The isotopic composition of cosmic-ray helium from 123 to 279 MeV per nucleon - A new measurement and analysis

A description is given of an experiment which was designed to measure the spectra and composition of cosmic-ray hydrogen and helium isotopes in the interval 120-280 MeV per nucleon to compare with the recent values observed at low energies and also with the only earlier measurement in this high-energy regime. The results presented were obtained during a high-altitude balloon flight from Thompson, Manitoba, in 1973 August. This flight occurred during the period in which the presence of the anomalous component in the helium spectrum became apparent, and provides the opportunity of determining whether this component extends to higher energies than those studied by satellite experiments. The results place an upper limit on the flux of 'anomalous' He-4 (which appeared at low energy in 1972) of at most 15% in the energy range of this experiment, and combined with the low-energy observations of the Chicago group, show an energy dependence of the average ratio of the helium isotopes in the modulated cosmic rays which is difficult to reconcile with current modulation models.