Validation of the implicit correlation method in mcpnx-polimi usingplutonium cross-correlation measurements

Monte Carlo particle transport codes used to accurately model organic liquid scintillator detector response are traditionally run in fully analog mode. Analog simulations of cross-correlation measurements with these codes areextremely time-consumingbecause the probability of correlated detection is extremely small, approximately equal to the product of the probabilities of a single detection in each detector. The new "implicit cor-relation" method described here increases the number of correlation event tallies thereby,reducing variance and required computation times. The cost of the implicit correlation method is comparable to the cost of simulating single event detectionin the lowest effi-ciency detector. This method is especially useful in the nuclearnonproliferation and safeguards fields for simulating correlation measurements of shielded special nuclear material A plutonium metal sample wasmeasured by a fast-neutron multiplicity counter for veri-fication of MCNPX-PoliMi calculations and the new method. The fast-neutron multiplicity counter was used with pulse shape discrimination techniques to produce a neutron-neutron cross-correlation distribution. The new method was implemented in MCNPX-PoliMi with a Pu-240spontaneous fission sourcein the measurement geometry. The method demon-strated good agreement with analog simulation and measurement results and aspeed-up of a factor of 500 over analog calculations.