Toward the Simulation of Photofission for Nuclear Material Identification

The detection of shielded highly enriched uranium is a challenging problem that is being addressed by numerous researchers. The measurement systems that are being investigated require fast, accurate, and simple read-out responses to be used at ports of entry. A number of currently proposed techniques make use of photon sources to induce fission in the nuclear material and to detect the subsequent gamma rays and neutrons from fission. The design of such devices and the analysis of the measurement results rely on Monte Carlo codes to simulate the interaction of neutrons and photons with the nuclear material, the shielding, and the radiation detectors. However, currently available Monte Carlo codes lack the ability to accurately simulate the emission of secondary particles from photonuclear events. This ability is vital for investigating novel efficient techniques capable of identifying nuclear materials and of estimating their amount. The goal of this study is to provide a tool that addresses and solves these deficiencies. This paper presents a methodology that relies on the acquisition of correlated signals from prompt neutrons and photons emitted by photonuclear interactions. The proposed measurement system makes use of organic scintillators for measuring the signals from the fast neutrons and photons. In this paper, we describe the currently implemented photofission simulation in MCNPX and propose an extension to improve the simulation of secondary particles. Our approach is based on the use of two existing and well-benchmarked codes, MCNPX and MCNP-PoliMi.