Neutron Capture Gamma-Ray Multiplicity Analysis for Characterization of Moderated Special Nuclear Material

Neutron multiplicity analysis is a commonly employed technique for estimating the mass and multiplication of special nuclear material (SNM) 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 times and larger uncertainty in mass and multiplication estimates based on standard neutron multiplicity analysis. Measuring the characteristic gamma rays produced through thermal neutron capture provides a way to recapture the lost efficiency in heavily moderated systems to better estimate SNM mass and multiplication. Because these gamma rays are produced by the absorption of a neutron, they are representative of the neutron population and can therefore be used as neutron surrogates in standard multiplicity analysis. These gamma rays also effectively extend the range of the neutrons, as they are born at the point of neutron capture at relatively high characteristic energies (e.g., 2.2 MeV in the case of capture on hydrogen) with higher likelihoods of escaping the moderator. Recent modeling studies indicate that time-tagged gamma-ray counts in the 2.2-MeV hydrogen capture peak can be used to accurately estimate the multiplication and mass of plutonium shielded by high-density polyethylene (HDPE). These simulations are being validated through the measurement of Beryllium-Reflected Plutonium (BeRP) Ball (a 4.5 kg sphere of plutonium) shielded by various thicknesses of HDPE. This talk will present the outcome of these measurements and will discuss the ongoing development of the neutron capture gamma-ray multiplicity technique. This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories, a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.