Optimization of Scintillation Light Collection Efficiency in BGO Crystals With Surface Diffusion for Enhanced Sensitivity in the KAPAE

The Kyungpook National University (KNU) Advanced Positronium Annihilation Experiment (KAPAE) aims to search for rare decays of positronium. The Phase I detector of KAPAE, equipped with bismuth germanate (Bi4Ge3O12, BGO) scintillation crystals with a polished surface, was designed to observe rare visible decays. The BGO scintillation crystals were chosen for their high gamma detection efficiency, high density, and high effective atomic number, making them suitable for gamma-ray detection in various applications, including medical imaging and high-energy physics experiments. However, due to its high refractive index of 2.15, BGO experiences considerable internal light trapping, which significantly reduces the efficiency of scintillation light collection. The Phase II detector focuses on detecting invisible decays of positronium by improving the light collection efficiency and energy resolution while minimizing dead areas using surface-diffused BGO crystals. This article investigates the light collection efficiency of $3\times 3\times 15$ cm3 surface-diffused BGO crystals, demonstrating an approximately 22% higher light yield compared to polished surfaces. This improvement is confirmed by Monte Carlo simulations using the Geant4 toolkit, which predict similar enhancements in the light collection efficiency. The energy resolution is improved from an average of 12% to 10.8% with the diffused BGO crystals at 662 keV. These results enhance the sensitivity of the KAPAE Phase II detector to invisible decays of positronium. These improvements in the light collection efficiency and energy resolution of surface-diffused BGO crystals make them suitable materials for broader applications in experiments that require precise gamma energy measurement. In this article, we present the optimization of the KAPAE detector, providing insights for the design and enhancement of scintillation detector performance in particle physics and other high-energy applications.