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 \sim 38.5 km with an average atmospheric overburden of \sim 3.9 g cm ^ { -2 } .
Individual elements are clearly separated with a charge resolution of \sim 0.15 e ( in charge units ) and \sim 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 \pm 0.02 for protons from 2.5 TeV to 250 TeV and –2.58 \pm 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 \pm 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 } .