Simplified Neutron Scatter Camera for Nuclear Safeguards Applications

Abstract

The field of nuclear safeguards is a diverse domain of international and national security that combines state-level and international policies with technologies and systems to prevent the proliferation of nuclear weapons while still respecting rights for peaceful uses of nuclear technologies. Key in the international safeguards regime is the use of radiation detectors to track and characterize nuclear material. A new area of interest in detector design is directional detectors: detectors that can report information on radiation source locations and distributions along with data on source strength and identity. Neutron scatter cameras are a type of directional neutron detectors that rely on multiple neutron scatters to generate images that can reveal the direction and distribution of neutron sources.

Fast neutron cameras historically rely on multiple detector volumes and make use of neutron time-of-flight measurements. These designs, though effective in localizing the source direction, rely on a large amount of detection and electrical equipment thus increasing the size, cost, and complexity of the systems to unreasonable levels for many applications. This project seeks to develop a compact scatter camera that is smaller and less expensive than systems relying on multiple detector volumes. Crucially, two components and capabilities are needed to achieve this: fast scintillation detection materials and picosecond electrical pulse timing. Utilizing such materials and electronics, distinguishing between scintillation light pulses generated by the same neutron within one detector volume is possible. An MCNPX-PoliMi model of such a system was developed to guide the design of this system. A prototype single volume scatter camera using only six standard photomultiplier tubes was constructed. An algorithm for the identification of neutron double scattering events and neutron source localization through probability cone back-projection was developed for use in tandem with the scatter camera. The camera and algorithm’s imaging resolution and efficiency are quantified. A unique cone down-selection method based on an initial direction guess vector was developed to improve image quality.

In addition to constructing and demonstrating a simplified neutron scatter system, this project also proposes scenarios relevant to nuclear security and safeguards where such a system could be useful. These measurement scenarios include limited-area searches for unknown sources, identification of nuclear material for treaty verification, external reactor core monitoring, inspections of nuclear material, and mapping contamination.