Numerical simulation of nuclear materials detection, imaging and assay with MEGa-rays

Once fully operational, LLNL’s Nuclear Photonics Facility is expected to be capable of generating tunable, mono-energetic gamma-ray (“MEGa-ray”) beams with energies of ~ 0.5 – 2.5 MeV and spectral intensities many orders of magnitude beyond those of current (3 rd generation) synchrotron light sources. MEGa-ray beams will allow us to exploit a physical process known as nuclear resonance fluorescence (NRF), in which an energetic photon is absorbed by a nucleus, which then decays to its ground state by emitting one or more characteristic gamma rays. NRF has already been demonstrated as a potentially viable technique for detecting shielded nuclear materials in singlephoton-counting experiments done with high-resolution (e.g. HPGe) detectors; however, the maximum count rates that these energy-differential (spectroscopic) detectors can sustain (e.g. < 20 kHz for moderate-sized detectors) effectively precludes their use with high-intensity photon beams such as MEGa-ray. In this paper we will present the conceptual design of an energy-integrated (i.e. nonspectroscopic), “Dual-Isotope Notch Observer” (DINO) NRF detection system which should be capable of detecting, imaging and assaying shielded nuclear materials (e.g. 235 U) irradiated by photon beams of arbitrary intensity by comparing the photon yields emitted at back angles from a pair of resonant (e.g. 235 U) and non-resonant (e.g. 238 U) “witness foils” located in a heavily-shielded environment downstream from the object under inspection. The ratio of the total, energy-integrated signals from scintillators recording emissions from the resonant and non-resonant foils can be used to define a robust “decision metric” that can be used in search scenarios to detect the presence of the resonant material or, given a suitable detector calibration procedure, provide accurate estimates of the aerial density ([gm/cm 2 ]) of the resonant material along the incident beam path. We have used detailed numerical simulations to investigate a number of different detection and/or imaging scenarios; however, in this paper we will focus on the potential for using MEGa-ray beams and DINO detector systems to assay conventional UO2 fuel rods in scenarios where other assay techniques might not be as reliable or even feasible