On The Light Collection Efficiency And Long-term Stability Of The Scintillation Detector In KM2A Of LHAASO Project

To study the origin of cosmic rays, The Large High Altitude Air Shower Obser-vatory (LHAASO) project was proposed to build a large one-square-Kilometer com-pound Extensive Air Shower (EAS) array. As one main detector array of LHAASO, one square Kilometer EAS array (KM2A) consists of5137Electromagnetic particle Detec-tors (EDs) and1209Muon Detectors(MDs) distributing within a region of560m ra-dius. The ED demands to fulfill the two main performances; one is high-level detection capability of KM2A, such as particle resolution25%@1particle and less than2ns of time resolution for each ED, while the other one is long term stability of the ED operat-ing. This thesis is going to introduce in rich details the research on performance of the plastic scintillator used in detector.There exist so far two kinds of ED designs, respectively achieved by coupling of scintillator tiles and wavelength shift (WLS) fibers, coupling of scintillator tiles and air guide. To study their light collection efficiency, a set of tests related to performance of detector materials have been done in experiment, such as test of different size of scintillator tiles, WLS fibers, reflective materials and different coupling methods.At the same time, a corresponding simulation packages based on GEANT4have been successfully developed so as to understand the whole process, from the light gen-eration and the transfer process to the photoelectric conversion at the cathode of pho-tomultiplier tube (PMT). This thesis focused on elaborating the simulation of ED pro-totypes with air guide, whose results are comparatively in line with the experimental ones. In effect, good cost performance, response uniformity and simple workmanship will be the final criterions to obtain optimization design of EDs. Given that EDs operate in complex environment the use of long-lifetime scintil-lator is generally considered to be the key of their long-term stability and fine per-formance. Several types of scintillator tiles were tested through two kinds of aging experiments and it turns out that the both current candidate scintillators, ST-401and HND-S2have about10-year lifetime which is able to meet the needs of the experiment.