Cosmic-Ray Electrons between 12 Mev and 1 Gev in 1967

Abstract

Observations of cosmic-ray electrons in the energy range of 12 Mev to 1 Gev were made in 1967 in a series of high-altitude balloon flights with a detector consisting of a scintillation-counter telescope, gas Čerenkov counter, and lead-plate spark chamber. Three flights were launched from Fort Churchill, Manitoba, in the summer of 1967, to measure the vertically incident primary electron flux; a fourth flight gave a direct measurement of splash albedo electrons. In April 1967 return albedo electrons were observed on a flight launched from Palestine, Texas. We derive 2σ upper limits for the flux of primary electrons at the top of the atmosphere of 20, 9, and 13 electrons/m^² sec ster in the energy intervals 17–57, 57–112, and 112–374 Mev respectively. Between 374 and 1060 Mev we find 16 ± 14 electrons/m^² sec ster. Above 100 Mev these results lie significantly below fluxes reported for 1966 by other observers. Comparison between the observed upper limits to the primary flux and a calculation of the flux of galactic secondary electrons indicates an absolute solar modulation of electrons below 300 Mev by at least a factor of 3. We observed splash albedo fluxes of 94 ± 16, 47 ± 11, 27 ± 9, and 2_(−2)^(+4) electrons/m^² sec ster in the energy intervals 12–50, 50–100, 100–350, and 350–1000 Mev, respectively, near Fort Churchill, Manitoba. Near Palestine, Texas, we find 60 ± 26 return albedo electrons/m^² sec ster between 25 and 65 Mev. At higher energies the observed flux of downward-moving electrons is consistent with being atmospheric secondaries; we give 2σ upper limits to the return albedo flux of 22, 12, and 6 electrons/m^² sec ster in the energy intervals of 65–131, 131–411, and 411–1149 Mev, respectively. These return albedo fluxes are significantly lower than corresponding fluxes previously reported by S. D. Verma but are consistent with results of a calculation by C. J. Bland. Comparison between our observations at Fort Churchill and observations at Palestine indicates a significant contribution to the splash albedo flux from primary particles with rigidity below 4.5 Gv.