The Kyungpook National University Advanced Positronium Annihilation Experiment (KAPAE) aims to study ortho-positronium annihilation including invisible and visible exotic decay processes. The detector for this experiment consists of two main parts: the trigger part and the $\gamma$-ray detection part. In this study, we focus on the development of the trigger and $\gamma$-ray detection setup for KAPAE. In the trigger setup, the suitable thickness of the plastic scintillator for the positron trigger is determined. The aerogel and nitrogen gas environment in which ortho-positronium can reduce the pick-off effect for 3-$\gamma$ annihilation is created. The $\gamma$-ray detection part consists of 196 bismuth germanate (BGO) crystals and reflectors surrounding them. The light yield uniformity of BGO crystals and the best suitable reflector type are evaluated. The source position dependency test for the BGO crystals are also evaluated. Single layer of polyethylene naphthalate (PEN) film plastic scintil- lator and aerogel with the continuous flow of nitrogen gas are found well suited for the positron trigger and annihilation environment, respectively. The light yield and its uniformity of 196 BGO crystals confirm its application for $\gamma$-ray detection in KAPAE. Moreover, the VM2000 are found the best suitable reflector due to the small thickness and good reflectivity as compared to Teflon reflector. Finally, when the detector is upgraded for the invisible decay study, the $\gamma$ detection efficiency was compared through Geant4 simulation. This efficiency is used to calculate the invisible decay sensitivity of the KAPAE detector. It is expected that for 1 year of data, the upper limit of the sensitivity of total invisible decay will be $9.90 \times 10^{−9}$. This sensitivity can be improved up to $6.44 \times 10^{−9}$ by improving the data transfer rate. After upgrading the detector, the sensitivity may improve from $5.91 \times 10^{−13}$ to $3.85 \times 10^{−13}$. The upper limit of the sensitivity of partially invisible decay will be $10^{−7}$ to $10^{−8}$ orders depending on the emitted $\gamma$ energy.
Thesis Advisor: Prof. Hongjoo Kim