Pulsed-Neutron Die-Away Experiments for Plastics and Neutron Thermal Scattering Laws

¹ Nuclear Criticality Safety Division, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA
² School of Nuclear Science and Engineering, Oregon State University, 1701 SW Western Blvd. Corvallis, OR 97331, USA
³ Naval Nuclear Laboratory, PO Box 79, West Mifflin, PA 15122, USA

^* e-mail: siefman1@llnl.gov

Published online: 26 May 2023

Abstract

Pulsed-neutron die-away (PNDA) experiments can be useful benchmarks to validate neutron thermal scattering laws (TSLs). The experiment uses a neutron generator to impinge a short (∼10⁻⁴ s), mono-energetic neutron pulse on a target sample. After the pulse, the neutron population within the sample moderates and reaches thermal equilibrium with a fundamental spatial mode and characteristic decay-time eigenvalue. The eigenvalue can be extracted from the experimental measurements of the neutron flux and used as an integral parameter in validation. For certain materials and geometric configurations, the eigenvalue is heavily influenced by thermal neutron scattering of only the target material. For that reason, a PNDA experiment can have a higher sensitivity to TSLs than is commonly available in critical experiments. Herein, we present results for a series of new PNDA experiments conducted at Lawrence Livermore National Laboratory with plastic materials, e.g. high-density polyethylene and Lucite. We compare the experimental integral parameters to simulated results and report trends in the biases. We evaluate the bias with different Monte Carlo transport codes (MCNP6.2 and Mercury) and show no significant differences between the results of the two codes.