Full Dynamic Range Energy Calibration of CALET onboard the International Space Station
2017 August 16
In August 2015, the CALET (CALorimetric Electron Telescope) instrument, designed for long exposure observations of high energy cosmic rays, docked with the ISS (International Space Station) and shortly thereafter began to collect data. CALET will measure the cosmic ray electron spectrum over the energy range of 1 GeV to 20 TeV with a very deep calorimeter with both total absorption and imaging (TASC and IMC) units. Each TASC readout channel must be carefully calibrated over the extremely wide dynamic range of CALET that spans six orders of magnitude to obtain a degree of precision necessary to achieve high energy resolution. The entire dynamic range is covered by four different gain ranges, based on two photon detectors - an avalanche photodiode (APD) and a regular photodiode (PD) - in conjunction with a shaper amplifier with two gain ranges. The energy calibration process consists of three steps. First step is determination of conversion factor between ADC units and the energy deposit. By this way, the conversion factor of APD high gain is determined. Next step is linearity measurements over each gain range. UV pulse laser calibrations were performed on ground to confirm this linearity. Third step is correlation measurements between adjacent gain ranges. After confirming the input-output linearity of each readout channel of TASC, using the conversion factor of APD high and correlations, the conversion factor of all readout channels can be determined. Finally, using the estimated calibration errors and measured detector responses, such as the pedestal noise, the errors in the energy deposit sum were calculated for simulated electron events from 1 GeV to 20 TeV. As a result, 2% precision level energy calibration was achieved over the entire dynamic range above 10 GeV.