Voyager Measurements of the Charge and Isotopic Composition of Cosmic Ray Li, Be, and B Nuclei and Implications for Their Production in the Galaxy

The relative abundance and the isotopic composition of Li, Be, and B in cosmic rays is important for understanding the propagation of these particles and the long-term production of these isotopes in the Galaxy. In this paper we present an analysis of 21 years of Voyager 1 and 2 spacecraft data. A total of ~600 Be nuclei are measured, along with corresponding larger numbers of Li and B nuclei. A subset of this data was also obtained at a significantly lower solar modulation level than corresponding measurements at the Earth. Our results are compared with propagation calculations using the latest (1998) cross sections. The agreement is excellent, including even the ⁷Be/⁹Be ratio, which has not been well predicted using earlier cross sections. The abundances of the isotopes ⁶Li, ⁷Li, ⁷Be, ⁹Be, ¹⁰B, and ¹¹B are important for studies of early-epoch nucleosynthesis in our Galaxy, and the ⁷Li abundance has cosmological implications. We present a calculation of the production of these isotopes in the Galaxy using a Galactic diffusion propagation model. The results of these calculations for cosmic-ray production agree well with the data but differ considerably from calculations of the production of these isotopes appearing in the literature used for nucleosynthesis studies that consider the total production of these nuclei from both the cosmic-ray nuclei process described here and the inverse process involving cosmic-ray H and He nuclei interacting with interstellar C, N, and O. Some suggestions are made as to the origin of these differences.