PNNL-13501 - Pacific Northwest National Laboratory

A next-generation technology is needed to address the
problem of plutonium mass measurement in arms control
applications. Despite the tremendous inertia behind the
established technique of coincidence counting, there is a
growing acknowledgment of the general failure of
neutron-based methods to properly support the United
States arms control objectives. Further, it is likely that a
simple but effective technology such as this could actually
accelerate progress in negotiations such as START III,
START III+, and Mayak. The amount of necessary
measurement hardware would be greatly reduced, with a
single measurement involving a single detector and single
information barrier potentially being sufficient. The
timeline for development and testing of measurement
systems would be greatly accelerated.
Approach
The approach to this problem necessarily involves the
development of a model for the plutonium configuration,
with subsequent evaluation of the errors resulting from
the use of the model. Two major simplifications are
necessary for such a model to proceed. First, the set of
shielding materials can be reduced to three basic types
with quantifiable properties. High-Z shielding accounts
for substances such as lead and uranium, medium-Z
shielding accounts for substances such as iron, low-Z
shielding accounts for all hydrogenous substances and
materials with atomic numbers below roughly aluminum.
A plot of mass attenuation coefficients (as a function of
energy) for the simplified model is shown in Figure 1.
Figure 1. Mass attenuation coefficients for selected
materials as a function of photon energy
The second major simplification involves object
geometry. Calculations are most easily completed for
slab (planar) geometries, while real components
incorporate more complex geometries. Our initial
algorithm assumes slab geometry. Future work will
assess improvements, if any, from incorporating a nonslab
geometry. Perfectly accurate geometries are not
possible because nuclear weapons are constructed in a
variety of shapes, sizes, and designs. We anticipate that
the uncertainties associated with using this simplification
will be small. Estimates accurate to within 25 to 50%
may be more than satisfactory for arms control
applications.
This work complements and extends the Sandia National
Laboratory algorithm work, called “Minimum Mass
Estimate (MME)” (Mitchell and Marlow 2000), by the
use of a larger part of the gamma-ray spectrum, an
elaborate shielding and geometric model, and
sophisticated statistical methods.
Results and Accomplishments
Our approach to development and testing of the initial
mass estimate algorithms was based on the use of
increasingly complicated gamma-ray spectra. Initially,
simplified spectra were generated using the SYNTH
gamma-ray generating software, followed by spectra
collected in the laboratory from a small (~100 gram)
weapons grade plutonium source. A wide variety of
absorbers (that spanned a broad range of atomic number
and density) were then used to attenuate the gamma-rays
from the plutonium both individually and in combinations
with other absorber materials. Measurements were made
using both short and long acquisition times so that we
could determine the sensitivity of the algorithms to the
statistics of the input spectra.
The key to the probabilistic approach to this problem is
that the expected (average) gamma-ray signals, as a
function of energy, are completely determined by known
physics. Under the absorber and geometric assumptions
discussed above, the expected gamma-ray signal at a
spectrometer has been described mathematically using
first-order effects.
The current algorithm assumes known thickness of the
low, medium, and high barriers. This assumption will be
relaxed in future development and the mathematics of
mass estimation in the presence of absorber-thicknessuncertainty
is developed.
Summary and Conclusions
The first round of algorithm development has yielded
very promising results. We were able to successfully
calculate an accurate plutonium mass from experimental
data given the composition of a combination of
Sensors and Electronics 407