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° E, 30.102° N, 4300 m above sea level, atmospheric depth： 606 g/m2） 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×50 cm and distributed in a grid with an effective area of 10 m2. 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 m2. The inner 100 detectors of 80 cm×50 cm each are deployed in a 10×10 matrix with a 1.9 m separation; the outer 24 detectors of 100 cm×50 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 5× 1013 eV and 1016 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.
