The energy spectrum of cosmic-ray electrons from 10 to 100 GeV observed with a highly granulated imaging calorimeter

Cosmic-ray electrons 1 have been observed in the energy range from 12 to ∼00 GeV with a new balloon-borne payload, the Balloon-borne Electron Telescope with Scintillating Fibers (BETS). This is the first publication of the absolute energy spectrum of electrons measured with a highly granulated fiber calorimeter. The calorimeter makes it possible to select electrons against the background protons by detailed observation of both the longitudinal and the lateral shower development. The performance of the detector was calibrated by the CERN-SPS accelerator beams: electrons from 5 to 100 GeV, protons from 60 to 250 GeV. The balloon observations were carried out twice, in 1997 and 1998, at the Sanriku Balloon Center (Institute of Space and Astronautical Science) in Japan. The observation time was ∼13 hr in all at an altitude above 34 km. A total of 1349 electron candidates were collected, and the 628 events with energies above 12.5 GeV, well above the geomagnetic rigidity cutoff of ∼10 GV, have been used to compose a differential absolute energy spectrum at the top of the atmosphere. The energy spectrum is described by a power-law index of 3.00 ± 0.09, and the absolute differential intensity at 10 GeV is 0.199 ± 0.015 m^-2 s^-1 sr^-1 GeV^-1. The overall shape of the energy spectrum in 10 ∼ 100 GeV can be explained by a diffusion model, in which we assume an energy-dependent diffusion coefficient (∝ E^0.3) for an injection spectrum, E^-2.4.