Measurement and interpretation of the isotopic composition of hydrogen and helium cosmic-ray nuclei below 75 MeV/nucleon

Abstract

The differential energy spectra of the hydrogen and helium isotopes of the galactic cosmic radiation have been measured in 1965 and 1966 by the highly eccentric satellites OGO I and IMP III. The energy ranges were respectively 20–50 and 30–90 MeV/nucleon.The measured ratio Γ_d = d/⁴He shows a very steep positive slope [Formula: see text] as a function of energy between 20 and 50 MeV/n. At 50 MeV/n, Γ_d is roughly 0.25. These results are in qualitative agreement with the measurements performed by the University of Chicago group on IMP III. The measured ratio Γ_He = ³He/⁴He is roughly 0.05 at 60 MeV/n, and decreases at lower energies.In order to understand the energy dependence of Γ_He and especially of Γ_d (Γ_d is not affected by the solar modulation), we investigated the creation of deuterons and ³He through fragmentation of cosmic-ray and interstellar ⁴He nuclei, and proton–proton reactions (assuming no injection of these isotopes at the source).We have used a Monte Carlo technique to propagate the cosmic-ray protons and ⁴He nuclei from the source to the earth. A critical survey of the available cross-section data has been made. The ionization loss, the energy dependence of the cross sections, and the reaction kinematics have been taken into account. The elastic scattering, acceleration in space, and solar modulation have not.We have systematically tried to fit the data with different source spectra and propagation models. Our present results completely rule out a total-energy, and favor a kinetic-energy source power spectrum. We discuss the bearing of different nonlocal propagation models (open model, closed model, injection at one time; different energy dependences of the mean "vacuum path-length") on Γ_d and Γ_He.