Multiplicity Analysis during Photon Interrogation of Fissionable Material

For accelerator-based active interrogation systems, direct simulation of multiplicity distributions with the Monte Carlo method is difficult because (i) secondary particles from gamma interactions are not treated correctly on an event-by-event basis and (ii) each source event is treated individually. The Monte Carlo code MCNP-PoliMi corrects the first deficiency. In order to overcome the second issue, the time width and intensity of the interrogation pulse - which specify how many photons arrive at the target simultaneously - must be incorporated into the calculation. To accomplish this, a subroutine has been developed which operates on the MCNP-PoliMi output file. The purpose of this subroutine is to assemble the source events into groups corresponding to the number of interactions which would occur during a given pulse. This reordered output file is used with the MCNP-PoliMi detection post-processor to compute a multiplicity distribution. The multiplicity distributions calculated using this new algorithm capture the higher-order multiplets present due to multiple reactions occurring during a single accelerator pulse. Plans are underway to gather relevant experimental data to validate the methodology developed and presented here. Analysis of this information will determine the feasibility of using multiplicity distributions as an identification tool for special nuclear material. Once validated, this capability will enable the simulation of a large number of materials and detector geometries.