Portal monitors for use in nuclear security typically use large volume detectors such as plastic scintillator detectors fabricated from polyvinyl toluene (PVT). Recent advancements in plastic scintillators have produced new scintillator materials which respond to both neutron and gamma radiation. These materials show pulse shape discrimination (PSD) properties which can b used to
distinguish between the fast neutron and gamma events. However it is observed that PSD performance generally degrades with increasing scintillator size. In this work we aim to understand the optical transport processes in PSD-sensitive plastic scintillator and how these are potentially
important for future portal monitors. Geant4 is a Monte Carlo radiation transport toolkit which is often used to model energy deposition in detector systems. However it also has the ability to ray-trace and track optical photons and tally
their arrival time at a collecting surface. In addition the performance of the reflective coating and of the scintillator geometry can be altered to study different detector configurations. Optical photons generated in a scintillation event can be sampled from a distribution with multiple exponential decay constants allowing the dependency of PSD on material properties to be studied. This paper will demonstrate Geant4 modelling of optical photon arrival times for a variety of material properties and geometries of plastic scintillator. These phenomena directly effect the time profiles and shapes of the detected optical pulses. We will discuss the effect of optical photon transport on the PSD performance of plastic scintillator, and the underlying causes of reduced PSD performance in large volume scintillators.