Cosmic ray nuclei of charge Z ≥ 3 during the period of low solar activity

Abstract

A detailed study of the composition and energy spectra of heavy nuclei of charge Z ≥ 3 in the primary cosmic rays has been made during the period of low solar activity, using two stacks of nuclear emulsions exposed in balloon flights from Fort Churchill, Canada, in June 1963. Each of the stacks was composed of 120 nuclear emulsions of three different sensitivities and was exposed at about 3.5 g. cm.^-2 of residual air for about 11.1 hr. Reliable resolution of charges of nuclei from lithium to oxygen was obtained; for heavier nuclei, charge groups were determined. From the analysis of 793 tracks of nuclei with Z ≥ 3, results on the following aspects were obtained: (1) The differential energy spectra of L (Z=3−5), M (Z=6−9) and H (Z=10−28) nuclei were measured in the energy intervel 150-600 MeV/nucleon; integral fluxes were obtained for energy >600 MeV/nucleon; (2) The energy dependence of the L/M ratio at the top of the atmosphere was determined; the ratios were obtained as 0.45 ± 0.06 and 0.29 ± 0·03 in the energy intervals of 200-575, and >575 MeV/nucleon respectively; (3) Relative abundances of individual nuclei of Li, Be, B, C, N and O at the top of the atmosphere were determined as 36, 29, 55, 100, 60 and 106 respectively in the energy interval 150-600 MeV/nucleon; corresponding values were also obtained for energy >600 MeV/nucleon. (4) The differential fluxes of multiply charged nuclei measured by us and by other investigators were used to determine the solar modulation between solar maximum to solar minimum. It was found that solar modulation of the fluxes of M and He nuclei were consistent with Rβ dependence and that the modulation parameter Δη between 1965 and 1957 was about 1.1. The implications of these results are discussed.