Application of the Cohn-α Method on Bare Highly Enriched Uranium Using Organic Scintillators

Neutron noise measurements are a modality of non-destructive assay to measure fission chain kinetics. Noise measurements can be used to verify the existence of special nuclear material (SNM) or to monitor the state of a multiplying system, such as a nuclear reactor or any other fissile assembly. Neutron noise measurements exploit the temporal fluctuations present in multiplying systems. Parameters such as the prompt neutron decay constant (α), can be used to calculate the lifetime of prompt fission chains within a multiplying system. Our work analyzes data from a series of measurements on a fast, bare system consisting of highly enriched uranium (93% 235U) hemisphere shells known as the Measurements of Uranium Subcritical and Critical (MUSiC). We measured the system with a three-by-four array of 5.08-cm-length by 5.08-cm-diameter cylindrical trans-stilbene organic scintillators (OSCAR) to obtain the gamma-ray and fast neutron signals. Pulse shape discrimination was used to isolate the neutron time-series data. The Cohn-α method was used to infer the prompt neutron period of a super-critical case, which had a reactivity 30 cents or 0.3βeff (βeff = 650 pcm) above critical. This configuration was comprised of 61 kg HEU and was driven by a 252Cf spontaneous fission source. Cohn-α transforms the time-series data to the frequency domain and may be used to infer other reactor kinetic parameters such as βeff. The results using Cohn-α suggest this configuration has a prompt neutron period (α-1) of 1,743 ± 1 ns. This result was compared to a high-fidelity MCNP6 KCODE simulation with an estimate of the prompt neutron period at delayed critical (αDC-1) of 1,020 ± 3 ns. This simulation result motivates further analysis of MUSiC data at a similar reactivity to extrapolate αDC-1 at delayed critical for direct comparison.