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

Abstract：

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 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.