Design and performances of electromagnetic particle detector for LHAASO-KM2A

In the Large High Altitude Air Shower Observatory（LHAASO） project,the one kilometer square extensive air shower array（KM2A） is the the largest detector array in terms of effect ive area.It consists of 5635electromagnetic particle detectors（EDs） and 122 1 union detectors（MDs）.Each ED is composed of 16 scintillator tiles readout by wavelength-shifting fibers that are bundled and attached by a 25 mm PMT.The design of the unit and its performances,such as photoelectron yield,time resolution and uniformity,are discussed in detail.An assembling scheme for the whole ED is established to guarantee the uniformity throughout all 16 tiles in a single ED and all EDs in mass production.