Detection of Shielded Special Nuclear Material Using High Energy Gamma Ray Transmission Imaging and Cherenkov Detectors

Detection of shielded special nuclear material is a very difficult task, especially while in transit. Current active methods designed to probe through shielding typically suffer from high false alarm rates, slow scanning, large dose to cargo and potential stowaways, and/or large accelerators with massive shielding requirements. To resolve this, we are investigating a novel approach to low dose active interrogation during transit for detection of shielded nuclear material in a mobile platform that can be quickly deployed and easily relocatable. The system rapidly scans containers using mono-energetic, high-energy gamma rays coupled with an array of Cherenkov threshold detectors to avoid low energy background from scatter while providing the ability of high contrast transmission imaging, thus confirming the presence of high-Z materials which are potentially fissile. This technique exploits the large absorption difference between high-Z materials and low- and mid-Z materials due primarily to the difference between pair production and Compton scattering cross-sections to produce a high contrast low dose planar image of the cargo. This paper will primarily concentrate on the design and fabrication of a transmission imager based on Cherenkov detectors. The custom design of Cherenkov based detectors can be finely tuned using material properties, such as refractive index, to adjust the lower threshold of the detector allowing for only detection of photons above a desired energy. Using Cherenkov detectors, we are able to significantly reduce background and scattering effects allowing for faster signal acquisition and processing. These detectors are inherently directionally dependent due to the characteristic Cherenkov cone of photons and dominance of forward scattering during high-energy Compton scattering events. The time-gated system and the speed of Cherenkov detectors will allow for very precise measurements of delayed gamma rays from fission. The detectors are versatile enough to be able to analyze a large flux of gamma rays over a broad range of energy. A container that has special nuclear material should be rare so extra attention is given to a data-driven approach to reduce false alarms, improving the speed and reliability of the system.