Development of advanced methods for detecting special nuclear materials in nuclear safeguards and security applications

QC 2026-10-01

Abstract

The detection, localization, and characterization of special nuclear materials (SNM) and other actinides are essential for nuclear security, safeguards, radioactive waste management, emergency response, and environmental radiological surveying. However, conventional passive radiation detection methods often face challenges in identifying and localizing fissile and other neutron-emitting materials, particularly when only small quantities are present, the materials are heavily shielded, or the measurement environment is complex. This thesis addresses these challenges through the development of Neutron–Gamma Emission Tomography (NGET), a novel imaging technique based on the detection of correlated fast neutrons and gamma rays emitted in fission events.

The research includes the theoretical development, experimental validation, optimization, and application of NGET using a detector system based on fast liquid organic scintillators. By exploiting characteristic time and energy correlations between emitted particles, this technique employs Bayesian inference and enables rapid three-dimensional (3D) localization of neutron-emitting sources. Experimental measurements and Monte Carlo simulations were performed to investigate the influence of detector geometry on imaging performance and spatial resolution, demonstrating that detector configurations should be optimized according to the intended application.

The developed technique was implemented in an imaging radiation portal monitor prototype system capable of detecting and localizing small quantities of neutron-emitting materials, including californium and representative special nuclear materials, with high efficiency and low false-alarm rates. In addition, the applicability of NGET to the nondestructive assay of radioactive waste was demonstrated through measurements of shielded legacy waste in collaboration with the company AB Svafo at the Studsvik nuclear decommissioning site, showing its ability to localize actinides that are difficult to detect using conventional passive nondestructive assay techniques. 

The results demonstrate that NGET provides a versatile and scalable approach for three-dimensional radiation imaging, offering significant improvements in sensitivity, localization accuracy, and operational flexibility compared with established methods. The work presented in this thesis shows considerable potential for applications in nuclear security, safeguards, radioactive waste characterization, nuclear emergency preparedness, and environmental monitoring, while providing a foundation for future developments in correlated gamma-neutron imaging.

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