CsI(Tl) photon detector with PIN photodiode readout for a K-mu 3 T-violation experiment

The π0 detector constructed for the T-violation search experiment E246 at KEK is described. It consists of 768 CsI(Tl) crystals with PIN photodiode readout. The crystals are assembled in a tight barrel with holes for muons entering the magnetic spectrometer. The design and performance of the calorimeter are presented. The average light yield of 11 000 p.e./MeV and average equivalent noise level of 63 keV (bench test) were obtained for the CsI(Tl) modules used in the calorimeter. The coherent noise was found to be about 11 keV per a module. A readout electronics has been specifically developed for the high counting rate environment of the experiment. A π0 invariant mass resolution of 9.9 MeV (σ) was obtained for the whole detector, including regions around the muon holes. A time resolution of 3.8 ns (rms) was achieved in the photon energy range of 10–200 MeV.