Characterization of a 4He Scintillation Detector and its Applications in Nuclear Material Assay

Abstract

Helium-4-based scintillation detectors have been developed as a gamma-insensitive alternative to organic scintillators for fast neutron detection and spectroscopy. While these detectors hold significant promise for applications in extreme environments where organic scintillators are not suitable, there has been relatively little scholarship on their fundamental characteristics and applications.

The only commercially available He-4-based detector is the Arktis S670. This detector has a unique construction, containing three optically segmented regions, each providing four signal outputs. An algorithm was developed to condense these four outputs into a single data list, showing an improved time resolution compared to a separate analysis of the outputs, and an energy calibration using monoenergetic 2.45 MeV and 14.1 MeV neutrons was demonstrated.

Another experiment was performed to characterize the response of the He-4 detector to monoenergetic nuclear recoils up to 9 MeV. The He-4 detector was positioned in the center of a semicircular array of organic scintillation detectors operated in coincidence, in conjunction with monoenergetic neutron sources. The measured detector response provides evidence for scintillation linearity and was used to develop an energy resolution function applicable to this energy range, enabling high-fidelity detector simulation for future applications.

Precise knowledge of the temporal output profile of a pulsed neutron generator is beneficial for several neutron active interrogation techniques. Using the He-4 detector, the primary temporal profile of a short-pulsed deuterium-tritium neutron generator was isolated from scattered and induced contaminants by spectral thresholding. This method can be used to continuously monitor the pulse characteristics and quality for various neutron active interrogation techniques.

The production of U-233 poses unique challenges for nuclear safeguards, as it is associated with an extreme gamma ray environment from U-232 contamination, as well as more conservative accountability requirements than for U-235. The He-4 detector and an array of He-3 detectors were used to demonstrate three advanced neutron signatures of U-233 in oxide. These measurements provide a benchmark for future nondestructive assay instrumentation development and demonstrate a set of key neutron signatures to be leveraged for nuclear safeguards in the thorium fuel cycle.

The use of photon active interrogation for the detection of special nuclear material has held significant promise; however, neutrons produced by photonuclear reactions in the accelerator target, collimator, and environment can obscure the fission neutron signal. High-pressure He-4-based scintillation detectors are well-suited to photon active interrogation, as they can measure the fast neutron spectrum, but show little response to gamma rays. A photon active interrogation system utilizing a He-4 scintillation detector and a 9 MeV linac-bremsstrahlung X-ray source was experimentally evaluated. It was shown to be capable of detecting photofission neutrons from U-238 based on their clear spectral separation from (gamma,n) neutrons produced in lead, a common shielding material.

This collection of research serves to deepen the understanding of He-4-based detector characteristics and pioneer some of their promising applications in nuclear safeguards and security. While additional work remains to develop the demonstrated applications into operational systems, this work has significantly broadened the plausible application space for He-4 scintillation detectors.