Polarization of Compton-Scattered Prompt Gamma-Ray Technique for HEU Detection at 186 keV

Prompt gamma-ray polarization is a practical method for detecting highly enriched uranium (HEU) in concealed sources. It also provides information on their geometry, magnetic fields, and radiation mechanisms. However, prompt gamma-ray polarization measurements have rarely been applied in nuclear nonproliferation areas to detect HEU. In this study, the feasibility of detecting the characteristic energy peak of 186 keV, which is associated with the asymmetry of the activation mechanism and the detection of energy-dependent polarization from concealed HEU sources, was evaluated using the Compton scattering approach. A Monte Carlo N-particle transport code simulation was used to realize the activation mechanism of HEU via two 1.4-mm strips of converter material [i.e., cesium lead tribromide (CsPbBr₃)], transported by secondary scattered gamma rays during the three-stage process of Compton scattering, polarization, and detection.

This paper presents the mathematical model, the physics of Compton scattering, and the polarization mechanism for the detection technique. In this case, the physics is relevant to both processes in which the emitted secondary scattered gamma rays undergo initial orthogonal polarization. Specifically, to meet the objective of testing the technical protocol for the enhanced detection of energy peaks associated with HEU, particularly 186 keV, simulations were conducted to quantify the HEU volume and neutron source strength in the MCNP data card to perform error analysis. The detector system had the potential to acquire good resolved photopeak with a 4.5% relative error or less, with a 1 Ci source activity, and a peak-to-background ratio of 1.15. This resolution took 163 s for high-purity germanium detection, which is comparable to current methods used for material detection placed within 100 to 900 s to completion. The small error difference was due to the attributes of the phenomenal enhancement properties of cesium tribromide and polarimetry. The identified photo peaks included K-shell X-rays from ²³⁵U, 61 keV from fission, 511-keV annihilation, and the peak of interest at 186 keV.

The result from spectral analyses showed clear signatures related to pure and adulterated HEU. HEU detection with the low neutron yield and the easiness of shielding the yield of the HEU sample showed that the HEU characterization was feasible when shielded, with the highest success rate under both enhancement approaches. The optimization and scale-up of this technique are expected to enhance its applications in a large-scale HEU detection design.