Relative abundances of the isotopes of galactic cosmic ray B, C, N, and O nuclei have been measured using the balloon-borne High Energy Isotope Spectrometer Telescope (HEIST). Analysis of data collected during the 1988 HEIST flight from Prince Albert, Saskatchewan, has resulted in mass histograms containing ~890 boron, ~3100 carbon, ~910 nitrogen, and ~3300 oxygen nuclei. Masses were derived using both the Cerenkov-Energy and ΔE-E' techniques, achieving a resulting rms mass resolution of ~0.26 amu. These isotopic composition measurements correspond to energy intervals at the top of the atmosphere of ~400-650 MeV/nucleon for boron, 430-670 MeV/nucleon for carbon, 440-680 MeV/nucleon for nitrogen, and 450-780 MeV/nucleon for oxygen, higher than previous direct isotope measurements for these elements.
The abundance ratios of carbon, nitrogen, and oxygen at the top of the atmosphere have been interpreted using an interstellar propagation model that includes improved fragmentation cross sections. Because cosmic ray boron is used as a "secondary tracer," the calculated isotope ratios of interest are insensitive to the value chosen for the solar modulation parameter, ø. The resulting abundance ratios for cosmic ray source material include ^(14)N/O = 0.042 ± 0.014 and ^(15)N/O ≤ 0.040, favoring no ^(15)N at the source. The carbon and oxygen isotopes at the cosmic ray source are ^(13)C/^(12)C = 0.005 ± .011 and ^(18)O/^(16)O = 0.0115 ± .0038, compared to solar system values of ^(13)C/^(12)C = 0.011 and ^(18)O/^(16)O = 0.0020. The derived cosmic ray source abundances show a possible enhancement of ^(18)O/^(16)O over the solar system value and a ^(13)C/^(12)C ratio consistent with solar system material. Taking a weighted average of our result with previous high resolution measurements of oxygen results in ^(18)O/^(16)O = 0.0075 ± 0.0024, an enhancement in ^(18)O of 3.75 times the solar system value.
Current isotope results are compared with models of cosmic ray origin. Both the supermetallicity model and the "anomalous" solar system model predict an ^(18)O excess in cosmic rays, however, the "anomalous" solar system model also predicts an excess in ^(13)C. The Wolf-Rayet model fits many of the currently observed isotopic excesses in cosmic rays, but the predictions for ^(18)O/^(16)O and the elemental N/O ratio are still in question. We conclude that although further refinements in the Wolf-Rayet model may explain ^(18)O and N/O, none of the presently available models account quantitatively for all of the observed differences in composition between cosmic rays and solar system material.
