Direct Measurement of the Cosmic-Ray Helium Spectrum from 40 GeV to 250 TeV with the Calorimetric Electron Telescope on the International Space Station
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Author/Creator ORCID
Date
2023-04-27
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Citation of Original Publication
Adriani, O., et al. "Direct Measurement of the Cosmic-Ray Helium Spectrum from 40 GeV to 250 TeV with the Calorimetric Electron Telescope on the International Space Station" Phys. Rev. Lett. 130, 171002 (27 April, 2023). https://doi.org/10.1103/PhysRevLett.130.171002.
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This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.
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Abstract
We present the results of a direct measurement of the cosmic-ray helium spectrum with the CALET
instrument in operation on the International Space Station since 2015. The observation period covered by
this analysis spans from October 13, 2015, to April 30, 2022 (2392 days). The very wide dynamic range of
CALET allowed for the collection of helium data over a large energy interval, from ∼40 GeV to
∼250 TeV, for the first time with a single instrument in low Earth orbit. The measured spectrum shows
evidence of a deviation of the flux from a single power law by more than 8σ with a progressive spectral
hardening from a few hundred GeV to a few tens of TeV. This result is consistent with the data reported by
space instruments including PAMELA, AMS-02, and DAMPE and balloon instruments including
CREAM. At higher energy we report the onset of a softening of the helium spectrum around 30 TeV
(total kinetic energy). Though affected by large uncertainties in the highest energy bins, the observation of a
flux reduction turns out to be consistent with the most recent results of DAMPE. A double broken power
law is found to fit simultaneously both spectral features: the hardening (at lower energy) and the softening
(at higher energy). A measurement of the proton to helium flux ratio in the energy range from 60 GeV/n to
about 60 TeV/n is also presented, using the CALET proton flux recently updated with higher statistics.