Development of electromagnetic calorimeter using GSO and LYSO crystals for the J-PARC muon-to-electron conversion search experiment

An electromagnetic calorimeter (ECAL) using an inorganic scintillating crystal has recently been developed for the COMET experiment at J-PARC in Japan, which searches for muon-to-electron conversion of a charged lepton flavor violating process. The ECAL is required to have an energy resolution better than 5% at 105 MeV, position resolution shorter than 1 cm, and time response faster than 100 ns which can be operated in a vacuum and a magnetic field of 1 T. A prototype ECAL was developed with the candidate crystals of GSO (Gd₂SiO₅) and LYSO (Lu_2-xY_xSiO₅). A beam test was carried out using an electron beam of 65 - 145 MeV/c at the Research Center for Electron Photon Science at Tohoku University in Japan. The results were as follows: Both the crystals have good linearity. GSO (LYSO) has energy resolution of 5.50 ± 0.02(stat) ± 0.04(syst) % (4.91 ± 0.01(stat) ± 0.07(syst) %), and position resolution of 8.1 mm (7.9 mm) at 105 MeV/c. The energy and position resolution depend on the beam incident position on the ECAL.