Low power compact dual mode detectors for nuclear security applications

Bennett, David Matthew Alan (2026) Low power compact dual mode detectors for nuclear security applications. PhD thesis, University of Glasgow.

Abstract

The transportation and use of illicit Special Nuclear Material is a challenge that must be addressed in an increasingly nuclear world. To this end, different monitoring techniques of key nuclear signatures, such as neutron and gamma radiation, are being investigated to improve the likelihood of the detection of this material.

This thesis details the work undertaken to achieve the goal of creating a low power and sturdy detector solution that can be used for longer term passive monitoring or active field interrogation. This is possible using dual (or even triple) mode detectors, which are detectors that can use pulse shape discrimination (PSD), to separate gamma and neutron radiation species, using different timing characteristics that arise from each form of radiation. The detectors chosen for use in this investigation were the inorganic scintillation materials CLLBC and CLYC, and the organic scintillation materials EJ-276 and EJ-299-50.

Inorganic scintillators are primarily sensitive to gamma radiation and thermal neutrons, with these materials commonly having a high 6Li enrichment, allowing for the thermal capture of these neutrons. Organic scintillators are sensitive to gamma radiation and fast neutrons, which have energies in excess of 1MeV. The EJ-299-50 material alters slightly from these definitions, as whilst it is organic in nature, it also has a high enrichment of 6Li, allowing it to act as a triple mode detector.

Both photomultiplier tubes (PMTs) and silicon photomultipliers (SiPMs) were used as photon detection technologies for recording scintillation photons throughout this investigation, with PMTs forming the significant majority due to operational difficulties with the available SiPMs. This was undertaken to evaluate the performance of each technology, as SiPMs are the natural choice for lower power performance. This is due to the lack of vacuum components and dynode chains required by a PMT, and a lower operating voltage of ∼40V. This is a significantly lower voltage than the ∼2kV required by a PMT. It was therefore necessary to determine the differences between each technology and how these would impact later designs.

Both digital and analogue processing methods were investigated, with digital methods expressed as discrete values processed using high speed digitisers, and analogue methods consisting of continuous voltage waveforms that would only be expressed digitally at a final output stage. The decision was made to attempt a comparative study, with the results recorded using digital methods in combination with a PMT considered the benchmark. This was due to the greater signal stability presented by the PMT, whilst the SiPM and analogue methods presented more opportunities for noise or faults to enter the system. Unfortunately each of three SiPM arrays used at various points during this investigation failed for separate reasons, and it was not possible to take sufficient SiPM results to determine a clear comparison between the technologies.

Each detector material was investigated and characterised to determine which material would make for the most effective prototype. Key results such as the detector energy resolution and gamma efficiency were also calculated. For CLLBC, CLYC, EJ-276 and EJ-299-50 the energy resolution at 662keV was determined as 5.6%±7.6 × 10−3%, 9.3%±0.04%, 20%±0.26% and 22.2%±0.21% respectively. The intrinsic gamma efficiency, also at 662keV, was determined to be 17.6%±8.8% and 13.6%±6.8%, for CLLBC and CLYC respectively. The use of a time-of-flight measurement allowed for the determination of neutron efficiency for the CLLBC, CLYC and EJ-276 materials, as EJ-299-50 was not available at the time of this measurement, with neutron efficiencies of 0.22%±0.05%, 0.31%±0.07% and 0.53%±0.12% determined respectively. This efficiency was calculated across the entire available neutron energy range which would affect each efficiency, due to the speciality of each material at detecting different neutron energies. The neutron efficiency of EJ-299-50 was unable to be calculated as there was not a characterised neutron source of known neutron emission rate available when this material was available, and as such would be an area of future interest.

The digital methods were used as a benchmark, with traditional figure-of-merit (FOM) analysis, which calculates the value of separation between the respective neutron energy regime and gamma species for each material, determining a FOM of 3.69±0.029 for CLLBC, 1.82±0.096 for EJ-276, 2.02±0.025 for EJ-299-50 and 2.5±0.1 for CLYC. However, the decision was made during this work to focus upon the analogue electronic methods, as it was believed the less intensive computing demands would decrease the power requirements, and allow for a better continuous passive interrogation prototype.

Electronic simulation results demonstrated that each of the chosen analogue PSD methods functioned as intended. These methods were the time-over-threshold method, which records the time a waveform is above a specified level, the zero-crossing method, which compares the time at which different waveforms cross a zero point, and the charge comparison method, which compares the amount of charge contained within predetermined time intervals. However due to manufacture delays and unforeseen noise contributions, the constructed prototypes did not produce results that matched the simulated outcomes. This meant that the current prototypes needed an additional round of redesign to prioritise noise suppression and signal amplification.

Overall, each of the available scintillation materials have been characterised to a greater extent which can be applied to later analogue PSD designs. This can be seen in combination with the electronic simulations that demonstrate the efficacy of each method, meaning that the choice of detector material and analogue PSD method can be made based on the user requirements. The designed analogue solution in this case did not operate as intended, but each method showed great promise for future development and investigation of low power analogue PSD solutions.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Supported by funding from AWE Nuclear Security Techologies and the NuSec Organisation.
• Subjects: Q Science > QC Physics
• Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
• Funder's Name: AWE Nuclear Security Technologies, NuSec Organisation
• Supervisor's Name: Seitz, Dr. Bjoern and Thomson, Dr. Frank
• Date of Award: 2026
• Depositing User: Theses Team
• Unique ID: glathesis:2026-85832
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 20 Mar 2026 11:57
• Last Modified: 22 Mar 2026 09:59
• Thesis DOI: 10.5525/gla.thesis.85832
• URI: https://theses.gla.ac.uk/id/eprint/85832