Optimizing Inspection Parameters for Long Stand-Off Detection of SNM

Detection of special nuclear material (SNM) at extended ranges (> 100 m) through the utilization of high energy (> 20 MeV) bremsstrahlung photons requires optimizing the structure and interrelation of irradiation (beam-on) and detection (counting) periods. Conventional inspection schemes at lower energies and smaller distances primarily operate by pulsing an accelerator at frequencies of 0.1-1 kHz while collecting emitted radiation from the target under inspection for the few milliseconds in between pulses. Simulation and experimental results for long stand-off scenarios with source photons > 20 MeV, however, indicate that two primary phenomena - (1) induced photoneutrons in proximity to the accelerator and (2) beam induced activation of air and soil - preclude the use of conventional inspection schemes. By considering the time structure and magnitude of the beam-induced photon and neutron backgrounds, signals of interest from the target, and natural backgrounds, inspection schemes have been developed to maximize signal to noise ratios (SNR). Analysis of the data indicates that the highest SNR values are found with short (2-5 s) irradiations followed by a 1-2 s period of collecting emitted neutron and photon signatures.