Temperature effects on light yield and pulse shape discrimination capability of siloxane based scintillators

In this work, we report the study of temperature effects on light yield of siloxane-based scintillators and on n/gamma discrimination capability, in terms of pulse shape discrimination (PSD). The solid scintillators are composed of phenyl containing polysiloxane (PMPS100), as a base polymer, loaded with moderate amounts (6 wt%) of 2,5-diphenyloxazole (PPO) as a primary dye and Lumogen Violet (LV) as waveshifter. The samples were heated in the range of 60-150 degrees C and scintillation performance were tested both after annealing for 24 h and in real time during heating. Light yield of siloxane-based scintillators containing 6 wt% PPO heated at 100 degrees C is very close to the room-temperature value, while heating at 120 degrees C causes a decrease of light yield (LY) of 17%. In addition, the figure of merit (FoM) for n/gamma discrimination of the scintillator shows a sensible worsening of the discrimination performances in case of prolonged treatment at 120 degrees C. Similar tests are made using the commercial plastic scintillator EJ-299 (currently named EJ-276), based on polyvinyltoluene (PVT). In this case, the light yield undergoes a much more rapid deterioration with annealing temperature, and at 70 degrees C it is reduced to 60% of the original value. The discrimination capability of EJ-299 decreases upon heating at 70 degrees C as well, with a 20% reduction of FoM; meanwhile for T > 70 degrees C the mechanical and optical features are remarkably degraded. The mass loss of primary dye PPO from the siloxane scintillator as a function of treatment temperature and initial dye concentration has been evaluated and compared to the behaviour of EJ-299. This measurement allows to single out and characterize a series of processes occurring during heating, which are relevant to the whole performance of the system under study, such as sublimation at the interface, thermally induced photooxidation of components, diffusion of fluorophores from the polymer bulk to the surface. The variation in luminescence characteristics have been analyzed by excitation/fluorescence spectroscopy and time-resolved fluorescence spectroscopy, in order to correlate the annealing treatment with the primary dye loss by sublimation, formation of superficial aggregates and/or degradation of the scintillator components in the synthesized siloxane scintillator.