OBSERVATIONS OF COSMIC-RAY ELECTRONS AND POSITRONS USING AN IMAGING CALORIMETER

A balloon-borne magnet spectrometer system was flown for 5.5 hr at an altitude of more than 117,000 feet from Prince Albert, Saskatchewan (Canada), on 1989 September 5, when the Newark neutron monitor rate was 2952. The instrument was a modified version of the one used to observe antiprotons in 1979. The most significant modification was the addition of an imaging calorimeter, 7.33 radiation lengths thick. Inclusion of the calorimeter has significantly improved the ability to distinguish electrons and positrons from the other constituents of the cosmic rays. The absolute electron flux has been determined in the energy interval 1.3-26 GeV. The electron spectrum at the top of the atmosphere was found to be J(e-) = 177E(-(3.15+/-0.13)) electrons/ (m(-2) sr s GeV) in the energy range 4.0-26 GeV. Below 4 GeV, the spectrum showed flattening, which is consistent with the effect of solar modulation. The e(+)/(e(+) + e(-)) ratio was found to be (0.11 +/- 0.03) in the energy range 5.2-13 GeV.