Charge and Energy Spectra of Trans-Iron Cosmic Rays

Abstract

A 22-${\mathrm{m}}^2$ detector array consisting of $G-5$ nuclear emulsion, fast-film Čerenkov detector, and 40 sheets of plastic detectors was exposed for ∼ 60 h at a mean atmospheric depth of ∼3.7 g/${\mathrm{cm}}^2$ and a rigidity of <2.5 GV. The detector was designed to determine both the charge and energy of trans-iron nuclei. For the first time it has been possible to determine cosmic-ray abundances in the difficult region $30<Z\le 42$ as well as to study the heavier nuclei. Over the entire periodic table from Ne on, the trend of cosmic-ray abundances is similar to that expected from sources with solar abundances, after nuclear reactions in interstellar space have been taken into account. The abundance ratios $[Z\ge 90]:\left[81\le Z\le 83\right]:\left[74\le Z\le 80\right]$ are not inconsistent with solar abundances if Grevesse's photospheric abundances of Th and U, adjusted for radioactive decay, are adopted instead of meteoritic abundances. These ratios agree somewhat better with an $r$-process composition and a leakage lifetime of several million years. No trans-uranic nuclei were observed in the present flight. Serious consideration must be given to the possibility that most of the cosmic rays with $Z<\mathrm{}60\mathrm{}$ originate from material of solar composition. One of the major new results of the study was the determination of an energy spectrum for the nuclei with $Z>\mathrm{}60\mathrm{}$. If the integral-energy spectrum is given by $N\mathrm{}(>E)\mathrm{}\propto {(E+M_0{c}^2)}^{-\eta }$ where $E$ is the kinetic energy per nucleon, then the spectral index for high-$Z$ nuclei is $\eta =2.75\mathrm{}\pm 0.53$ for $0.5 \frac{\mathrm{GeV}}{\mathrm{amu}}\le E\le 2.0 \frac{\mathrm{GeV}}{\mathrm{amu}}$. In the same energy interval the index for helium nuclei is $\eta =1.3\mathrm{}$. This may imply a different origin for high- and low-$Z$ cosmic rays.