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Plutonium Isotopic Composition by
Gamma-Ray Spectroscopy
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Plutonium Isotopic Composition
by
Gamma-Ray Spectroscopy
S.-T.HtiM
T.
E.
Sampion
J.
L.
Parker
S.S.Johnson
D.
F.
Bowersox
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PLUTONIUM ISOTOPIC COMPOSITION
8V
GAMMA-RAY SSECTROSCOPY
S.-T. Hsue,
T. E.
Sampson,
J. L.
Parker,
S. S*
Johnson, and 0.
F.
Bowersox
ABSTRACT
Me discuss the general approach, computerized oata analysis
methods, and results of measurements to determine the isotopic compo-
sition of plutonium by gamma-ray spectroscopy. The simple techniques
are designed to be applicable to samples of arbitrary size, geometry,
chemical and 'isotopic composition that have attained; 2*lp
u
_237(j
equilibrium. The combination of the g&mma spectroscopic measurement
of isotopic composition
7
coupled with caioriaetric measurement of
total sample power is shown to give a totally nondestructive deter-
mination of sample Pu mass with a precision of 0.6k for 2O00-Q sam-
ples of PUO2 with 12X 24qp
u
content. The precision of isqtopic meas-
urements, depends upon many factors including sample size, sample
geometry, and isotopic content. Typical ranges are found to be
238p
U|
<
1.10X;
23*>
u
0.1-0.5%; 240p
u
, 2-5X: 241pu, 0.3-0.7*;"
(oetermined by isotopic corfelation); and
241/*,,
0.2-10%.
I. INTRODUCTION
Work
at
Los Alamos Scientific Laboratory (LASL)
in the area
of
plutonium isotopic analysis
by
gamma-
ray spectroscopy started
in
1974 when
0. L.
Parker
and
T. 0.
Reilly outlined
a
methoo using ratios
of
neighboring peaks with relative efficiency correc-
tions determined from
the
spectrum under stuoy.
These techniaues have received wide acceptance
and
have been
put
into routine
use at
several labora-
tories.
2
'
3
Results from
an
interiaboratory
com-
parison have been published.* Different labora-
tories generally
use the
same techniques with
the
main di fference being: the methods used
for
deter-
mining photopeak areas.
A somewhat different approach
has
been used
by
Gunnirk
5
who
fits! the complex 100-keV region with
response functions
of
each isotope. This method
has been used very successfully
for
the case
of di-
lute solutions for which attenuation corrections are
small.
Gunnink and coworkers,
6
'
7
and Cowder
et
al.,
8
have used gamma rays
in the <
60-keV region
for
isotopic analysis
on
frsshly separated solutions
in
the 100
to
300-gA range. Use of .•gmm rays
in the
< 60-keV region
has
also been discussed
by
Unezama
et
al.
9
Techniaues similar
to
Reference
1
have been
de-
scribed
by
Dfagnev
and
coworkers,
and
Reilly
etal.
13
; .- . '
}
/
At LASL
the
press
of
other projects
has pre-
venteo
us
from fully developing the methods proposed
over six years ago. Only
in
the past year has there
been
a
mandate
to
refine these techniaues for appli-
cation
at
LASL. The work reported here will enable
routine application
of
this method for verification
of
Pu
isotopic composition
ana
determination
of Pu
isotopic composition
for
proper interpretation
of
total
Pu
measurements
by
calorimetry
or
neutron co-
incidence counting.
The procedures
to be
described reouire
no
peak
fitting thus minimizing computer core and speed
re-
auirements.
Little training
is
nee
ed for
the me-
thod
to be
routinely used
by
technicians. The me-
thod represents
a
simple, and perhaps the aret ver-
satile single detector approach
to Pu
isotopics.
In addition
to the Pu
isotopic fractions
the
procedure also computes specific power
for
inter-
pretation
of
calorimetry mamureaanU, and effective
2
*°Pu for neutron coincidence counter application*.
Algorithms are included that produce reasonably good
estimates
of the
statistical precision
of
these
Quantities.
II.
GENERAL APPROACH
The philosophy that
has
governed this approach
is one
of
simplicity.
We
wish
to
see
how far
this
technique
can be
taken using
the
simplest data
ac-
auisition
and
analysis techniques
and yet
have
the
widest applicability
to
arbitrary sample configura-
tions.
The atom ratio
of
isotopes
1
and
2 is
determined^
from
a
gamma ray spectrum
by
means
of
N,
C,
(1)
where
N - Number of atoms of indicated isotope
C - Photopeak cants from selected gamma ray
from indicated isotope
T
l/2 - Half-life of indicated isotope
B - Branching ratio of selected gamma ray from
indicated isotope
RE - Relative efficiency at selected gamma ray
energy including geometry, sample self
absorption, attenuation, and detector ef-
ficiency .
Half lives and branching ratios are taken from
the literature.
14
'
15
The relative efficiency is
determined from the spectrum under study by deter-
mining the quotient of the photopeak counts and the
branching ratio for a series of ganma rays from one
of the isotopes in the sample.
239
Pu,
241
Pu,
and
241
Pu-
237
U gamma rays are used in this method with
the
241
Pu and
241
Pu-
237
U relative efficiency points
being normalized to those from
239
Pu.
Photopeak areas are determined by region of in-
terest (HOI) summation.
16
Background regions are
selected above and below each photopeak. A linear
background is interpolated under the photopeak from
the centroids of the background regions. Background
regions are carefully selected to avoid neighboring
peak interferences, particularly from
241
Am which
can vary greatly fro* sample to sample.
Digital gain and zero stabilization is used in
the oata acquisition cltctronics. This is important
to ensure that the peaks don't wander out of their
assigned flOIs. The 129.3 keV and 413.7 keV peaks
239
of
Pu
are used for zero and gain stabilization.
The ROI summation method puts great emphasis
on
good detector resolution
in
order
to be
able
to
resolve
the
peaks
of
interest from close lying
neighbors.
A
high resolution planar detector
of
1200
mm
2
x 10 mm
deep
is
used
for
these measure-
ments.
Detectors
of
this type can have resolutions
of
<
500
eV at
122
keV
although the detector used
in
this program has
a
resolution
of
540 eV. The best
possible resolution should
be
utilized.
The
major
disadvantage
to
this detector type
is
its
lorn
effi-
ciency
at
higher energies. With such
a
planar de-
tector
one is
unable
to
utilize
the
potentially
useful 600 keV region.
The analysis routines propagate the statistical
uncertainties\ in
the
photopeak areas
to
give
the
statistical uncertainties
in
the final
Pu
isotopics.
Because
the
isotopic ratios
are not
completely
in-
dependent,
the
error propagation
may not be
rigor-
ously exact.
It
has proven, however,
to be a
useful
estimate
of
the observed precision
of
repeated runs
on the same sample.
The techniques discussed here are applicable
to
a very wide range
of
sample types. Sample size
is
only limited
by
count rate
and
counting time con-
siderations. Samples
as
small
as
0.25
g
have been
measured; however,
a few
grams seems
to be a
mere
practical minimun size. Sample geometry has
a
great
influence
on
count rate
and can be
more important
than
Pu
mass alone. Samples
can be as
large
as
criticality considerations allow. For large samples
count rates
are
tailored
by
choice
of a
suitable
collimator.
We
attempt
to
keep count rates
at
about
10 kHz
as a
compromise between optimal data collec-
tion rates .(higher than
10
kHz) and best resolution
(lower).
Counting times are influenced
by
the sta-
tistical precision desired
and
the ultimate appli-
cation
of the
isotopic results. Simple verifica-
tions of, say, the 239/241 ratio may take only
a few
minutes.
Applications that require the
23a
Pu and/or
2
*
t
Vu isotopics generally require
at
least several
hours.
Cd
and Ta
filters are usad
to
reduce the count
rate from
^Am at
59.5 kev
and the
100
ke¥
x-ray
complex
in
order
to
remove
any
pile
up
pamks
Tim
the 150-165 keV region.
The specific algorithms
OISCUSSM
in this report
apply only & samples which have attained
2
*W*
237
U
equilibrium. The 6.75 day half life of
a
'\S con-
trols this time with 99% of equilibrium being
reached 45 aays after chemical separation of the
237
U.
SimiltarsRalyjsi^methods can be used for npn-
eauilibriu*
materials but"arfeKflo4.discussed here.
The methods are applicable to samples of.,
arbi-
trary composition and geometry containing Am and up
to MO uCi/g Pu of fission products. Me have not
yet examined mixed U-Pu samples. Some modifications
to these methods may be necessary if significant
amounts of
235
U are present.
One additional requirement for this method is
that tf4 isotopic distribution of all Pu in the
sample must be homogeneous. The sample itself may
contain a nonhomogeneous Pu distribution, but all
Pu should have the same isotopic composition.
One weakness of the current methods is that the
low efficiency of the planar detector does not en-
able one to use the information available from the
600-keV region. This region can potentially provide
increased precision for iPu in larger samples,
a needed improvement for calorimetry and coincidence
counting applications. The precision of the deter-
mination of low concentrations of *Am would also
improve using the 600-keV region; however, this is
not vital for calorimetry interpretation because Am
contributes only a few per cent of the total power
for concentrations below 1000 ppm. Use of a second,
larger detector
2
violates our ground rules of
simplicity and low cost. A single large detector
could measure both the high energy and low energy
regions simultaneously, but the poorer resolution
of a larger detector makes the simple HOI summation
method.less applicable to the low energy data from
a large detector than that from a higher resolution
planar.
The algorithms to be described represent a sim-
ple,
and perhaps the most versatile single detector
approach to Pu isotopics. Even if the 600 keV re-
gion were utilized it would not be applicable to
samples with incomplete fission product separation,
because of gamma ray interferences, or to small
samples, because of intensity problems.
III.
ANALYSIS METHOD
Data is accumulated in Canberra Series 80 MCA
interfaced to a PDP-11 series computer operating
under RT-11 version 38. All analysis routines are
written in FORTRAN, and are integrated into a ver-
satile user-oriented package suitable for either
production or R and D work.
• Relative efficiency values are calculated from
peak area/branching ratio for ' Pu lines at
129.3,
143.4 • 144.2, 171.3, 179.2,
189..J,
195.7,
203.5, 255.4, 297.5, 345.0, 375.0, and 413.7 keV.
All relative efficiencies are normalized to a value
of 1.0 at 413.7 keV. Not all of these points are
used in the suhseouent analysis.
Next the
9
Pu relative efficiency values at
345.0 and 375.0 keV are linearly extrapolated to
give values at 332.4 and 335.4 keV. The peak comp-
lexes at 332 and 335 keV contain contributions from
2Z
»W
237
U;,
241
Am,
and
239
Pu.
The
239
Pu component
is subtracted from both complexes using the
345.0-keV
239
Pu line. The remaining peak areas at
332 and 335 keV contain contributions from
24il
Pu-
237
U and
24i
Am.
Assuming
241
Pu-
237
U eouilibrium the
two peak areas and two isotopic unknowns are used to
solve for the * '•fmn T>U ratio. This
L
(m/
24i
Pu ratio is used to correct other
2ill
Pu-
237
u
peaks at 164.6, 208.0, 267.5, 332.4, 335.4, 368.6,
and 370;9 keV for their
241
Am content.
1
The magni-
tude of this correction as a function of time since
chemical separation of Am and
237
U is shown in
Fig.
1. Relative efficiency values at 148.6 keV
from
241
Pu and at 164.6, W.0, 267.5, and 332.4 keV
from
2
*
1
Pu-
237
U are normalized to the
239
Pu values
at 332.4 keV. Tht resulting relative efficiency
curves for two sample sizes and a 200 mm
2
x 10 mm
planar detector, are shown in Fig. 2.
In keeping with our goal of simplicity, me oo
not attempt to fit the entire curve. Interpolation
and extrapolation over limited ranges are used to
calculate the neeoed relative efficiency values.
Efficiencies at 152.7 C
23
^), 160.3 C
2
*^),
and 161.5 keV
(
23
^Pu)
are determined by linear
interpolation between 148.6 and 164.6 keV. The ef-
ficiency at 169.6 *c«v
C
2
*
1
*)
is set equal to
that at 171.3. Efficiency points at 2Q».O, 3*7.5,
and 332.4 keV arc fit «lth a Quadratic caution and
the fit is extrapolated to recaapute the efficiency
