Sandia Labs FY22 Laboratory Directed Research & Development Annual Report

FY22 ANNUAL REPORT

NUMEROUS MISSION APPLICATIONS MAY BENEFIT FROM NEUTRON-CAPTURE GAMMA MULTIPLICITY COUNTING MEASUREMENTS.

Neutron multiplicity analysis is a technique used for estimating the mass and multiplication of special nuclear material configurations. However, in heavily moderated configurations, neutron leakage is reduced due to thermalization and subsequent capture of neutrons. As neutron leakage is reduced, measurement efficiency decreases, resulting in increased measurement

various moderator configurations and accurately estimated assembly multiplication. These results, which demonstrate significant promise for potential application to the emergency response, warhead verification, and nuclear safeguards communities, were presented at the Institute of Nuclear Materials Management Annual Meeting. (PI: Kyle Polack)

times and larger uncertainty in mass and multiplication estimates based on standard neutron multiplicity analysis. In this collaborative project, which included researchers at Oak Ridge and Los Alamos National Laboratories, the team experimentally validated the viability of the neutron capture gamma multiplicity signature through a series of experiments in which they measured a 4.5 kg mass of weapons-grade plutonium in

Measurement of a 4.5 kg sphere of weapons grade plutonium moderated by 6” of high-density polyethylene (HDPE) using an array of sodium iodide detectors (rear). In this photo, the moderating sphere is half assembled, which allows the plutonium and the nested HDPE shells to be seen. The measurements were performed using a fully assembled moderating sphere. The nested shells made it possible to assess the performance of the technique using a parametric study over a range of HDPE thicknesses.

A cartoon depicting a plutonium source surrounded by an increasing amount of moderator. As the polyethylene (PE) surrounding the plutonium (Pu) grows, the neutron leakage increases due to rising neutron multiplication but eventually diminishes as neutrons are absorbed to emit characteristic gammas. These capture gamma rays can act as surrogates for captured neutrons.

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