The depth of maximum shower development and its fluctuations: cosmic ray mass composition at E_0 >= 10^17 eV

TL;DR

This study analyzes Cherenkov light data from the Yakutsk array to estimate cosmic ray composition across a wide energy range, revealing a transition from heavy to light nuclei dominance as energy increases.

Contribution

It provides new measurements of $ ext{xmax}$ and its fluctuations over a broad energy spectrum, offering updated insights into cosmic ray mass composition using the QGSJet II model.

Findings

Heavy nuclei dominate at around 10^{17} eV.

Light nuclei dominate at around 10^{19} eV.

The $ ext{xmax}$ distribution shape supports the composition transition.

Abstract

We present new data on Cherenkov light observations obtained during the period 1994-2009, after a modernization of the Yakutsk EAS array. A complex analysis of $\xmax$ and its fluctuations $\sigma(\xmax)$ was performed over a wide energy range. With the new data, according to QGSJet II model, an estimation was made of the cosmic ray mass composition for $\E \sim 10^{17} - 3 \times 10^{19}$ eV. The result points towards a mixed composition with a large portion of heavy nuclei at $\E \sim 10^{17}$ eV and the dominance of light nuclei at $\E \sim 10^{19}$ eV. The analysis of $\sigma(\xmax)$ energy dependence for the same energies qualitatively confirms this result. The shape of the $\xmax$ distribution at fixed energy $10^{18}$ eV is analysed to make more precise conclusions on cosmic ray mass composition.