Optimizing the light output of a plastic scintillator and SiPM based detector through optical characterization and simulation: a case study for POLAR-2

The combination of plastic scintillators with Silicon Photo-Multipliers (SiPMs) is widely used for detecting radiation in high-energy astrophysics, particle physics, neutrino physics, or medical physics. An example of application for this kind of detectors are Compton polarimeters such as POLAR-2 [1-3] or LEAP [4-6] for which a low-Z material is needed for the Compton effect to be dominant down to as low energy as possible. Such detectors aim to measure low energy Compton depositions in organic scintillators which produce small amounts of optical light because of the low energy deposited and the low scintillation efficiency, and for which optimizing the instrumental optical properties consequently imperative. The light collection efficiency of such a device was studied with a focus on the POLAR-2 Gamma- Ray Burst polarimeter. POLAR-2 consists of a segmented array of 6400 elongated plastic scintillators divided into 100 modules, all read out by SiPMs. The conversion of incoming gamma-rays into readable signal goes through the production and collection of optical light, which has been optimized both through measurements and simulations. The optical elements of the POLAR-2 polarimeter prototype module were optically characterized and an optical simulation based on Geant4 was developed to fully model its optical performances. The results from simulations were used to optimize the design and finally to verify its performance. The study resulted in a detector capable of measuring energy depositions of several keV. In addition, an important finding of this work is the impact of the plastic scintillator surface roughness on the light collection. It was found that a plastic scintillator with a higher scintillation efficiency but made of a softer material, hence with a rougher surface, was not necessarily the best option to optimize the light collection. Furthermore, in order to optimize the optical crosstalk between different channels, a production technique for very thin (similar to 150 mu m) and reusable silicone-based optical coupling pads was developed. This method can be adapted to produce either standalone pads or to directly mold a layer on any SiPM to be later coupled to the scintillators. After an introductory discussion describing the need of a large scale GRB polarimeter like POLAR-2, the optical design and characterization of the polarimeter modules that compose its sensitive part are described. The Geant4-based optical simulations of the POLAR-2 modules and the impact of the optical properties of its various elements on the light collection efficiency of the instrument are later presented. The work is finally summarized and an outlook is given on the potential applications of the POLAR-2 optical characterization and simulation work to other experiments employing similar elements.