Development of a liquid Xe photon detector for μ→eγ decay search experiment at PSI

We are developing a new type of photon detector for an experiment to search for muons decaying into a positron and gamma ray. Our sensitivity on the branching ratio is expected to reach 10^-14 by use of the world´s most intense continuous muon beam at Paul Scherrer Institut (PSI). In this experiment, the photon detector will utilize liquid Xe as a scintillation material because of its fast response, large light yield, and high density. The scintillation light emitted in liquid Xe will be directly observed by photomultipliers (PMTs) located in the liquid without the use of a transmission window. In order to study the detector response to gamma rays, we have constructed a prototype with an active volume of 2.3 × 10³ cm³ surrounded by 32 PMTS. The PMT is a new development enabling operation at temperatures as low as liquid Xe (165 K). Energy, position, and timing resolutions have been evaluated using gamma-ray sources ranging from 320 keV to 1835 keV. The performance of the prototype is presented