Observations of high-energy electrons with the bets instrument and the future prospects

Abstract We have developed the balloon-borne electron telescope with scintillating fibers (BETS) for the observation of cosmic-ray electrons in 10 –100 GeV. The detector is an imaging calorimeter consisting of the scintillating-fiber belts of 36 layers (each 280 mm wide) and the 8 plates of lead (each 5mm thick ). The rejection of the background protons was performed with an efficiency of ∼2000 using the shower imaging capability with high granulation. The balloon observations were carried out in 1997 and in 1998 at Sanriku Balloon Center (ISAS) in Japan. The observed electron flux is consistent around a few 10 GeV with the recent results reported by the HEAT group. Comparing the flux with theoretical expectations by a diffusion model, the best fit is obtained for the model of a diffusion coefficient of 2 × 10 28 ( E/GeV ) 0.3 cm b /sec in the SN rate of once per 30 years in the Galaxy. In January, 2003, a newly-developed scintillating fiber detector will be borne on the Polar Patrol Balloon (PPB) in Antarctica and exposed for 30 days to the cosmic radiations. We can increase the statistics of electron number in 10–1000 GeV by 10 times comparing to the present data.