Detection of shielded special nuclear material (SSNM) while in transit is a very arduous task. Current active interrogation methods for probing through shielding typically suffer from high false alarm rates, slow scanning, large dose to potential stowaways, and/or large accelerators with restrictive shielding requirements. To resolve this, we are investigating a novel approach to low dose active interrogation during transit for detection of SSNM in a mobile platform that can be rapidly deployed and easily relocatable. The system quickly scans cargo containers using dual mono-energetic, high-energy gamma rays (4.4 MeV and 15.1 MeV) coupled with an array of Cherenkov detectors. Using the Cherenkov threshold characteristics we are able to avoid low energy background while providing the ability to produce high contrast transmission imaging, thus confirming a material’s fissile nature. Our technique uses the large absorption difference, via pair production, between high-Z materials from low- and mid-Z materials due primarily to the difference between pair production and Compton scattering cross sections to produce a high contrast planar image of the cargo.
Cherenkov based detectors are used because they 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 using state of the art digital equipment. 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. Our design focuses on a cost efficient yet durable design that is resilient against temperature effects. 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.
The placement of detectors in this array is carefully designed to reduce detector crosstalk while maximizing efficiency and image resolution. The geometry of these detectors has a large influence on the desired output characteristics. Cross talk is reduced through spacing as well as detector cross-sectional area but this must be balanced against the need for image resolution. Through various crosstalk experiments we are able to reduce crosstalk via signal processing algorithms.
Currently the LANNS group is working towards implementation of solid state photon counters, silicon photomultipliers, for signal acquisition due to the superior low light collection abilities, enhanced quantum efficiency, and resolving time measured in the picosecond range which better compliments the near instantaneous nature of Cherenkov radiation. The group is creating mini-arrays of different designs for evaluation to be tested with a particle accelerator to produce images.