Measurement of High-energy Cosmic-Ray Proton Spectrum from the ISS-CREAM Experiment

Abstract

The Cosmic Ray Energetics And Mass for the International Space Station (ISS-CREAM) experiment successfully recorded data for 539 days from 2017 August to 2019 February. We report the energy spectrum of cosmic-ray protons from the ISS-CREAM experiment at energies from 1.60 × 10 3 to 6.55 × 10 5 GeV. The measured spectrum deviates from a single power law. A smoothly broken power-law fit to the data, including statistical and systematic uncertainties, shows the spectral index change at 9.0 × 10 3 GeV from 2.57 ± 0.03 to 2.82 ± 0.02 with a significance of greater than 3 σ . This bump-like structure is consistent with a spectral softening recently reported by the balloon-borne CREAM, DAMPE, and NUCLEON, but ISS-CREAM extends measurements to higher energies.