Rapid Analysis of the SNM Smuggling Threat Space for Active Interrogation Using a Greens Function Approach

Detection of smuggled special nuclear material (SNM) in truck orsea containers is essential for ensuring the security of the UnitedStates from radiological threats. Radiation transport simulationis a widely used tool for evaluating the probability of detectiongiven some SNM, smuggler strategy, detector system, and alarmalgorithm. Inherent in these calculations is the tradeoff betweenthe speed and the fidelity of the computation. Full Monte Carlo(MC) radiation transport captures the true physics and geometricrichness of the system; however, doing this in a reasonableamount of time for a spanning set of threats requires vast computingresources. Simplifications such as 1-D deterministic approximationscan reduce the computation time drastically, butmay miss important physical and geometric phenomena. Wepresent a method that uses Green’s Functions, which are computedonce up front and are then stored for future use, to rapidlyanalyze many possible scenarios with the fidelity approachingthat of full 3-D MC transport but with a computational time onthe order of seconds. Using this technique, we model an activeinterrogation (AI) photon source incident on a cargo containercontaining some SNM and analyze the time dependent flux ofneutrons at the detector. We present an illustrative application ofour technique on the neutron background from cosmic radiation.First, we model cosmic interactions in the atmosphere to obtainthe neutron flux as a function of both energy and direction at150m above the ground. We break the subsequent transport ofthe neutrons into several distinct regions: neutrons incident onthe ground per neutron at 150m, neutrons reflecting back andexiting the ground per neutron incident on the ground, neutronsin the detector per neutron leaving the ground, as well as thedirect path of neutrons into the detector per neutron at 150m.We are able to vary parameters such as ground composition, aircomposition and humidity, detector height above ground, anddetector type. With this method we have achieved a 7.3 percentroot mean square difference between our spectral neutron fluxand that obtained from a full MCNPX calculation.