Optimization and Underground Implementation of the KAPAE Phase II Detector for an Invisible New Particle Search in Positronium Decay

RESUMO

The KAPAE Phase II detector was developed to search for invisible decays of positronium as a probe of physics beyond the Standard Model. The detector geometry was optimized with Geant4 simulations, and the scintillation and readout performance were evaluated experimentally. The temperature dependence of the BGO scintillator and several readout configurations were studied to find the operating condition giving the best energy resolution. The background level was then measured both at the surface and underground. At the surface, the positron-gamma coincidence background was of order 1 Hz, with occasional bursts up to about 65 Hz. Underground, it decreased to below about 3 \times 10^{-4} Hz, more than three orders of magnitude lower.