Light transport modelling of pulse shape discrimination within plastic scintillators.

Plastic scintillators are used in scenarios where gamma-ray identification is necessary and can be scaled to large sizes for use in applications such as particle physics and nuclear security. In particular, modern plastic materials with the ability to detect neutrons via Pulse Shape Discrimination (PSD) are providing exciting new avenues of research for the field. The pulse shapes for these materials have characteristic decay time constants based on the radiation interaction causing scintillation emission. The materials exhibit this capability with volumes of a few cubic centimetres, but when scaled to larger volumes, the PSD performance diminishes. By identifying the cause of this deterioration, methods can be determined and implemented to increase PSD performance. Through the course of the work reported on, methods for increasing the performance are identified. In one instance, the detector configuration should be chosen based upon the application. The accompanying PSD algorithm should be selected based on the configuration and application environment to achieve maximum performance. When designing the detector configuration, simulations should be used as a tool to help maximise performance. The simulations presented in this thesis have identified that pulse shapes change based on many aspects of the scintillation system. By modelling the light transport, it has been found that the emission direction of scintillation light alters the pulse shape decay times. The light arrives in different distributions based on the directionality. Moreover, these distributions change based on reflector type, the percentage reflectivity and location of the scintillation emission. PSD relies on differences in the decay time constants of pulse shapes. These factors outlined lead to additional time constants being convolved with the pulse shape. This creates variation in the PSD parameter calculated in the algorithms. This variation increases with the scintillator volume and is the cause of reduced performance with large-sized plastic scintillators