A balloon-borne electron telescope with scintillating fibers

A new balloon-borne cosmic-electron telescope incorporating a trigger system and an imaging calorimeter is described. The trigger system is adopted to select electrons with energies above 10 GeV, and to reduce the background protons to about 1%. The imaging calorimeter consists of scintillating-fiber belts, emulsion plates and lead plates (∼ 8 radiation length thick in total). Two-dimensional profiles of shower development are observed with the scintillating fibers. An analysis of the profiles might reject 95 % of the miss-triggered protons at 85 % electron efficiency. The telescope was launched at Sanriku in September 1995, and flown for twelve hours at the average altitude of 38 km. We observed about 700 electrons over 10 GeV under 4 g cm ^-2 of average residual atmosphere.