Observations of ultraheavy cosmic ray particles

Ultraheavy cosmic ray particles (UH) and Fe nuclei were observed in two balloon flights at the mean cutoff rigidity of 10 GV (corresponding to 3.5 GeV/amu for a particle with Z = 50) in 1976 and in 1982. The total collecting power of an array exposed in the 1976 observation, which was expanded vertically with an area of 50.4 m2, was 846.2 m2 · hr at a mean altitude of 11.2 g/cm2 residual atmosphere. The other exposed in 1982 was expanded horizontally with an area of 28.8 m2, was 238.5 m2 · hr at a mean altitude of 7.7 g/cm2 residual atmosphere. Each array was composed of plastic track detector stacks of 40 cm x 50 cm. To measure the flux of Fe, several small sized stacks of emulsion or CR-39 plates were also included in the arrays. Seven tracks produced by particles with Z > 45 were found in CN sheets. The fluxes of UH and Fe nuclei were 0.11 ± 0.01 particles/m2 · sr · sec and (3.9 ± 1.5) x 10-6 particles/m2 · sr · sec, respectively, at the top of the atmosphere. The flux ratio of [UH]/[Fe] being compared with other experimental results, it was shown that the ratio decreases with increasing energy above 1 GeV/amu. To examine this variation with energy, calculations for the propagation of cosmic rays based on the nested leaky box model were made for UH (Z > 40) and Fe group nuclei. The calculations showed similar decreases in the [UH]/[Fe] ratios between 800 MeV/amu - 10 GeV/amu. This variation is due to the behavior of spallation cross-sections of UH nuclei around 1 GeV/amu and the decrease of the escape meen free path of cosmic ray particles with increasing energy. (author)