Oservations of Cosmic Ray Electrons and Positrons Using an Imaging Calorimeter

On september 5, 1989 a ballon-borne magnet spectrometer system was flown for 5.5 hr at an altitude of more than 117,000 ft from Prince Albert, Saskatshewan (Canada). The instrument was a modified version of the one used to observe antiprotons in 1979. The most significant modulation 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 (corrected to the top of the atmosphere). At energies above 4 GeV, the electron spectrum at the top of the atmosphere can be represented by a power-law spectrum with constant spectral index. The electron spectrum was found to be J_e- =177 . E^-(3.15+-0.13) electrons/m^2-sr-s-GeV in the energy range 4.0-26 GeV. Below 4 GeV the spectrum showed significant flattening. The e^+/(e^+ +e^-) ratio was found to be (0.11+-0.03) in the energy range 5.2 - 13 Gev.