Abundance of light nuclei in the primary cosmic radiation

Abstract

The composition of the primary cosmic radiation is modified as a result of nuclear collisions which particles suffer in transit through the interstellar medium. The most sensitive indicator for the frequency of such collisions is the number of nuclear fragments corresponding to lithium, beryllium, and boron, which are found in the incident radiation close to the top of the atmosphere. It is, however, difficult to determine what fraction of these particles originated in outer space and what fraction is due to additional nuclear collisions in the uppermost layers of the atmosphere above the point of observation. In order to determine the relative importance of these two components, we have measured the percentage of light elements in the primary cosmic radiation as a function of the amount of air traversed by the particles. Our measurements are based on an analysis of 651 particle tracks recorded at geomagnetic latitude λ=41°. The relative intensities obtained under air masses varying from 8.5 to 30 g/cm² are in good agreement with individual values for vertically incident particles obtained by other workers. Our data were obtained at a rather great altitude (6.6 g/cm2 of residual pressure), so that they permit an extrapolation to the top of the atmosphere which is largely independent of assumptions regarding the collision cross sections and fragmentation probabilities of complex nuclei. We have obtained the following primary flux values, in particles/m² sec sterad: Li, Be, B (L nuclei) 0.55±0.60 C 2.65±0.40 N 1.90±0.35 O+F 3.00±0.45 C, N, O, F (M nuclei) 7.55±0.65 Z>~10 (H nuclei) 2.20±0.35 Thus Li, Be, and B nuclei represent but a small fraction of the flux of heavy primary particles. In order to account for this composition, the amount of interstellar gas traversed by the particles since their initial acceleration cannot have exceeded 1 g/cm² of hydrogen, as will be shown in detail in a separate paper. The smallness of this value imposes very stringent conditions on acceptable theories for the acceleration and subsequent diffusion of cosmic-ray particles.