Title: Multiplication and Presence of Shielding Material from Time-Correlated Pulse-Height Measurements of Subcritical Plutonium Assemblies,
Abstract: We present the results from the first measurements of the Time-Correlated
Pulse-Height (TCPH) distributions from 4.5 kg sphere of $\alpha$-phase
weapons-grade plutonium metal in five configurations: bare, reflected by 1.27
cm and 2.54 cm of tungsten, and 2.54 cm and 7.62 cm of polyethylene. A new
method for characterizing source multiplication and shielding configuration is
also demonstrated. The method relies on solving for the underlying fission
chain timing distribution that drives the spreading of the measured TCPH
distribution. We found that a gamma distribution fits the fission chain timing
distribution well and that the fit parameters correlate with both
multiplication (rate parameter) and shielding material types (shape parameter).
The source-to-detector distance was another free parameter that we were able to
optimize, and proved to be the most well constrained parameter. MCNPX-PoliMi
simulations were used to complement the measurements and help illustrate trends
in these parameters and their relation to multiplication and the amount and
type of material coupled to the subcritical assembly.