Cosmic-Ray Proton and Helium Spectra: Results from the JACEE Experiment

Measurements of the cosmic-ray hydrogen and helium spectra at energies from 20 to 800 TeV are presented. The experiments were performed on a series of twelve balloon flights, including several long duration Australia to South America and Antarctic circumpolar flights. No clear evidence is seen for a spectral break. Both the hydrogen and the helium spectra are consistent with power laws over the entire energy range, with integral spectral indices 1.80 +/- 0.04 and 1.68^+0.04_-0.06 for the protons and helium, respectively. The results are fully consistent with expectations based on supernova shock acceleration coupled with a ``leaky box'' model of propagation through the Galaxy.