Fast Rossi-alpha Measurements of Plutonium using Organic Scintillators

TL;DR

This paper compares organic scintillator and helium-3 detectors for rapid Rossi-alpha measurements of plutonium, demonstrating the potential of organic scintillators to estimate prompt neutron decay constants more effectively in certain assemblies.

Contribution

It introduces a fast measurement method using organic scintillators for estimating the prompt neutron decay constant in plutonium assemblies, offering an alternative to traditional helium-3 systems.

Findings

Organic scintillators estimated alpha as 47.4 ns with 9.37% error.

Helium-3 system estimated alpha as 37 μs, dominated by slowing down time.

Organic scintillators are suitable for assemblies with alpha much less than 35 μs.

Abstract

In this work, Rossi-alpha measurements were simultaneously performed with a $^3$He-based detection system and an organic scintillator-based detection system. The assembly is 15 kg of plutonium (93 wt$\%$ $^{239}$Pu) reflected by copper and moderated by lead. The goal of Rossi-alpha measurements is to estimate the prompt neutron decay constant, alpha. Simulations estimate $k_\text{eff}$ = 0.624 and $\alpha$ = 52.3 $\pm$ 2.5 ns for the measured assembly. The organic scintillator system estimated $\alpha$ = 47.4 $\pm$ 2.0 ns, having a 9.37$\%$ error (though the 1.09 standard deviation confidence intervals overlapped). The $^3$He system estimated $\alpha$ = 37 $\mu$s. The known slowing down time of the $^3$He system is 35-40 $\mu$s, 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 $\mu$s.