Properties of low energy ions observed in the Skylab cosmic ray experiment

Low energy cosmic ray nuclei in space were detected using a stack of Lexan polycarbonate exposed in free space outside the Skylab orbital station for 73 days (from 22 Nov. 1973 to 3 Feb. 1974, a solar ''quiet'' period). Since at the time of exposure the sun was typically quiet, it has become possible to study the low energy particles during solar minimum, that is free from contamination of solar particles. This is also the longest exposure in space for a retrievable detector with a thin shielding of only about 25 mg/cm2 of aluminium so that low intensity low energy particles could be recorded in appreciable numbers. Energy spectra and relative abundance of carbon to nickel nuclei have been derived from the measurements of each track in plastic sheets. The following important information emerges out of the work : (i) A high flux of oxygen group (C, N, O) nuclei in the energy interval 10.25 MeV/amu having unusual composition (with oxygen much more enhanced relative to carbon) was observed. (ii) At higher energies of 25.50 MeV/amu the flux of oxygen group nuclei falls off rapidly and it again rises around 75 MeV/amu. (iii) Additional data obtained recently indicates the enhanced abundances of Ca-Cr nuclei relative to iron group (charge Z = 25-28) in the region of E = 25-100 MeV/amu as was observed before. These nuclei (Ca-Cr) are very much enhanced by an order of magnitude compared to the galactic cosmic rays. (author)