PNNL-13501 - Pacific Northwest National Laboratory

Study Control Number: PN00074/1481
Plutonium Mass Estimation via Gamma-Ray Spectrometry
Walter K. Hensley, Anthony J. Peurrung, Debra S. Barnett, Dale N. Anderson
The measurement of plutonium mass is a critical requirement in nuclear arms control applications. A computational
algorithm is being developed and optimized for the estimation of plutonium mass with sufficient accuracy, while
alleviating the drawbacks of traditional neutron-based methods of coincidence counting, for use in a variety of arms
control applications.
Project Description
The purpose of this project is to demonstrate that gammaray
based plutonium mass estimation offers sufficient
accuracy to mandate its use for a variety of arms control
applications, including those now using neutron
coincidence counting. The information contained in a full
gamma-ray spectrum can be used to obtain an estimation
of plutonium mass because of the different attenuation to
which gamma rays of different energies are subjected.
These differential attenuations allow considerable
inference to be drawn about the shielding external to any
plutonium and about the plutonium itself. This
information, in turn, can be used to formulate a mass
estimate along with an associated error. This project is
exploring this process in detail, developing an optimal
algorithm, determining the various errors associated with
mass estimates, and demonstrating mass estimation using
a variety of relevant spectra. The potential savings in
time and money that would result from the use of spectral
mass estimation are sufficient to significantly accelerate
progress toward current and future arms control
objectives.
406 FY 2000 Laboratory Directed Research and Development Annual Report
Introduction
Currently, plutonium mass measurements are performed
almost entirely using the neutron-based method of
coincidence counting. Although the neutron-based
method is capable of accurate mass measurements, it
suffers from a number of drawbacks which are completely
alleviated with gamma-ray based plutonium mass
measurements. Table 1 compares the features of neutronbased
and gamma-ray spectrometry methods for
estimating plutonium mass in a nuclear weapon.
While gamma-ray estimation of plutonium mass does
have the disadvantage of reduced accuracy, this is readily
tolerable for arms control applications. Frequently, arms
control measurements entail demonstration that an item or
component merely exceeds a threshold amount of
plutonium. The thresholds are typically set so low that
any conceivably valid item or component will pass
inspection.
Table 1. Comparison of spectrometric methods for assessing plutonium mass
Issue Neutron-based Methods Gamma-ray Methods
Cost/unit Roughly one million dollars. Long lead times commonly required for Post-development cost is nearly zero. Existing
new application system development.
software, computers, and sensors can be used.
Portability Large number of rigid moderators required leads to high weight and
devices often best suited to fixed installations.
Geometry Inflexible, each counting system is limited to particular item
geometry. Nuclear weapons vary in shape and size.
Evasion Although fully calibrated systems are hard to evade, issues with
confusion by alternate neutron sources such as 252 Cf, AmLi, and
PuBe exist.
Matrix effects Strange or unknown geometries in arms control measurements,
especially those involving moderator, can seriously affect accuracy.
Criticality
Safety
Substantial amounts of required moderator precludes some plutonium
measurements.
Laptop computer and hand-held High Purity
Germanium (HPGe) sensor required.
Accommodates any size or shape.
No issue of spoofability. The method directly
quantifies 239 Pu, the isotope of interest for arms
control applications.
An issue of this research.
No moderator required.