A balloon experiment using CALET prototype (bCALET-2)

CALET (CALorimetric Electron Telescope) is a high energy cosmic-ray detector to be installed on International Space Station in 2015 to carry out accurate measurements of high energy electrons and gamma-rays. For verification of the detector performance, we carried out balloon experiments using CALET prototype detectors in May 2006 (bCALET-1) and in August 2009 (bCALET-2). In this paper we mainly report about the second experiment using bCALET-2. bCALET-2 is a calorimetric instrument composed of a 3.58 radiation length thick tungsten-scintillating fiber imaging calorimeter (IMC) and a 13.4 radiation length thick bismuth-germanium-oxide calorimeter (TASC). The concept of the structure is similar to that of CALET, but the number of sensors and the thickness of materials were optimized for the balloon experiment. The observation was carried out at the Taiki Aerospace Research Field of JAXA in Hokkaido, and the detector was flown successfully for 2.5 h at a level altitude of 35 km. The observed events were analyzed by methods developed through Monte Carlo simulations, and the energy spectra of electrons and atmospheric gamma-rays in the energy range of 1-30 GeV were obtained and compared to the results of previous experiments.