Cosmic-Ray Proton and Helium Spectra from the First CREAM Flight

Cosmic-ray proton and helium spectra have been measured with the balloon-borne Cosmic Ray Energetics And Mass experiment flown for 42 days in Antarctica in the 2004-2005 austral summer season. High-energy cosmic-ray data were collected at an average altitude of similar to 38.5 km with an average atmospheric overburden of similar to 3.9 g cm(-2). Individual elements are clearly separated with a charge resolution of similar to 0.15 e (in charge units) and similar to 0.2 e for protons and helium nuclei, respectively. The measured spectra at the top of the atmosphere are represented by power laws with a spectral index of -2.66 +/- 0.02 for protons from 2.5 TeV to 250 TeV and -2.58 +/- 0.02 for helium nuclei from 630 GeV nucleon(-1) to 63 TeV nucleon-1. They are harder than previous measurements at a few tens of GeV nucleon-1. The helium flux is higher than that expected from the extrapolation of the power law fitted to the lower-energy data. The relative abundance of protons to helium nuclei is 9.1 +/- 0.5 for the range from 2.5 TeV nucleon(-1) to 63 TeV nucleon(-1). This ratio is considerably smaller than the previous measurements at a few tens of GeV nucleon(-1).