Aiming at the observation of cosmic-ray chemical composition at the knee energy region, we have been developinga new type air-shower core detector (YAC, Yangbajing Air shower Core detector array) to be set up at Yangbajing (90.522$^circ$ E, 30.102$^c
irc$ 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 thick 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 $10^{6}$ MIPs. The first phase of this experiment, named YAC-I, consists of 16 YAC detectors each having the size 40 cm $times$ 50 cm and distributing 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 about 500 m$^2$. The inner 100 detectors of 80 cm $times $ 50 cm each are deployed in a 10 $times$ 10 matrix from with a 1.9 m separation and the outer 24 detectors of 100 cm $times$ 50 cm each are distributed around them 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 proton, helium and iron nuclei between 5$times$$10^{13}$ eV and $10^{16}$ eV covering the knee and also being connected with direct observations at energies around 100 TeV. We present the design and performance of YAC-II in this paper.
