The effect of γ-radiation and temperature on the spectral-luminescent properties of UPS-923A plastic scintillator

This work presents a comprehensive experimental study of the effects of γ-irradiation and temperature on the spectral-luminescent properties of the UPS-923A plastic scintillator based on polystyrene doped with p-terphenyl C 18 H 14 (pTP) and diphenyloxazolylbenzene C 24 H 16 N 2 O 2 (POPOP). The main objective of the study was to separate the contributions of direct radiation damage and thermally activated processes to the degradation of the material. To this end, the samples were γ-irradiated in the dose range of 5 × 10⁵–1 × 10⁷ rad at two fundamentally different temperatures—273 K and ∼313 K (irradiation channel temperature)—and the effect of purely thermal treatment of unirradiated samples in the range of 303–353 K was also investigated. It was experimentally established that at an identical γ-irradiation dose (10⁷ rad), the decrease in photoluminescence intensity at 273 K does not exceed ∼20–25%, whereas at the channel temperature, degradation reaches ∼80%, especially for the spectrum-shifting POPOP dopant. At the same time, purely thermal treatment up to 313–323 K does not lead to irreversible changes in luminescence characteristics, which rules out the possibility of explaining the observed degradation solely by a thermal effect. Based on experimental data and a quantitative assessment performed within the framework of Arrhenius kinetics, a mechanism of radiation-thermal synergism is proposed. It is shown that a moderate temperature increase from 273 K to ∼313 K leads to an exponential increase in the mobility of radiation-induced radicals (by approximately two orders of magnitude), which accelerates their migration and involvement in secondary radiation-chemical reactions— interactions of radicals with the polymer matrix and luminescent dopants, leading to their chemical modification or destruction. First and foremost, such processes affect POPOP molecules and disrupt the pTP→POPOP energy transfer. The results demonstrate the key role of temperature as a kinetic modifier of radiation damage and emphasize the need for strict control of the temperature regime during radiation testing and operation of plastic scintillators in detector systems operating under simultaneous radiation and thermal exposure.