Enhanced abundances of Ca, Ti and Cr in low energy cosmic rays in Skylab experiment

Abstract

WE report here the results of the measurements of the abundances of Ca, Ti and Cr nuclei relative to Fe, in low energy cosmic rays in 25–100 MeV/a.m.u. interval, which were measured in a dielectric detector assembly exposed on the outside of Skylab III for 73 d. The collecting power (defined as the product of area and exposure time) of the lexan detector in this experiment was the largest so far used in free space—1.1 × 10⁹ cm² s and the period of exposure from November 22, 1973 to February 3, 1974 was exceptionally ‘quiet’ (free from solar activity). These factors enabled us to make a reliable study of the abundance of these low energy cosmic-ray nuclei. The results on the measurements of the low energy oxygen group of nuclei in the same detector have been reported earlier¹. We found an interesting feature in the enhanced abundances of low energy Ca–Cr nuclei relative to Fe, which are different from the composition of high energy (> 1 GeV/a.m.u.) cosmic rays, but are similar to those of low energy cosmic-ray nuclei observed in interplanetary space^2,3. In the present work, however, observations have been made down to an energy of 25 MeV/a.m.u., which is a factor of two lower than those in earlier studies.