Voyager Measurements of the Mass Composition of Cosmic-Ray Ca through Fe Nuclei

We present measurements of the charge and isotopic composition of cosmic-ray Ca through Fe nuclei made on the Voyager spacecraft. We have analyzed 18 years of data in the energy range 100-300 MeV nucleon^-1 collected by the High Energy Telescope of the cosmic-ray subsystem experiment on the Voyager 1 and 2 spacecraft. The average solar modulation level for this measurement is 480 MV. These data have several times the statistical accuracy and have mass resolution comparable to the best previously published measurements covering this charge range from the ISEE cosmic-ray experiment. Many of the isotopes of these charges are dominated by secondary production from cosmic rays traversing the interstellar medium, and for these isotopes our measured isotopic fractions are generally consistent to within 10%, with the exception of certain K-capture isotopes, with those calculated for secondary production using the latest propagation codes and cross sections. For five isotopes, ⁴⁰Ca, ⁵²Cr, ⁵⁵Mn, ⁵⁴Fe, and ⁵⁸Fe, we can determine the cosmic-ray source composition to an improved accuracy over previous values. None of these isotopes show any significant differences with respect to the solar composition. This result, taken with other recent Voyager measurements of the isotopic composition of charge Z = 6-16 nuclei as well as Co and Ni nuclei, now provide the source composition of ~20 individual cosmic-ray isotopes with Z between 6 and 28 measured by the same instrument under the same conditions. This source composition is remarkably solar-like with only ¹³C, ¹⁴N, and ²²Ne isotopes showing significant abundance differences. This degree of similarity is not well explained by present specific models for the cosmic-ray origin and is discussed further in this paper.