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We present the Time of Flight Fixed by Energy Estimation (TOFFEE) as ameasure of the fission chain dynamics in subcritical assemblies. TOFFEE is thetime between correlated gamma rays and neutrons, subtracted by the estimatedtravel time of the incident neutron from its proton recoil. The measuredsubcritical assembly was the BeRP ball, a 4.482 kg sphere of alpha-phaseweapons grade plutonium metal, which came in five configurations: bare, 0.5, 1,and 1.5 in iron, and 1 in nickel closed fitting shell reflectors. We extend themeasurement with MCNPX-PoliMi simulations of shells ranging up to 6 inches inthickness, and two new reflector materials: aluminum and tungsten. We alsosimulated the BeRP ball with different masses ranging from 1 to 8 kg. Atwo-region and single-region point kinetics models were used to model thebehavior of the positive side of the TOFFEE distribution from 0 to 100 ns. Thesingle region model of the bare cases gave positive linear correlations betweenestimated and expected neutron decay constants and leakage multiplications. Thetwo-region model provided a way to estimate neutron multiplication for thereflected cases, which correlated positively with expected multiplication, butthe nature of the correlation (sub or super linear) changed between materialtypes. Finally, we found that the areal density of the reflector shells had alinear correlation with the integral of the two-region model fit. Therefore, weexpect that with knowledge of reflector composition, one could determine theshell thickness, or vice versa. Furthermore, up to a certain amount andthickness of the reflector, the two-region model provides a way ofdistinguishing bare and reflected plutonium assemblies.

Characterizing Subcritical Assemblies with Time of Flight Fixed by Energy Estimation Distributions