Pulse Shape Discrimination is a technique used extensively for the detection and analysis of neutron emissions in mixed fields of ionising radiation. Pulse Shape Discrimination utilises a single detector and digital processing setup that can discriminate between incident γ-rays and neutrons based on the decay rates in the scintillation light caused by the different radiations interacting in the detector media. Extensive work has been carried out on optimising Pulse Shape Discrimination through improvements in digital signal processing, however few works focus on the physical geometry of the detection medium and how this influences discrimination.
This work uses Monte Carlo codes (OpenMC, FLUKA, GEANT4) to model the ratio of energy deposition from incident gamma and neutron isotropic radiation fields (1 MeV – 9 MeV range) in a series of EJ299 plastic scintillator geometries. The general shape types simulated included, spherical, cuboid, cylindrical, and conical geometries in a range of volumes. The simulated shapes were compared with regard to their volume and their surface-area-to-volume (SAV) ratio. Where SAV offers a normalised parameter to assess the cross-section of the detector covering the solid angle of the emitted radiations.
Simulated results indicate that at lower particle energies (< 3 MeV) conical detectors demonstrate a lower gamma/neutron energy deposition ratio, i.e. a higher proportion of neutron energy is deposited compared to the gamma energy deposition, when compared to other detector shapes with similar volumes and SAV ratios. A conical detector with an approx. 40 cm3 volume (height 6 cm, 2.5 cm base radius) has a gamma/neutron energy deposition ratio of 8.31%, compared to 10.21% for a cylindrical detector (height 2 cm, 2.5 cm base radius). Similar differences are found throughout a range of comparable volumes. Though modest, this difference would stimulate greater production of optical photons with a slower decay times which enhance the ability of digital signal processors to discriminate between the signals produced by the γ -rays and neutrons. The difference in energy deposition ratios between shape geometries diminishes as radiation energy increases.
Further work is being conducted to model additional geometric refinements, different detector media, and simulations of optical photon production occurring as a result of the energy deposition.