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We present the results from the first measurements of the Time-CorrelatedPulse-Height (TCPH) distributions from 4.5 kg sphere of $\alpha$-phaseweapons-grade plutonium metal in five configurations: bare, reflected by 1.27cm and 2.54 cm of tungsten, and 2.54 cm and 7.62 cm of polyethylene. A newmethod for characterizing source multiplication and shielding configuration isalso demonstrated. The method relies on solving for the underlying fissionchain timing distribution that drives the spreading of the measured TCPHdistribution. We found that a gamma distribution fits the fission chain timingdistribution well and that the fit parameters correlate with bothmultiplication (rate parameter) and shielding material types (shape parameter).The source-to-detector distance was another free parameter that we were able tooptimize, and proved to be the most well constrained parameter. MCNPX-PoliMisimulations were used to complement the measurements and help illustrate trendsin these parameters and their relation to multiplication and the amount andtype of material coupled to the subcritical assembly.

Multiplication and Presence of Shielding Material from Time-Correlated Pulse-Height Measurements of Subcritical Plutonium Assemblies