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 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 $10\mathrm{GeV}/n$ to $2.2\mathrm{TeV}/n$ 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 $200\mathrm{GeV}/n$ established with a significance $>3\sigma$. They have the same energy dependence with a constant $\mathrm{C}/\mathrm{O}$ flux ratio $0.911\pm 0.006$ above $25\mathrm{GeV}/n$. 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.

• Received 28 July 2020
• Revised 1 September 2020
• Accepted 19 November 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.251102

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