FAST ROSSI-ALPHA MEASUREMENTS OF PLUTONIUM USING ORGANIC SCINTILLATORS

¹ Department of Nuclear Engineering and Radiological Sciences, University of Michigan, 2355 Bonisteel Blvd, Ann Arbor, MI 48109, USA
² NEN-2: Advanced Nuclear Technology Group, Los Alamos National Laboratory, PO Box 1663, MS B228, Los Alamos, NM 87545, USA

mikwa@umich.edu

Published online: 22 February 2021

Abstract

In this work, Rossi-alpha measurements were simultaneously performed with a ³He-based detection system and an organic scintillator-based detection system. The assembly is 15 kg of plutonium (93 wt% ²³⁹Pu) reflected by copper and moderated by lead. The goal of Rossi-alpha measurements is to estimate the prompt neutron decay constant, alpha. Simulations estimate k_eff = 0:624 and α = 52:3 ± 2:5 ns for the measured assembly. The organic scintillator system estimated α = 47:4 ± 2:0 ns, having a 9.37% error (though the 1.09 standard deviation confidence intervals overlapped). The ³He system estimated α = 37 μs. The known slowing down time of the ³He system is 35-40 μs, which means the slowing down time dominates and obscures the prompt neutron decay constant. Subsequently, the organic scintillator system should be used for assemblies with alpha much less than 35 μs.