Charge and Energy Spectrum of Heavy Nuclei during the Solar Minimum, 1965

The differential energy spectra of $H_1$ ($Z\ge 20$) and $H$ ($Z\ge 10$) nuclei were measured in a balloon flight at a time close to the solar minimum using a nuclear emulsion stack flown at an altitude of 2.7 g/${\mathrm{cm}}^2$ from Fort Churchill, Manitoba, Canada on 30 June 1965. $\delta$-ray density and residual range measurements on tracks, and measurements of change of $\delta$-ray density with track length, were used to determine the charge and energy of the nuclei. The measured differential fluxes of $H_1$ ($Z\ge 20$) and $H$ ($Z\ge 10$) nuclei are 0.0013±0.0006 and 0.0045±0.0010 particles/(${\mathrm{m}}^2$ sr sec MeV per nucleon) in the energy range 300-500 MeV per nucleon and 0.0011±0.0004 and 0.0049±0.0008 particles/(${\mathrm{m}}^2$ sr sec MeV per nucleon) in the energy range 500-800 MeV per nucleon. These results are in agreement with satellite and rocket measurements made in the similiar energy range, and, when compared with low-energy measurements, indicate an energy spectrum of nucle that is flatter than those observed during earlier years.