Development of Yangbajing air shower core detector for a new EAS hybrid experiment

Aiming at the observation of cosmic-ray chemical composition in the "knee" energy region, we have been developing a new type of air-shower core detector (YAC, Yangbajing Air shower Core detector array) to be set up at Yangbajing (90.522 degrees E, 30.102 degrees N, 4300 m above sea level, atmospheric depth: 606 g/m(2)) in Tibet, China. YAC works together with the Tibet air-shower array (Tibet-III) and an underground water Cherenkov muon detector array (MD) as a hybrid experiment. Each YAC detector unit consists of lead plates of 3.5 cm thickness and a scintillation counter which detects the burst size induced by high energy particles in the air-shower cores. The burst size can be measured from 1 MIP (Minimum Ionization Particle) to 106 MIPs. The first phase of this experiment, named "YAC- I", consists of 16 YAC detectors each with a size of 40 cm x50 cm and distributed in a grid with an effective area of 10 m(2). YAC- I is used to check hadronic interaction models. The second phase of the experiment, called "YAC- II", consists of 124 YAC detectors with coverage of about 500 m(2). The inner 100 detectors of 80 cmx 50 cm each are deployed in a 10 x10 matrix with a 1.9 m separation; the outer 24 detectors of 100 cm x50 cm each are distributed around these to reject non-core events whose shower cores are far from the YAC- II array. YAC- II is used to study the primary cosmic-ray composition, in particular, to obtain the energy spectra of protons, helium and iron nuclei between 5x 10(13) eV and 10(16) eV, covering the "knee" and also connected with direct observations at energies around 100 TeV. We present the design and performance of YAC- II in this paper.