Low-Energy Cosmic-Ray Composition and Energy Spectra Measured in June 1965

Sounding rockets carrying nuclear emulsions to study the composition and energy spectra of the lowenergy cosmic rays were flown on 17 and 23 June 1965 from Fort Churchill, Canada. The results show that the intensity of the heavy nuclei ($Z\ge 3$) was about the same as that in July 1964 and below that measured by other observers in May and early June 1965, before the Forbush decrease on 15 June 1965. The heliumnucleus flux between 15 and 22 MeV/nucleon was found to be significantly lower on 17 June 1965 immediately after the Forbush decrease than on 23 June 1965. The charge spectrum indicates that in the region from 40 to 90 MeV/nucleon the ratio of light- to medium-nucleus fluxes is 0.19±0.07, well below the value of 0.5 found at intermediate energies (200 to 400 MeV/nucleon), and that the carbon-to-oxygen ratio is about 1, rather than 1.6 as found at higher energies. A strong preference for even nuclei relative to odd is seen for charges 9 to 14 in the 60- to 180-MeV/nucleon range.