PROMPT NEUTRON DECAY CONSTANT FROM FEYNMAN VARIANCE FITTING

The use of fast-response liquid scintillators for passive measurements with highly enriched uranium (HEU) castings provides information on the timescale of the fission chain multiplication process for HEU. Thus, these detectors produce time responses characteristic of the HEU and not the moderator around 3He counters. Up to five HEU castings were assembled and the passive time coincidence measurements performed. The prompt neutron decay constants inferred from the Feynman variance data with the arrays of large liquid scintillators are compared with decay constants obtained from previously presented Feynman variance data collected in active interrogation measurements with these same HEU castings. The active measurements used plastic scintillation detectors adjacent to the castings to eliminate time-of-flight effects and showed that the prompt neutron decay constant is clearly not exponential. However, time-of-flight effects in the passive measurements result in exponential behavior and allow the Feynman variance data to be fit with the well-known point kinetics analytical formula. The characterization of the prompt neutron decay constant is investigated by comparing the results of the passive and active measurement data.