Simulation study of the CALET instrument at the Japanese experiment module on International Space Station

We have carried out simulation study on the CALET (CALorimetric Electron Telescope) instrument, es- pecially for heavy payload, to optimize the performance for electron measurement up to 10 TeV. The CALET for heavy payload consists of a sampling imaging calorimeter brought from the balloon-borne electron telescope with scintillating fibers (BETS) and a total absorption calorimeter with BGO log arrays. By the analysis of the simulated events, we have confirmed the performance of the CALET as follows. The proton rejection power is as much as 10 6 with the BGO thick- ness of 35 cm (32 r.l.). The energy resolution is estimated to be 30%= p E(GeV) and the incident angle is determined by the imaging with an angular resolution less than 1 deg. marizing briefly the detector, the sampling imaging calorime- ter is stacks of 70cm 70cm planar scintillating fiber detector arrays with a 1.0 mm square cross section aligned in X and Y directions, interleaved with lead plates. The total thickness of lead plates is 13 radiation lengths (r.l.). The total absorp- tion calorimeter consists of 2.5cm 2.5cm 35cm BGO logs which are placed aligned in X and Y directions layer by layer and the thickness is 32 r.l. The total thickness is 45 r.l. and 2.1 nuclear mean free path for protons. From this design, it is possible to measure the full shower development of electrons up to 10 TeV and to detect a considerable part of secondary hadrons produced by the nuclear interactions of protons. In this paper we present the simulated performance of the CALET, especially for heavy payload design, which has a ca- pability to observe cosmic ray electrons in the energy range from several GeV to 10 TeV. We also report about the per- formance for measuring the gamma-rays from 1 GeV to 1 TeV.