Ultra-high energy cosmic rays: 40 years retrospective of continuous observations at the Yakutsk array: Part 2. Mass composition of cosmic rays at ultra high energies

In the paper, we describe methods for the analysis and present results for the mass composition of cosmic rays, obtained by using these techniques over a large time span. The data were obtained at the Small Cherenkov array over a 20 – year period of continuous observation and 40 – years of observations at the main Yakutsk array. Our experimental data indicate a change in the mass composition in the energy range 1016–1018 eV and is confirmed by independent results obtained by other EAS arrays