The isotopic composition of hydrogen and helium in solar cosmicrays provides a means of studying solar flare particle accelerationmechanisms since the enhanced relative abundance of rare isotopes, suchas 2H, 3H and 3He, is due to their production by inelastic nuclearcollisions in the solar atmosphere during the flare. In this work theCaltech Electron/Isotope Spectrometer on the IMP-7 spacecraft has beenused to measure this isotopic composition. The response of the dE/dx-Eparticle telescope is discussed and alpha particle channeling in thindetectors is identified as an important background source affectingmeasurement of low values of (3He/4He).
The following flare-averaged results are obtained for theperiod, 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 significantlylower than the same ratios at higher energies, suggesting that thedeuterium and tritium spectra are harder than that of the protons. Theyare, however, consistent with the same thin target model relativisticpath length of ~ 1 g/cm2 (or equivalently ~ 0.3 g/cm2 at 30 MeV/nuc) whichis implied by the higher energy results. The 3He results, consistent withprevious 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 more3He 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 arecomparable, 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 limitis marginally consistent with Ramaty and Kozlovsky's thick target modelwhich accounts for such events by the nuclear reaction kinematics anddirectional properties of the flare acceleration process. The 5September event was particularly significant in that much more 3He wasobserved than 4He and the fluxes of 3He and 1H were about equal. Therange 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 isdiscussed.
