Solving the Rubik’s Cube: two dimensional neutron
radiography with superheated droplet detectors

We have previously studied Zero-Knowledge-Protocol (ZKP) differential neutron radiography for warhead verification, using superheated droplet (bubble) detectors. We successfully showed the proof-of-concept using one dimensional radiography experiments. In the present study, we conducted calculations and preliminary experiments with a test object with full 3D structure (4×4×4 = 64 voxels) combined with 2D radiography. The baseline target object is a (4”×4”×4”) cube, which consists of eight 1” stainless steel cubes (2×2×2 = 8 voxels) in the center surrounded by 58 1” high density polyethylene cubes. We swapped some of steel cubes with aluminum and polyethylene blocks to test the sensitivity of the system. A total of 16 bubble detectors were located behind the target in a (4×4) grid. We used 14 MeV neutrons coming from the “EXCALIBUR” neutron source available at the Princeton Plasma Physics Laboratory, which allows rapid measurements. Theoretically, measurements from two sides should provide enough data to solve for the opacity of the 8 central blocks. Initial experimental results are in reasonable agreement with MCNP calculations for the overall bubble count rates, and general trends can be seen. Results with higher total counts and accurate inter-detector calibration will be presented.