Clues for the origin of cosmic rays

Possible origins for cosmic rays are reviewed, noting that samples of every element of the periodic table have been detected in cosmic rays. Differences in cosmic ray elemental and isotopic abundances from solar system and interstellar matter abundances have been examined. Studies of relative abundances of atomic nuclei between numbers 26 and 40, based on high-resolution data from the ISEE-3 spacecraft, revealed a cosmic ray neutron-rich isotope ratio 1.6 times that of the solar system ratios. Spectroscopic examinations of interstellar clouds have revealed Si-29 and Si-30 enhancements, similar to cosmic ray abundances. Measurements of the chemical abundance distributions of rare ultraheavy elements on the HEAO-3 and Ariel 6 spacecraft yielded results consistent with r-process nucleosynthesis, similar to solar system material. More accurate measurements, including isotopic ratios of the ultraheavy elements, are recommended.