Multiplicity Analysis During Photon Interrogation of

For accelerator-based active interrogation systems, direct simulationof multiplicity distributions with the Monte Carlo method isdifficult because (i) secondary particles from gamma interactionsare not treated correctly on an event-by-event basis and (ii) eachsource event is treated individually. The Monte Carlo codeMCNP-PoliMi corrects the first deficiency. In order to overcomethe second issue, the time width and intensity of the interrogationpulse—which specify how many photons arrive at the targetsimultaneously—must be incorporated into the calculation. Toaccomplish this, a subroutine that operates on the MCNP-PoliMioutput file has been developed. The purpose of this subroutine isto assemble the source events into groups corresponding to thenumber of interactions that would occur during a given pulse.This re-ordered output file is used with the MCNP-PoliMi detectionpost-processor to compute a multiplicity distribution. Themultiplicity distributions calculated using this new algorithmcapture the higher-order multiplets present due to multiple reactionsoccurring during a single accelerator pulse. Plans are underwayto gather relevant experimental data to validate themethodology developed and presented here. Analysis of thisinformation will determine the feasibility of using multiplicitydistributions as an identification tool for special nuclear material.Once validated, this capability will enable the simulation of alarge number of materials and detector geometries.