The depth of maximum shower development and its fluctuations: cosmic ray mass composition at E0≥1017 eV

We present new data on Cherenkov light observations obtained during 1994–2009 period, after a modernization of the Yakutsk EAS array. A complex analysis of xmax and its fluctuations σ(xmax) was performed in a wide energy range. With the new data, according to QGSJet II model, an estimation was made of cosmic rays mass composition for E0~1017–3×1019 eV. The result points towards a mixed composition with a large portion of heavy nuclei at E0~1017 eV and the dominance of light nuclei at E0~1019 eV. The analysis of σxmax energy dependence for the same energies qualitatively confirms this result. A shape of xmax distribution at fixed energy 1018 eV is analysed to make more precise conclusion on cosmic ray mass composition.