Photofission Signatures for the Detection of Highly Enriched Uranium

Linear-accelerator (LINAC) driven active interrogation systems are under increasing investigation for the detection and characterization of nuclear materials, such as uranium-235. Prompt-neutron emission during active interrogation is not unique to fissionable material; photoneutron reactions occur in common materials such as lead. Consequently, accurate simulation of the yield, multiplicity, and energy spectra of photoneutrons, as well as the detector response, is paramount in designing an effective interrogation system. Advanced Monte Carlo codes such as MCNPX-PoliMi are ideal for simulating these quantities. In this paper, we present new results of simulations performed with MCNPX-PoliMi using LINAC-based interrogating sources on HEU, depleted uranium, and lead.