First preliminary results of the balloon-borne experiment SPHERE-2 on the allnuclei primary cosmic rays (PCR) spectrum and primary composition are presented. The primary spectrum in the energy range 10 16 -5 · 10 17 eV was reconstructed using characteristics of Vavilov-Cherenkov radiation of extensive air showers (EAS), reflected from a snow surface. Several sources of systematic uncertainties of the spectrum were analysed. A method for separation of the primary nuclei' groups based on the lateral distribution function' (LDF) steepness parameter is presented. Preliminary estimate of the mean light nuclei' fraction f30−150 at energies 3 · 10 16 -1.5 · 10 17 eV was performed and yielded f30−150= (21±11) %.

Status of the SPHERE experiment Related contentResults on the primary CR spectrum and composition reconstructed with the SPHERE-2 detector R A Antonov, S P Beschapov, E A Bonvech et al. Here is presented the current state of the SPHERE-2 balloon-borne experiment. The detector is elevated up to 1 km above the snow surface and registers the reflected VavilovCherenkov radiation from extensive air showers. This method has good sensitivity to the masscomposition of the primary cosmic rays due to its high resolution near the shower axis. The detector consists of a 1500 mm spherical mirror with a 109 PMT cluster in its focus. The electronics record a signal pulse profile in each PMT. In the last 2 years the detector was upgraded: time resolution of pulse registration was enhanced up to 12.5 ns, channel sensitivity was increased by a factor of 3, a new LED-based relative PMT calibration method was introduced, and new hardware and etc. was installed.