Monte Carlo Method for Calorimetric NRF Cargo Screening

Abstract

A number of fields in nuclear security require isotopic analysis and identification. Nuclear resonance fluorescence(NRF) has provided a non-intrusive isotope-sensitive measurement technique to detect special nuclear material in cargo [8], and has been proposed to be used as a verification technique in arms control treaty verification [41]. Standard methods of performing NRF involve the use of expensive HPGe detectors to detect a scattered signal to discriminate between isotopes of special nuclear materials. Furthermore these require a continuous wave (CW) beam, which currently can be delivered only by large and static accelerators [40]. We propose a system using an energy-modulating chopper wheel and a simpler, pulsed electron accelerator beam as the radiation source. This work builds upon a concept presented by Kemp et al. [24], with the difference of a measurement of NRF in a scattering mode. In this approach the chopper wheel serves as a switch effectively modulating the beam to include or exclude photons of NRF energies for interrogating the test object. Comparison between the chopper "On" and "Of" will provide a differential signal which upon integration can allow inference of special nuclear materials based on their NRF signals. The approach places integrating calorimetric Cherenkov detectors at a back-scattered angle which will eliminate much of the background typically found in a transmitted spectra. Cherenkov detectors will replace the HPGe detectors in effort to decrease the low energy background. We present a thoroughly tested Monte Carlo model to compare with experimental testing using Cherenkov detectors and nuclear resonance fluorescence to discriminate between isotopes of special nuclear material. Preliminary simulation results show that a uranium interrogation object could not be determined within a 5 minute interrogation.