Undoped CsI calorimeter for the KL0→π0νν¯ experiment at KEK-PS

An electromagnetic calorimeter consisting of 576 undoped CsI crystals has been used to search for the KL0→π0νν¯ decay at the KEK 12-GeV proton synchrotron (E391a). The calorimeter is placed in a vacuum of lower than 10−1 Pa. Special energy and timing calibrations have been performed. The energy calibration started from a check of the linearity for nine sample crystals using an electron beam. Then, after mounting all crystals in the E391a detector, we made an in situ calibration of the energy by using cosmic ray and punch-through muons. The gain constants obtained from both muons agree with each other with an accuracy of 2% (σ). They were further refined by using γγ samples from π0's produced off an aluminum plate (5-mm thick) hit by neutral beam particles. Using the final gain constants, we obtained a KL0 mass resolution of 4.3 MeV/c2(σ) for the KL0→π0π0π0 decay. Also, using the timing constants measured for cosmic rays, we obtained a resolution of 0.51 ns (σ) for the timing difference among six γ's in the KL0→π0π0π0 decay.