ИСТИНА

The approach to cosmic ray study with reflected optical Vavilov-Cherenkov radiation (Cherenkov light) was proposed long ago. At present the SPHERE-2 detector is the only ex- isting apparatus that have detected a significant sample of extensive air showers by means of this method. At the same time the recorded data allows detailed reconstruction of EAS lateral distribution function used to study primary CR mass composition. We report on the status and results of the SPHERE experiment with the emphasis on the peculiarities of the reflected Cherenkov light technique. Detector response simulation was performed by means of full direct Monte Carlo simulation with account of realistic background and noise pat- terns recorded during the observational runs. Instrumental acceptance was simulated for various energies, charge numbers and zenith angles of primary nuclei. Primary energy of observed showers was estimated with a typical statistical uncertainty 10–20% depending on the primary nucleus parameters. The typical systematic uncertainty of the estimated energy vs the primary charge number was found to be below 3%. The primary all-nuclei spectrum was reconstructed. The fraction of light nuclei vs energy in the energy range 10–100 PeV was estimated by means of an event-by-event approach using the LDF steepness parameter.