MIP calibration and the long-term stability of CALET onboard the International Space Station

In August 2015, the CALorimetric Electron Telescope (CALET) docked with the International Space Station (ISS). CALET will measure the cosmic ray electron spectrum over the energy range of 1 GeV to 20 TeV with a very high resolution of 2% above 100 GeV, based on a dedicated instrument incorporating an exceptionally thick 30 radiation-length calorimeter with both total absorption and imaging units (TASC and IMC). Each TASC readout channel must be carefully calibrated to obtain the degree of precision necessary to achieve the high energy resolution. This report describes the specific calibration methods, focusing on the calibration of the energy deposit of each channel to obtain an ADC unit to energy conversion factor using Minimum Ionizing Particles (MIP), known as "the MIP calibration." To fully calibrate each lead tungstate (PWO) log of the TASC, it is first necessary to correct the position dependent effects so as to equalize the response along its length. In addition, because both the PWO light yield and the APD gain will vary with temperature, it is also required to correct for this temperature dependence. Following these corrections for the position and temperature dependence, and also using events extracted using event selection based on likelihood analysis, it was possible to find the energy conversion factor. With the excellent agreement between the conversion factors obtained from proton and helium MIP data, the validity of the absolute calibration of the energy conversion factor was confirmed. In the end, this report describes the analysis of the long term stability of the MIP calibration, from which it was concluded that the time dependence of the MIP peak value was successfully removed.