Calibration by cosmic rays

All of the CsI($Tl$) counters were calibrated using cosmic rays before assembled in the support structure. The counters were put into a light-shielded box and placed in a measuring system called a "cosmic-ray calibration stand", comprising 8 layers of drift chambers for tracking and two layers of scintillation counters for triggering and timing. Since these can trace cosmic-ray tracks with a position resolution of better than 1 mm, the position and path lengths of cosmic rays in a crystal can be calculated precisely. The signals from the preamplifiers of two photodiodes after being summed were then shaped and amplified with a shaper having the time constant of 1 $\mu$s, and digitized by LeCroy MQT300A and 3377 TDC. A typical distribution of pulse heights per path length which corresponds to a minimum ionization loss of about 5.67 MeV/cm has the energy resolution of about 25 %. The systematic measurement error was estimated to be less than 1 %.