Candidate: Charles Sosa

Title: The Importance of Light Collection Efficiency in Radiation Detection Systems that use Organic Scintillators

Chair: Sara Pozzi

Abstract: An organic scintillator is a transparent material that fluoresces when ionizing radiation interacts with it, making it a suitable radiation detector when coupled to a light-readout electronic device, such as a photomultiplier tube (PMT). The most commonly used organic scintillator shape is a cylinder: one face is coupled to the PMT, while the remaining surfaces are covered with a reflective material to increase the light collected by the PMT. This work demonstrates how modifying the shape and reflective-boundary of organic scintillators can improve light-collection efficiency (LCE) and, by extension, the performance of radiation detection systems, particularly for applications within nuclear nonproliferation and safeguard. Efforts to improve detector performance have historically focused on increasing light output through chemical means, the detection efficiency of light- sensing technology, and methods in data acquisition and processing. While vast research in these areas has shown improvements to detector performance, less research exists on the impact of the organic-scintillator shape and reflective-boundary conditions. This work compared the performance of conical and cylindrical organic scintillators of two materials (EJ200 and trans-stilbene) in three key areas of detector performance: energy resolution, timing resolution, and particle identification; often referred to as pulse-shape discrimination (PSD). Conical EJ200 and stilbene outperformed their cylindrical counterparts in LCE by 17.6% and 18.0%, respectively. Gains in energy resolution were shown to be strongly dependent on the light output of the scintillator material and the quality of the spectral match between the wavelength emission and of the scintillator material and PMT response. Conical EJ200 outperformed cylindrical EJ200 by 34% in time resolution. And conical stilbene outperformed cylindrical stilbene by 23% in PSD within a light-output range of 25 keVee to 100 keVee. The work also developed and validated a Geant4 model to study differences in the light-collection process for both geometries.