Proton and Helium Spectra from the First Flight of the CREAM Balloon-Borne Experiment
Abstract
Cosmic-ray proton and helium spectra have been measured with the balloon-borne Cosmic Ray Energetics And Mass (CREAM) 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 ∼38.5 km with an average atmospheric overburden of ∼3.9 g/cm<super>2</super>. Individual elements are clearly separated with a charge resolution of ∼0.15e (in charge units) and ∼0.2e, respectively, for protons and helium nuclei. The measured spectra at the top of the atmosphere are represented by a power law 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 to 63 TeV/nucleon. The measured proton and helium spectra are harder than previous measurements at a few tens of GeV/nucleon. Possible explanations of this spectral hardening could be the effect of a relatively nearby source or the effect of spectral concavity caused by interactions of cosmic rays with the accelerating shock. The helium flux is higher than that expected from extrapolation of a power-law fit to the lower-energy data. The relative abundance of protons to helium nuclei is about 8.8 ± 0.5 in the range from 2.5 TeV/nucleon to 63 TeV/nucleon. In this thesis, the analysis of proton and helium spectra will be discussed.