Isotopic composition of galactic cosmic ray boron, carbon, nitrogen, and oxygen

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 {dollar}sim{dollar}890 boron, {dollar}sim{dollar}3100 carbon, {dollar}sim{dollar}910 nitrogen, and {dollar}sim{dollar}3300 oxygen nuclei. Masses were derived using both the Cerenkov-Energy and {dollar}Delta{dollar}E-E{dollar}spprime{dollar} techniques, achieving a resulting rms mass resolution of {dollar}sim{dollar}0.26 amu. These isotopic composition measurements correspond to energy intervals at the top of the atmosphere of {dollar}sim{dollar}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 at the top of the atmosphere have been interpreted using an interstellar propagation model that includes improved fragmentation cross sections and uses boron as a "secondary tracer." The resulting abundance ratios for cosmic ray source material include {dollar}sp{lcub}14{rcub}{dollar}N/O = 0.0042 {dollar}pm{dollar} 0.014 and {dollar}sp{lcub}15{rcub}{dollar}N/O {dollar}le{dollar} 0.040, favoring no {dollar}sp{lcub}15{rcub}{dollar}N at the source. The carbon and oxygen isotopes at the cosmic ray source are {dollar}sp{lcub}13{rcub}{dollar}C/{dollar}sp{lcub}12{rcub}{dollar}C = 0.005 {dollar}pm{dollar}.011 and {dollar}sp{lcub}18{rcub}{dollar}O/{dollar}sp{lcub}16{rcub}{dollar}O = 0.0115 {dollar}pm{dollar}.0038, compared to solar system values of {dollar}sp{lcub}13{rcub}{dollar}C/{dollar}sp{lcub}12{rcub}{dollar}C = 0.011 and {dollar}sp{lcub}18{rcub}{dollar}O/{dollar}sp{lcub}16{rcub}{dollar}O = 0.0020. The derived cosmic ray source abundances show a possible enhancement of {dollar}sp{lcub}18{rcub}{dollar}O/{dollar}sp{lcub}16{rcub}{dollar}O over the solar system value and a {dollar}sp{lcub}13{rcub}{dollar}C/{dollar}sp{lcub}12{rcub}{dollar}C ratio consistent with solar system material. Taking a weighted average of our result with previous high resolution measurements of oxygen results in {dollar}sp{lcub}18{rcub}{dollar}O/{dollar}sp{lcub}16{rcub}{dollar}O = 0.0075 {dollar}pm{dollar} 0.0024, an enhancement in {dollar}sp{lcub}18{rcub}{dollar}O of 3.75 times the solar system value.; Current isotope results are compared with models of cosmic ray origin. The Wolf-Rayet model fits many of the currently observed isotopic excesses in cosmic rays, but the predictions for {dollar}sp{lcub}18{rcub}{dollar}O/{dollar}sp{lcub}16{rcub}{dollar}O and the elemental N/O ratio are still in question. We conclude that although further refinements in the Wolf-Rayet model may explain {dollar}sp{lcub}18{rcub}{dollar}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.