Experimental Examination of Low-Energy Cosmic-Ray Heavy Nuclei

In a sounding-rocket experiment flown from Fort Churchill on 4 September 1963, a finite flux of cosmic-ray nuclei with charges greater than that of helium was detected at energies below the experimental energy cutoff of balloon-borne experiments. The particles were examined by extending large sheets of nuclear emulsions from the sides of the rocket during its period outside the atmosphere. The flux of medium nuclei (6≤Z≤9;Z=nuclearcharge) in the energy range from 30 to 150 MeV/nucleon was measured as 0.67±{}0.13 particles/(m²sr sec), and the flux of (10≤Z≤19) nuclei in the energy range from 40 to 190 MeV/nucleon was 0.31±{}0.09 particles/(m²sr sec). A finite flux of light nuclei (3≤Z≤5) also was seen in the 30 to 110 Mev/nucleon region. The abundances of medium and (10≤Z≤19) nuclei relative to helium nuclei in the same energy intervals were found to be significantly less than the relative abundances previously determined at high energies. However, within the statistical uncertainty, the relative differential flux values observed in the vicinity of the earth are consistent with the hypothesis that the helium, medium, and (10≤Z≤19) nuclei have the same source spectrum, at least above about 0.2 BeV/nucleon, for a wide range of source spectral shapes, and have interstellar path lengths of the order of those normally assumed (i.e., 2.5 g/cm² or slightly larger). Other possible interpretations also are considered.