Tracks of very heavy cosmic-ray particles have been recorded in nuclear emulsions and several different types of plastic detectors exposed during two high-altitude ballon flights in September 1968. Based on a total of 113 tracks, the following flux values have been derived for kinetic energies >1 GeV/nucleon: $J\left(35\mathrm{}\le Z\le 40\right)=(16\pm 4)\times {10}^{-6\mathrm{}}$ $J\left(41\mathrm{}\le Z\le 50\right)=(28\pm 6)\times {10}^{-6\mathrm{}}$ $J\left(51\mathrm{}\le Z\le 60\right)=(9\pm 2)\times {10}^{-6\mathrm{}}$ $J\left(61\mathrm{}\le Z\le 70\right)=(1.8\pm 0.7)\times {10}^{-6\mathrm{}}$ $J\left(71\mathrm{}\le Z\le 85\right)=(3\pm 1)\times {10}^{-6\mathrm{}}$ $J(Z\ge 86)=(1.0\pm 0.6)\times {10}^{-6\mathrm{}}$ The well-established presence of particles with $Z\ge 86$ requires a cosmic-ray source in which the $\mathcal{r}$ process of nucleosynthesis has been effective. The relative abundances over the charge region ($51\le Z\le 85$) show agreement with solar-system abundances, but there appear to be significant departures for the two lower-charge groups. Calibration of the detectors and experimental problems are also discussed.

• Received 12 April 1973

DOI:https://doi.org/10.1103/PhysRevD.8.1707