Cosmic-Ray Boron Flux Measured from 8.4 GeV / η to 3.8 TeV / η with the Calorimetric Electron Telescope on the International Space Station
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Date
2022-12-16
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Citation of Original Publication
Adriani, O., et al. “Cosmic-Ray Boron Flux Measured from 8.4 GeV=n to 3.8 TeV=n with the Calorimetric Electron Telescope on the International Space Station.” Phys. Rev. Lett. 129, 251103 (16 December 2022). https://doi.org/10.1103/PhysRevLett.129.251103
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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 measurement of the energy dependence of the boron flux in cosmic rays and its ratio to
the carbon flux in an energy interval from 8.4 GeV=n to 3.8 TeV=n based on the data collected by the
Calorimetric Electron Telescope (CALET) during ∼6.4 yr of operation on the International Space Station.
An update of the energy spectrum of carbon is also presented with an increase in statistics over our previous
measurement. The observed boron flux shows a spectral hardening at the same transition energy E0 ∼
200 GeV=n of the C spectrum, though B and C fluxes have different energy dependences. The spectral
index of the B spectrum is found to be γ ¼ −3.047 0.024 in the interval 25 <E< 200 GeV=n. The B
spectrum hardens by ΔγB ¼ 0.25 0.12, while the best fit value for the spectral variation of C is
ΔγC ¼ 0.19 0.03. The B=C flux ratio is compatible with a hardening of 0.09 0.05, though a single
power-law energy dependence cannot be ruled out given the current statistical uncertainties. A break in the
B=C ratio energy dependence would support the recent AMS-02 observations that secondary cosmic rays
exhibit a stronger hardening than primary ones. We also perform a fit to the B=C ratio with a leaky-box
model of the cosmic-ray propagation in the Galaxy in order to probe a possible residual value λ0 of the
mean escape path length λ at high energy. We find that our B=C data are compatible with a nonzero value of
λ0, which can be interpreted as the column density of matter that cosmic rays cross within the acceleration region