ABSTRACT
Positronium (Ps), a quasi-stable bound system of an electron and its antiparticle, the positron, has been used to study fundamental interactions, discrete symmetries and dark matter. There are two types of Ps, distinguished by the total spin: para-positronium (p-Ps, S=0) and ortho-positronium (o-Ps, S=1). Due to charge-conjugation conservation, o-Ps decays into an odd number of photons, while p-Ps decays into an even number. Both o-Ps and p-Ps can be utilized in dark matter searches by detecting at least one missing gamma ray in their decay processes.
This thesis details the design of an experiment that uses 22Na combined with linear alkyl-benzene (LAB) in acrylic containers to create positronia. The 22Na isotope has an 89.9% chance of decaying into 22Ne , accompanied by a positron, an anti-electron neutrino, and a 1.27-MeV photon. The calorimeter is designed to measure the energy deposition and impact position of each gamma ray. Two types of calorimeters, cross-type and offset-type, are constructed to cover a 4π solid angle using pure CsI crystals, each measuring 5 cm×5 cm×50 cm and wrapped in a 13-μm-thin mylar sheet.
Each type of calorimeter is a rectangular box. Each of the box is made of three layers of CsI crystal arrays, yielding a thickness of 15 cm. The cross-type calorimeter consists of six identical surfaces measuring 15 cm (thick) × 50 cm (wide) × 50 cm (tall) (wrapping thickness is ignored). The middle layer is rotated by 90 degrees relative to the other two layers, creating a cross-stacking arrangement. In the offset configuration, the middle layer of each surface is shifted by half a block relative to the front or back layer. Both configurations were evaluated using a Monte Carlo method via the Geant4 toolkit. Using 100,000 events of 22Na decay, the leakage of the 1.27-MeV gamma is (0.0159±0.0004) for the cross-type calorimeter and (0.0161±0.0004) for the offset-type calorimeter. Here, leakage refers to the probability of losing a particle with less than 80 keV energy deposition in the calorimeter. For the cross-type calorimeter, the gammaray leakages are (0.033±0.005)% for o-Ps and (0.126±0.012)% for p-Ps respectively. The missing-event fractions are (0.157±0.018)% for o-Ps and (0.438±0.033)% for p-Ps respectively.
For the offset-type calorimeter, the gamma-ray leakages are (0.030±0.005)% for o-Ps and (0.110±0.012)% for p-Ps respectively. The missing-event fractions are (0.161± 0.018)% for o-Ps and (0.406±0.032)% for p-Ps respectively. Notably, the gamma ray leakages for o-Ps are below 0.04%, and the missing event ratios for o-Ps are below 0.2% for both calorimeter designs, indicating that both configurations are adequate for detecting gamma ray energies. However, The missing-event fractions suggest a relatively high probability of background events that could mimic an exotic decay event, especially when compared to the world’s least sensitive upper limit of 2.0×10−4 [5].