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Title: The Cosmic-Ray Composition between 2 PeV and 2 EeV Observed with the TALE Detector in Monocular Mode
We report on a measurement of the cosmic-ray composition by the Telescope Array Low-energy Extension (TALE) air fluorescence detector (FD). By making use of the Cherenkov light signal in addition to air fluorescence light from cosmic-ray (CR)-induced extensive air showers, the TALE FD can measure the properties of the cosmic rays with energies as low as ~2 PeV and exceeding 1 EeV. In this paper, we present results on the measurement of ${X}_{\max }$ distributions of showers observed over this energy range. Data collected over a period of ~4 yr were analyzed for this study. The resulting ${X}_{\max }$ distributions are compared to the Monte Carlo (MC) simulated data distributions for primary cosmic rays with varying composition and a four-component fit is performed. The comparison and fit are performed for energy bins, of width 0.1 or 0.2 in ${\mathrm{log}}_{10}(E/\mathrm{eV})$, spanning the full range of the measured energies. We also examine the mean ${X}_{\max }$ value as a function of energy for cosmic rays with energies greater than 1015.8 eV. Below 1017.3 eV, the slope of the mean ${X}_{\max }$ as a function of energy (the elongation rate) for the data is significantly smaller than that of all elements in the more » models, indicating that the composition is becoming heavier with energy in this energy range. This is consistent with a rigidity-dependent cutoff of events from Galactic sources. Finally, an increase in the ${X}_{\max }$ elongation rate is observed at energies just above 1017 eV, indicating another change in the cosmic-ray composition. « less
Award ID(s):
2012934 1806797 1712517 2112904
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The astrophysical journal
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Sponsoring Org:
National Science Foundation
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