Design of novel compact detector based on the bismuth germanate scintillator and silicon photomultiplier for ortho-positronium physics

Positronium decay research is one of the sensitive probes to discover new physical phenomena. Because of the pairing system of electron (particle) and positron (antiparticle), we can study reactions which are forbidden by the Standard Model. In this study, we designed a novel compact detector for precise measurement of ortho-positronium (o-Ps) decay. Due to their compact size, silicon photomultipliers (SiPMs) are used for the photo-detection in the detector. The SiPM is well-known as a photodetector that can replace photomultiplier tube (PMT). In general, the size of the PMT is relatively large, so an SiPM was used to make compact trigger part for direct collection of scintillation light. In this research, the trigger part consists of plastic scintillator coupled directly to a single-channel SiPM to obtain the positron signal from the center of the detector. The trigger part is surrounded by the gamma-detection part to detect decay gammas in all directions. For dual readout, both sides of the Bismuth Germanate (BGO) scintillators are coupled with SiPMs. The designed frame and grid are used to support BGO scintillators and SiPMs. We obtained pretest data of a positron trigger signal and gamma-energy spectrum of ²²Na and ¹³⁷Cs radioactive sources for the novel compact detector. The detector will be used to study the C-parity violation, invisible, and rare decay search.