A Cherenkov light detection experiment at mount Chacaltaya to study nuclear composition of cosmic rays

Cosmic ray composition is very important to clarify the origin of cosmic rays. In particular, the composition in the energy region at and beyond the knee is of special interest, because it must be closely related to sites of cosmic ray productions and mechanisms of particle accelerations. In the BASJE (Bolivian Air Shower Joint Experiment) group, we constructed a new air shower array at Mount Chacaltaya (5,200m above the sea level) to observe cosmic rays with energies greater than 1015 eV. Moreover, we have designed Cherenkov light detectors operated together with the air shower array. It is known that the longitudinal development of a particle cascade in the atmosphere strongly depends on the type of the primary nucleus, and an air shower initiated by a heavier nucleus develop faster than that by a lighter primary. It has also been pointed out that the longitudinal development of an air shower can be measured by detecting Cherenkov light emitted from electrons in the shower. We carried out a Monte-Carlo studies of air showers and found that primary cosmic ray nuclei types can be distinguished by measuring lateral distributions and arrival timings (waveforms) of Cherenkov lights associated with air showers. In our experiment, we install ∼ 18 Cherenkov light detectors in the air shower array and will carry out a hybrid experiment to observe air showers by determining directions and primary energies of cosmic rays from air shower array data, and identifying primary particle type from Cherenkov light detector data.