The charge composition and energy spectra of cosmic-ray nuclei from 3000 MeV per nucleon to 50 GeV per nucleon

Abstract

We present new data on the energy spectrum and charge compositon of cosmic-ray nuclei with 3< or =Z< or =28. These data were obtained using a large-area multielement balloon-borne telescope containing scintillation counters, UVT Lucite Cerenkov detectors, and a Freon gas Cerenkov detector. The energy spectra and charge ratios have been measured over the energy range from approx.300 MeV per nucleon to approx.50 GeV per nucleon. We find that the data above approx.1 GeV per nucleon is consistent with a cosmic-ray escape length X (E) = (7.2 +- 1.2) E/sup -0.30plus-or-minus0.06/ g cm/sup -2/, where E is the kinetic energy in units of GeV per nucleon. This energy dependence can be reconciled with a Kolmogorov spectrum of interstellar scattering irregularities in a simple resonant scattering model for cosmic-ray diffusion in the Galaxy. The changes in primary-to-primary nucleus ratio we observe, including those involving Fe nuclei, are consistent with this escape-length variation and do not require the assumption of different source spectra for different nuclei. We note that because of this escape-length variation with energy, the exponent ..mu.. of the source spectrum of cosmic rays (q (E) approx.E/sup -mu/) must be 0.3 smaller than that of the higher energy spectra observedmore » at Earth. In analogy to the helium spectrum, it is concluded that the exponents of the source spectra of heavier cosmic-ray nuclei are probably not constant with energy but increase slowly from approx.1.95 at an energy approx.5 GeV per nucleon to 2.45 above 100 GeV per nucleon. Utilizing the individual charge ratios measured over a broad range of energies, we have determined a new set of cosmic-ray source abundances. The relative abundances of N, Na, Si, Ca and Fe differ from some earlier determinations. The behavior of the charge ratios below 1 GeV per nucleon is examined, and it is concluded that the evidence is contradictory regarding the continuation of an energy-dependent path length at lower energies.« less