Observation of Cosmic-Ray Particles withZ≥50and Interpretation of the Charge Spectrum

Abstract

Large areas of plastic detectors and nuclear emulsions were exposed to the primary cosmic radiation on two high-altitude ballon flights in May 1968 and September 1969. From measurements on the tracks found in the scanning of the plastic detectors, events with charges $Z>\mathrm{}50\mathrm{}$ were selected, and these data were consolidated with those from our earlier flights. Several conclusions can be drawn from the observed charge spectrum. The detection of trans-bismuth nuclei confirms earlier observations of these particles in the cosmic rays. However, no trans-uranium particles were observed. Detailed features of the charge spectrum cannot be explained by nuclei from $r$-process nucleosynthesis alone. Although the addition of particles following $s$-process abundances yields improved agreement, the spectrum appears more complicated than would result from a simple combination of $r$- and $s$-process abundances with identical propagation histories.