Observations of hydrogen and helium isotopes in solar cosmic rays

The isotopic composition of hydrogen and helium in solar cosmic rays provides a means of studying solar flare particle acceleration mechanisms since the enhanced relative abundance of rare isotopes, such as 2H, 3H and 3He, is due to their production by inelastic nuclear collisions in the solar atmosphere during the flare. In this work the Caltech Electron/Isotope Spectrometer on the IMP-7 spacecraft has been used to measure this isotopic composition. The response of the dE/dx-E particle telescope is discussed and alpha particle channeling in thin detectors is identified as an important background source affecting measurement of low values of (3He/4He). The following flare-averaged results are obtained for the period, October, 1972 - November, 1973: (2H/1H) = 7+10-6 X 10-6 (1.6 - 8.6 MeV/nuc), (3H/1H) less than 3.4 x 10-6 (1.2 - 6.8 MeV/nuc), (3He/4He) = (9 ± 4) x 10-3, (3He/1H) = (1.7 ± 0.7) x 10-4 (3.1 - 15.0 MeV/nuc). The deuterium and tritium ratios are significantly lower than the same ratios at higher energies, suggesting that the deuterium and tritium spectra are harder than that of the protons. They are, however, consistent with the same thin target model relativistic path length of ~ 1 g/cm2 (or equivalently ~ 0.3 g/cm2 at 30 MeV/nuc) which is implied by the higher energy results. The 3He results, consistent with previous observations, would imply a path length at least 3 times as long, but the observations may be contaminated by small 3He rich solar events. During 1973 three "3He rich events," containing much more 3He than 2H or 3H were observed on 14 February, 29 June and 5 September. Although the total production cross sections for 2H,3H and 3He are comparable, an upper limit to (2H/3He) and (3H/3He) was 0.053 (2.9-6.8 MeV/nuc), summing over the three events. This upper limit is marginally consistent with Ramaty and Kozlovsky's thick target model which accounts for such events by the nuclear reaction kinematics and directional properties of the flare acceleration process. The 5 September event was particularly significant in that much more 3He was observed than 4He and the fluxes of 3He and 1H were about equal. The range of (3He/4He) for such events reported to date is 0.2 to ~ 6 while (3He/1H) extends from 10-3 to ~ 1. The role of backscattered and mirroring protons and alphas in accounting for such variations is discussed.