Measurements of cosmic-ray Li, Be and B nuclei in the energy range 100 MeV/nuc to >22 BeV/nuc

Abstract

We report on new measurements of the spectra of Li, Be and B nuclei in the primary cosmic radiation in the energy range 100 MeV/nuc to >22 BeV/nuc. The differential spectrum of these light nuclei is found to have a maximum at ∼400 MeV/nuc in 1966. The L/M ratio is found to be equal to 0.25±0.01, constant over the entire energy range of the measurement. Atmospheric and solar modulation effects on the L nuclei and the L/M ratio are discussed. It is concluded that this ratio is representative of conditions in interstellar space. Using the most recently available fragmentation parameters gives a material path length of 3.6 g/cm² of hydrogen for the particles producing the L nuclei. The absence of any variation of the L/M ratio with energy places severe constraints on models for the propagation of cosmic rays. Models in which the material path length is a strong function of energy — or that exhibit an exponential path-length distribution for a fixed energy are incompatible with these results. An examination of the abundance ratios of the individual L nuclei separately reveals major discrepancies with the predictions of interstellar diffusion theory based on presently accepted fragmentation parameters. The constancy of the measured Li/M and B/M ratios with energy is not in accord with the large energy dependence of these ratios expected from the energy dependence of the fragmentation cross-sections. The low Li/M ratio and high B/M ratio to be expected if these nuclei are created at a much lower energy than we observe are also not found. This presents difficulties for theories which suggest that the passage through matter has occurred at low energies subsequently followed by considerable acceleration.

The Be/M ratio in cosmic rays is anomalous in that it is ∼40% larger than expected on the basis of the fragmentation cross-sections. Evidence presented here on the isotopic composition of Be nuclei suggests that this discrepancy is due to an enhanced abundance of Be⁹ or Be¹⁰ in cosmic rays. This discrepancy complicates the determination of a cosmic-ray ‘age’ using the decay of Be¹⁰ into B.

Nevertheless the Be/B ratio is observed to remain constant at 0.42±0.03 over the energy range from 100 MeV/nuc to over 10 BeV/nuc. Unless the fragmentation parameters into the various isotopes of Be and B are such that e.g. Δ(Be/B)<0.05 as a result of this decay, then the age of cosmic rays is either >3×10⁸ years or <10⁶ years. The further observation that the mass to charge ratio of all Be nuclei of energy ∼1 BeV/nuc is =2.05±0.1 suggests that Be¹⁰ is present at these energies. This supports the idea of a short lifetime.