Cosmic ray mass composition measurement in the energy range from $10^{16.5}$ eV to $10^{18.5}$ eV observed with the TALE hybrid detector

Abstract

We report on the cosmic ray mass composition measured by the Telescope Array Low-energy Extension (TALE) hybrid detector. The TALE detector consists of a fluorescence detector (FD) station with 10 FD telescopes located at the Telescope Array (TA) Middle Drum FD Station (itself made up of 14 FD telescopes), and a surface detector (SD) array of scintillators. The array consists of 40 SDs with 400 m spacing and 40 SDs with 600 m spacing. In this paper, we present results on the measurement of the depth of shower maxima ($X_\mathrm{max}$) in the energy range from $10^{16.5}$ eV to $10^{18.5}$ eV collected over five years of the TALE hybrid detector. The $X_\mathrm{max}$ distributions were analyzed and compared with Monte Carlo simulations of proton, helium, nitrogen, and iron primaries, using the QGSJet II-04 hadronic interaction model. Our results indicate that the elongation rate of the mean $X_\mathrm{max}$, which is defined as the slope of $\langle X_\mathrm{max} \rangle$ versus cosmic ray energy, exhibits a break around $10^{17}$ eV. Up to this energy, the composition becomes increasingly heavy, characterized by a growing dominance of heavy nuclei and a steadily decreasing fraction of light primaries. Beyond this energy, the proton fraction increases significantly with energy. These findings suggest a transition from Galactic to extra-Galactic cosmic ray sources around the so-called second knee.

Figures (32)

• Figure 1: Map of the Telescope Array detectors. Red diamonds represent the locations of each surface detector. Three FD stations are shown as blue hexagons. A transparent blue fan shape represents a field of view for each FD. The TALE SDs are deployed in the northwest part of the TA site, shown by magenta diamonds.
• Figure 2: Layout of the TALE detector. Open square boxes represent the locations of the TALE SDs, and a small filled circle corresponds to the location of the MD / TALE FD station. The arrows represent the azimuthal viewing ranges of the FDs. The curved boundary indicates the separation between the 40 SDs with 400 m spacing and those with 600 m spacing. The central data acquisition tower, labeled MDCT in the figure, stands for the Middle Drum Control Tower, which collects trigger and waveform data from the SDs.
• Figure 3: Left: Photograph of TALE telescopes at the FD station. Right: A deployed SD in the experimental site.
• Figure 4: Schematic comparison of monocular and hybrid shower geometry reconstruction. The relations between the measured values, $t_{i}$ and $\alpha_{i}$, and the fitting parameters, which are $t_{\rm{core}}$, $r_{\rm{core}}$ and $\psi$, are shown. In the FD monocular measurement on the left side, the expected detection timing of each PMT is expressed using these three parameters. In the hybrid geometry reconstruction on the right side, the parameter $t_{\rm{core}}$ is removed by two observables measured by an SD, $t_{\rm{SD}}$ and $r_{\rm{SD}}$.
• Figure 5: Primary dependence of the missing energy correction (ratio of the $E_{\rm{cal}}$ over the primary energy $E_{0}$) as a function of the $E_{\rm{cal}}$.