  We present the Time of Flight Fixed by Energy Estimation (TOFFEE) as a
measure of the fission chain dynamics in subcritical assemblies. TOFFEE is the
time between correlated gamma rays and neutrons, subtracted by the estimated
travel time of the incident neutron from its proton recoil. The measured
subcritical assembly was the BeRP ball, a 4.482 kg sphere of alpha-phase
weapons 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 the
measurement with MCNPX-PoliMi simulations of shells ranging up to 6 inches in
thickness, and two new reflector materials: aluminum and tungsten. We also
simulated the BeRP ball with different masses ranging from 1 to 8 kg. A
two-region and single-region point kinetics models were used to model the
behavior of the positive side of the TOFFEE distribution from 0 to 100 ns. The
single region model of the bare cases gave positive linear correlations between
estimated and expected neutron decay constants and leakage multiplications. The
two-region model provided a way to estimate neutron multiplication for the
reflected cases, which correlated positively with expected multiplication, but
the nature of the correlation (sub or super linear) changed between material
types. Finally, we found that the areal density of the reflector shells had a
linear correlation with the integral of the two-region model fit. Therefore, we
expect that with knowledge of reflector composition, one could determine the
shell thickness, or vice versa. Furthermore, up to a certain amount and
thickness of the reflector, the two-region model provides a way of
distinguishing bare and reflected plutonium assemblies.

Data Analysis, Statistics and Probability