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 similar to 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 similar to 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 log(10)(E/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 10(15.8) eV. Below 10(17.3) 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 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 10(17) eV, indicating another change in the cosmic-ray composition.