Analysing the impact of detector properties on pulse shape discrimination in plastic scintillation detectors using Monte Carlo simulations

Holroyd, Caroline Julie and Aspinall, Michael (2025) Analysing the impact of detector properties on pulse shape discrimination in plastic scintillation detectors using Monte Carlo simulations. PhD thesis, Lancaster University.

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Abstract

This thesis investigates how key detector parameters influence the pulse shape characteristics and pulse shape discrimination (PSD) performance of the commercially available plastic scintillator EJ-276. Plastic scintillators provide a cost-effective and physically robust alternative to conventional neutron detection technologies, such as 3He detectors, liquid scintillators, and crystal scintillators. However, previous studies have shown that PSD performance in plastic scintillators deteriorates as the scintillator length increases, posing a significant limitation for large-area applications, such as radiation portal monitors (RPMs) used in homeland security. To address this, a systematic study was conducted using the Monte Carlo simulation toolkit Geant4. This research evaluates how variations in scintillator geometry and length, optical surface treatment, and photodetector noise impact light collection efficiency (LCE), pulse shape formation, and PSD performance. The simulations incorporate recent updates to Geant4 which enable more accurate modelling of scintillation decay components and their particle-dependent intensities, building on prior work in the field. Results show that increasing the scintillator length reduces light collection efficiency (LCE) and introduces pulse shape artefacts. These distortions deviate from the Gaussian-like appearance observed in smaller scintillators and increase susceptibility to noise, leading to degraded PSD performance in elongated detectors. While highly reflective and specular surface treatments enhance LCE, diffuse reflectors better preserve pulse shape fidelity. These findings provide new insights into the causes of PSD degradation in elongated plastic scintillators.