# Mass composition of cosmic rays above 0.1 EeV by the Yakutsk array data

## TL;DR

This study analyzes the mass composition of ultra-high-energy cosmic rays above 0.1 EeV using Cherenkov radiation data from the Yakutsk array over four decades, revealing a change in composition with energy.

## Contribution

It provides a detailed analysis of cosmic ray mass composition at ultra-high energies based on extensive Cherenkov radiation observations and models, extending previous measurements.

## Key findings

- Displacement rate dXmax/dE is nonlinear and energy-dependent.
- Evidence of changing cosmic ray composition with energy.
- Fluctuations of Xmax support composition variation.

## Abstract

The paper presents the results of the longitudinal development of extensive air showers (X$_{max}$) of ultra-high energies and mass composition of cosmic rays. The measurements of X$_{max}$ are based on data from observations of the Cherenkov radiation at the Yakutsk array for the period 1974-2014. The cascade curves of individual showers and the depth of maximum X$_{max}$ were reconstructed over the energy range 10$^{16}$-5.7$\cdot$10$^{19}$ eV. It is shown that the displacement rate of the parameter dX$_{max}$ / dE in the atmosphere is nonlinear and depends on the energy. Such a feature indicates a change in mass composition, which is confirmed by fluctuations of X$_{max}$ in this energy region. The composition of cosmic rays was determined by interpolation using the QGSJetII-04 model.