Simulation and Analysis of Correlated Neutron and Gamma-ray Detection from Photofission

We describe Monte Carlo simulations of the photon interrogation of uranium and plutonium metal samples of varying mass and composition. The simulations are performed using a Monte Carlo package consisting of modified versions of the codes MCNPX and MCNP-PoliMi. The cross-correlation functions simulated consist of correlated detection of these neutrons and gamma-rays. In the first part of our paper, these physical mechanisms are described, and the ability of the resulting correlation signatures to characterize shielded special nuclear material (SNM) is discussed. The second part of our paper illustrates a methodology based on Monte Carlo simulations and artificial neural networks (ANN) that is aimed at determining the characteristics of the fissile material. In the simulations, we modeled a simple geometry consisting of fissile material shielded with CelotexTM, placed between two plastic scintillation detectors. We show that the methodology can reliably differentiate between highly enriched uranium and plutonium. Furthermore, the mass of the material can be determined with a relative error of about 7%.