Development of a versatile calibration method for electro-magnetic calorimeters using a stopped cosmic-ray beam

A new method using a stopped cosmic-ray beam for the calibration of an electro-magnetic calorimeter which consists of several hundreds of modules has been established. Cosmic muons stop in the calorimeter, and positrons (and electrons) from the muon decays with the maximum energy of 53 MeV are used for the energy calibration. These events could be identified as double pulses observed by a flash ADC. This technique has many advantages, (1) intermediate energy scale (<; 53 MeV) to interpolate between a few MeV for radioactive sources and 153 MeV, the muon from K
⁺
→ μ
⁺
n
_μ
, (2) efficient procedure to calibrate all modules at the same time and (3) versatile method applied for many types of electro-magnetic calorimeters. This method was checked in the J-PARC E36 experiment which was performed to precisely measure the ratio of γ(K+ → e+n
_e
)/γ(K
⁺
→ μ+n
_μ
). Double pulses from the CsI(Tl) photon detector were successfully observed and the waveform function was carefully studied to decompose the second pulse generated by the decay positron. The experimental energy spectrum is consistent with the simulated Michel spectrum taking into account the shower leakage from the CsI(Tl) module, which indicates the validity of the proposed calibration method.