Isotopic and elemental composition of the anomalous low-energy cosmic-ray fluxes

Abstract

Recent observations of cosmic rays below ~30 MeV nucleon^(-1) have revealed anomalous enhancements in the energy spectra of oxygen, nitrogen, and helium relative to those of other nuclei such as boron and carbon. We have measured the quiet-time fluxes of the elements hydrogen through oxygen in the ~4-30 MeV nucleon^(-1) energy interval during the period 1972 October through 1974 October. We find that the low-energy fluxes of Li, Be, B, and Care consistent with those expected from adiabatic deceleration and show no significant evidence for secondary fragmentation products arising from the enhanced nitrogen and oxygen fluxes. In the ~6-12 MeV nucleon^(-1) interval, the observed nitrogen and oxygen nuclei are predominantly ^(14)N and ^(16)O, with upper limits (84% confidence level) of ^(15)N/N ≤ 0.26, ^(17)O/O ≤ 0.13, and ^(18)O/O ≤ 0.12 for the other stable nitrogen and oxygen isotopes. These isotopic measurements are consistent with the presumed isotopic composition of the neutral interstellar medium, as would be the case if the anomalous component is locally accelerated. If, on the other hand, the enhanced low-energy cosmic rays result from unusual nucleosynthesis processes which enhance nitrogen and oxygen but not carbon, then these processes are constrained to producing predominantly ^(14)N and ^(16)O.