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 (3rd 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 single-photon-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. non-spectroscopic), "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 UO_2 fuel rods in scenarios where other assay techniques might not be as reliable or even feasible.
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