Direct Measurement of the Cosmic-Ray Carbon and Oxygen Spectra from 10 GeV$/n$ to 2.2 TeV$/n$ with the Calorimetric Electron Telescope on the International Space Station

TL;DR

This study reports precise measurements of cosmic-ray carbon and oxygen spectra from 10 GeV/n to 2.2 TeV/n using CALET on the ISS, revealing a spectral break around 200 GeV/n and consistent flux ratios.

Contribution

First detailed measurement of cosmic-ray carbon and oxygen spectra with CALET, identifying spectral hardening and flux ratios at high energies.

Findings

Spectral index change of ~0.15 around 200 GeV/n

Constant C/O flux ratio of 0.911 above 25 GeV/n

Flux normalization about 27% lower than AMS-02 but consistent with previous experiments

Abstract

In this paper, we present the measurement of the energy spectra of carbon and oxygen in cosmic rays based on observations with the Calorimetric Electron Telescope (CALET) on the International Space Station from October 2015 to October 2019. Analysis, including the detailed assessment of systematic uncertainties, and results are reported. The energy spectra are measured in kinetic energy per nucleon from 10 GeV$/n$ to 2.2 TeV$/n$ with an all-calorimetric instrument with a total thickness corresponding to 1.3 nuclear interaction length. The observed carbon and oxygen fluxes show a spectral index change of $\sim$0.15 around 200 GeV$/n$ established with a significance $>3\sigma$. They have the same energy dependence with a constant C/O flux ratio $0.911\pm 0.006$ above 25 GeV$/n$. The spectral hardening is consistent with that measured by AMS-02, but the absolute normalization of the flux is about 27% lower, though in agreement with observations from previous experiments including the PAMELA spectrometer and the calorimetric balloon-borne experiment CREAM.