Design and performances of prototype muon detectors of LHAASO-KM2A

In the Large High Altitude Air Shower Observatory project, 1221 muon detectors will be built to discriminate cosmic gamma rays from nuclei and between cosmic ray species with energies in 1013-1018eV. The muon detector has been designed to meet the physical requirements, according to which two muon detector prototypes have been built at the Yangbajing Cosmic Ray Observatory at the end of 2012 and in October 2014, respectively. The first one has been stably operated for more than 2 years. The performances of the two detectors have been evaluated, especially for position dependence and punch-through effect. In this paper, the design and the performances of the prototype muon detectors are presented.