We have performed a series of cosmic-ray electron observations using balloon-borne emulsion chambers since 1968. While we previously reported the results from subsets of the exposures, the final results of the total exposures up to 2001 are presented here. Our successive experiments have yielded a total exposure of 8.19 m2 sr day at altitudes of 4.0-9.4 g cm-2. The performance of the emulsion chambers was examined by accelerator beam tests and Monte Carlo simulations, and the on-board calibrations were carried out by using the flight data. In this work, we present the cosmic-ray electron spectrum in the energy range from 30 GeV to 3 TeV at the top of the atmosphere, which is well represented by a power-law function with an index of -3.28 ± 0.10. The observed data can also be interpreted in terms of diffusive propagation models. The evidence of cosmic-ray electrons up to 3 TeV suggests the existence of cosmic-ray electron sources at distances within 1 kpc and times within 1 × 105 yr ago.
ASJC Scopus subject areas