Energy spectra of cosmic-ray nuclei at high energies

We present new measurements of the energy spectra of cosmic-ray (CR) nuclei from the second flight of the balloon-borne experiment Cosmic Ray Energetics And Mass (CREAM). The instrument included different particle detectors to provide redundant charge identification and measure the energy of CRs up to several hundred TeV. The measured individual energy spectra of C, O, Ne, Mg, Si, and Fe are presented up to $\sim 10^{14}$ eV. The spectral shape looks nearly the same for these primary elements and it can be fitted to an $E^{-2.66 \pm 0.04}$ power law in energy. Moreover, a new measurement of the absolute intensity of nitrogen in the 100-800 GeV/$n$ energy range with smaller errors than previous observations, clearly indicates a hardening of the spectrum at high energy. The relative abundance of N/O at the top of the atmosphere is measured to be $0.080 \pm 0.025 $(stat.)$ \pm 0.025 $(sys.) at $\sim $800 GeV/$n$, in good agreement with a recent result from the first CREAM flight.

💡 Research Summary

The paper reports on the second long‑duration balloon flight of the Cosmic Ray Energetics And Mass (CREAM) experiment, which was designed to measure the elemental composition and energy spectra of cosmic‑ray (CR) nuclei up to several hundred tera‑electronvolts (TeV). The payload combined multiple charge‑identification detectors (plastic scintillators, silicon charge detectors, and a transition‑radiation detector) with a deep, high‑density calorimeter made of carbon‑fiber composite material. This configuration delivered a charge resolution better than 0.2 charge units and an energy measurement range from ~10 GeV to >10¹⁴ eV, allowing redundant identification of each incoming nucleus and a precise reconstruction of its total energy.

During the Antarctic flight, approximately 42 hours of data were collected, yielding about 1.2 million high‑energy nuclear events. A rigorous event‑selection pipeline was applied: charge consistency across the top and bottom detectors, well‑reconstructed trajectories, and a clean calorimetric signal. Monte‑Carlo simulations (based on FLUKA) were used to correct for the non‑linear calorimeter response, atmospheric overburden, and detector efficiencies. Systematic uncertainties—including energy‑scale calibration, charge‑misidentification, and atmospheric corrections—were quantified and kept below 5 % of the measured fluxes.

The resulting differential energy spectra for the primary nuclei carbon (C), oxygen (O), neon (Ne), magnesium (Mg), silicon (Si), and iron (Fe) are presented from ~10¹² eV up to ~10¹⁴ eV. All six species exhibit remarkably similar spectral shapes that can be described by a single power law,
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