The energy spectrum of cosmic-ray protons and helium near 100 GeV

A large ring-imaging Čerenkov telescope (RICH-II) was flown on a high altitude balloon from Fort Sumner, New Mexico, USA in October 1997. This instrument is designed to determine the energy spectra of light cosmic-ray nuclei over the energy range 30–150 GeV/n through a precise measurement of the angle of Čerenkov emission from each particle. We give details about the design and performance of the instrument and present results on the absolute intensity of cosmic-ray protons and helium nuclei. The observed ratio of proton to helium intensities does not change significantly over this energy range. We also find that the abundances of protons and helium nuclei at 100 GeV/n at the cosmic-ray source relative to elemental galactic abundances are much smaller than those of other elements with comparable first ionization potential.