Isotopic composition and energy spectrum of low-nergy helium nuclei in primary cosmic radiation

Abstract

The isotopic composition and the energy spectrum of low-energy helium nuclei have been investigated, using a nuclear emulsion stack flown from Fort Churchill, Canada, on August 4, 1962. The proportion of He3 nuclei at the top of the atmosphere was determined to be 18±5% within the kinetic energy interval of 160–370 Mev/nucleon and 24±8% within the rigidity interval 1.1–1.4 bv/c. The differential energy spectrum in the above energy interval was found to have a maximum around 275–300 Mev/nucleon. Assuming that there is no He3 at the cosmic-ray sources, and that the observed abundance reflects the interactions of the primary radiation in its passage through interstellar matter, we conclude that the low-energy primary radiation has traveled a total of 6±2 g/cm2 in this medium. This result is found to be consistent with the values obtained from the low-energy lithium, beryllium, and boron experiment. The implications of these results are examined on the basis of different models used for the propagation of cosmic radiation through interstellar matter.