Differential energy spectra of cosmic-ray protons and He nuclei have been measured for the first time by an ionization spectrometer flown at balloon altitudes. The energy range extended from 50 to > 1000 GeV. The observed differential intensities can be represented with power-law spectra with a slope of -2.75 ± 0.03 for protons and of -2.77 ± 0.05 for He nuclei. The proton-to-He ratio is 26 ± 3 at 40 GeV/nucleon and is constant within errors up to 400 GeV/nucleon.

• Received 25 February 1972

DOI:https://doi.org/10.1103/PhysRevLett.28.985