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

TL;DR

This paper reports measurements of cosmic-ray proton and helium spectra from a balloon experiment, revealing harder spectra at high energies and a lower proton-to-helium ratio than previously observed, extending our understanding of cosmic-ray composition.

Contribution

First detailed measurement of high-energy cosmic-ray proton and helium spectra from a balloon flight, showing spectral hardening and altered composition at TeV energies.

Findings

Spectral indices of -2.66 for protons and -2.58 for helium at TeV energies.

Helium flux exceeds extrapolated lower-energy power law.

Proton-to-helium ratio is 9.1, lower than at GeV energies.

Abstract

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 ~38.5 km with an average atmospheric overburden of ~3.9 g cm$^{-2}$. Individual elements are clearly separated with a charge resolution of ~0.15 e (in charge units) and ~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 to 63 TeV/nucleon. They are harder than previous measurements at a few tens of GeV/nucleon. 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 to 63 TeV/nucleon. This ratio is considerably smaller than the previous measurements at a few tens of GeV/nucleon.