10.414
Measured
wt% 235U
Measured
wt% 235U
Meas./Accept.
% RSD
0.3049
0.6938
1.9344
4.5710
10.246
0.9630
0.9724
0.9961
1.0244
1.0159
18.1
12.2
8.0
9.3
5.7
Average
0.9944
LA-UR-02-2530
Table VI. Twenty-Four 15 Minute Measurements
16 mm Steel Absorber
Sample
EC-171-031
A1-408-1
EC-171-194
EC-171-446
A1-324-1
Accepted
wt% 235U
0.3166
0.7135
1.9420
4.4623
10.086
13 mm Steel Absorber
Measured
wt% 235U Meas./Accept. % RSD
0.3131
0.7231
1.9495
4.5950
10.093
0.9889
1.0135
1.0039
1.0297
1.0007
Average
1.0073
37.4
16.9
18.6
20.9
14.9
Measured
wt% 235U
Meas./Accept.
% RSD
0.3556
0.7100
1.9968
4.3954
10.453
1.1232
0.9951
1.0282
0.9850
1.0364
27.9
14.1
15.7
11.9
8.0
Average
1.0336
Discussion
These measurements on uranium using a planar HPGe detector operating at 0.25 keV/channel (4096
channels spanning 0–1024 keV) were very difficult for FRAM to analyze. The spectra were weak because of the
small detector. Even more important was the low number of channels/keV resulting in very narrow peaks at low
channel numbers for the high-resolution planar detector. The full width at half maximum for the 185 keV 235U
peak was less than 3 channels.
With the weak, narrow peaks it was not possible to perform a shape calibration on each spectrum. We
used a fixed set of default peak shapes for the analysis of these data. This is standard procedure for analyzing
weak spectra with FRAM. We also used a beta release of v4 of FRAM to analyze the spectra. FRAM version 4
has several features that enhanced the analysis of these data. It incorporates a coincidence summing correction to
reduce the sample-detector distance sensitivity as well as using an improved physics-based relative efficiency
function in the data analysis [5,9,10]. We found that both of these features were necessary for the analysis.
The measurement results were nearly unbiased with quite large counting statistics uncertainties,
especially for the lowest enrichments at the shortest counting times. Of the 720 measurements tabulated in Tables
IV-VI, only one measurement was discarded for an obviously bad analysis, a tribute to the robustness of the
FRAM software. The discarded measurement was for 16mm steel and a 15 minute count time on the depleted
235
U = 0.31 wt%--the worst case studied.
Figures 6–8 display the 130 to 200 keV region of a 15 min measurement through 16 mm of steel on
samples with 0.31, 1.94, and 10.1% 235U respectively. The depleted U (0.31% 235U) sample is on the edge of
being able to be analyzed successfully as one of the 24 run sequence did not analyze correctly. Note the
background continuum levels above the 185-keV peak. For low enriched samples we expect these to be fairly
constant with respect to enrichment as this level is generally proportional to the 238U content, arising mainly from
bremsstrahlung from 238U daughters. These levels are seen to be approximately the same for the 0.31% and
1.94% samples. The level for the 10.1% sample is about twice as high as for the lower-enriched samples, arising,
we believe, from the fact that the 10.1% sample contains over five times as much uranium as the lower-enriched
samples. This observation leads us to believe that measurements on actual UF6 cylinders containing depleted
uranium will be more difficult than for the mockup measurements reported here because of the UF6 cylinder’s
much greater uranium content.
We also observe that the measurement repeatability for the depleted uranium sample, 16-mm steel, and
15-min count time was over 35% (1 RSD). The 2σ error bars for 15-min measurements on the depleted and
natural uranium samples essentially touch but do not overlap. This is close enough to suggest that the ability to
perform discrimination between depleted and natural uranium using planar HPGe detectors should be tested in the
field on actual UF6 cylinders.
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LA-UR-02-2530
1500
235
U = 0.31%
Fig. 6. Region of the spectrum
from sample EC-171-031 with 235U
= 0.31%. Count time was 15 min.
Steel thickness was 16 mm. This
case was the most difficult to
analyze as the 143- and 163-keV
peaks are essentially not visible.
Counts
1000
500
0
130
140
150
160
170
keV
180
190
200
10000
235
U = 1.94 %
8000
Fig. 7. Region of the spectrum
from sample EC-171-194 with 235U
= 1.94%. Count time was 15 min.
Steel thickness was 16 mm. This
spectrum is easily analyzed, as the
143-and 163-keV peaks are easily
visible.
Counts
6000
4000
2000
0
130
140
150
160
170
keV
180
190
7
200
LA-UR-02-2530
60000
235
U = 10.1 %
Fig. 8. Region of the spectrum
from sample A1-324-1 with 235U =
10.1%. Count time was 15 min.
Steel thickness was 16 mm. This
spectrum is easily analyzed, as the
143-and 163-keV peaks are easily
visible.
50000
Counts
40000
30000
The photopeaks are more intense
relative to the 1.94% sample
because of the higher enrichment
and physically larger sample.
20000
10000
0
130
140
150
160
170
keV
180
190
200
Summary
We have documented the performance of FRAM on extensive sets of data from various uranium items
including the first successful measurements reported using a planar HPGe detector in the 120–1024 keV range.
These measurements were undertaken to show the feasibility of IAEA use of existing planar detectors and 4096
channel MCAs for measurements on UF6 cylinders with wall thickness between 13 and 16 mm.
We have demonstrated that FRAM can successfully analyze planar HPGe detector (25 mm diam x 16
mm thick) data taken through as much as 16 mm steel in 4096 channels at a gain of 0.25 keV/channel. The
measurements were demonstrated for 235U enrichments ranging from 0.3% to 10% and counting times as low as
15 min. Measurement repeatability at the 1 RSD level for 235U ranged from a low of about 6.5% (one hour count
time, 13 mm steel absorber, and 10% enrichment) to a high of approximately 35% (15 minute count time, 16 mm
steel absorber, and 0.3% enrichment). The latter measurement likely bounds the range of conditions under which
planar detector measurements can be successful.
In comparison we have shown coaxial detector (approx. 25–30% efficiency) data taken in 8192 channels
on some of the same samples. Coaxial detector measurements show significantly improved repeatability when
compared with planar measurements. With coaxial detectors, measurements through 16-mm steel in 10–20 min
show repeatability at the 1 RSD level in the range of 20% (0.3% enriched) to 8% (2–4% enriched). The coaxial
detector repeatability is about a factor of two better than that from the planar detector.
We conclude that although coaxial detectors are preferred, one can successfully make UF6 cylinder
measurements with 25-mm diam by 16-mm thick planar HPGe detectors in count times as low as 15 min with
enrichments as low as 0.3% 235U. We suggest that the extremes of this range, depleted uranium with 15-min
count times, need further field testing on actual UF6 cylinders.
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LA-UR-02-2530
References
[1]
R. Gunnink, W. D. Ruhter, P. Miller, J. Goerten, M. Swinhoe, H. Wagner, V. Verplancke, M. Bickel, and
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[12]
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9