Isotopic Composition of Galactic Cosmic Ray Nuclei

Galactic cosmic rays represent a directly accessible sample of matter that originates outside the solar system, The elemental and isotopic composition of this sample of high-energy matter contains a record of nucleosynthesis in other regions of the galaxy, and of subsequent nuclear and electromagnetic interactions that have altered its composition. There have recently been significant new advances in reading this record, brought about in large part by the launch of new high-resolution instrumentation, In particular, measurements of the abundances of individual elements have now been extended into the upper 2/3 of the periodic table, and the individual isotopes of a number of heavy cosmic ray nuclei have recently been resolved. As a result, we can now specify the composition of cosmic ray source material in much greater detail. The primary focus of this report is recent progress in addressing the following question: In what ways is the composition of the matter that gets accelerated to be cosmic rays similar to, or different from, the composition of solar system matter? The answers to this question are providing important new quantitative clues to the origin and evolution of this high-energy sample of matter from the galaxy. The report is divided into three main sections, covering the composition of: 1) elements with nuclear charge Z ~ 28; 2) ultraheavy (Z 1 30) nuclei; and 3) cosmic ray isotopes. Each section summarizes observational advances, theoretical implications, and progress that can be expected within the next few years. I have limited the discussion to the energy region (-30 MeV/nucleon to -100 GeV/nucleon), where direct measurements of individual elements or isotopes are presently possible. Following the report is a bibliography, as well as references to other work cited in the text. Because the purpose of this report is to summarize U.S. contributions to this topic over the last four years (1979-1982), I will not report on important contributions that have recently been made outside of this country, although in many cases non-U.S. work will be cited in the context of the topics discussed.