Introduction of a thickness-dependent scintillator-PMT interface reflection coefficient to improve absolute light yield calculations for inorganic scintillators

Accurately determining the absolute light yield of scintillating crystals is of interest for many research fields and applications, including nuclear science and engineering, high-energy physics, and medical imaging. One way to determine the absolute light yield is to measure scintillating crystals of different thicknesses, calculate their light output as a function of thickness, and fit an analytical (model) function to the experimental data to estimate the absolute light yield [1,2,3]. There are currently two model functions used in literature, called the 2R-model [4] and the 2D-model [5]. In these models, the fit parameter (loss or absorption coefficient) is dependent on the scintillator-photomultiplier (PMT) coupling, which has a direct impact on the interface reflection coefficient. However, such interface reflection coefficient was not included in the aforementioned models. This coefficient is not negligible as it plays an important role in light extraction efficiency through the interface. Numerically estimating its value is important for many applications, including the absolute light yield measurements for inorganic scintillators and novel photonic-crystal nano-structures [6]. In this paper, we propose an extension to the 2D-model by including the scintillator-PMT reflection coefficient. We validated the new analytical model with measurements of four LYSO crystals with different thicknesses and in different coupling configurations. Furthermore, we numerically estimated the reflection coefficients of various optical coupling configurations and for different crystal thicknesses, and we hypothesized that such reflection coefficients should be dependent on the crystal thickness. We also showed that the model developed in this work (the extended 2D-model) predicts a 18%-20% higher light yield with the experimentally determined reflection coefficients in LYSO crystals. Finally, we compared the experimentally determined reflection coefficients to the theoretical (simulation) values and we observed more than 75% agreement.