Testing and analysis of the plastic scintillator units for LHAASO-ED

A total of 5195 electromagnetic particle detectors (EDs) are used in the 1-square-kilometer extensive air shower array (KM2A), which is a subarray of the Large High Altitude Air Shower Observatory (LHAASO). As the detection sensitive medium of the EDs, more than 20,000 plastic scintillator units (BC-408), produced by Saint-Gobain, are used in LHAASO. It is important to monitor the light output of the scintillator units among the units. To improve the efficiency, a sampling inspection scheme (misjudgment rate of less than 5%) was designed, and a batch test system was developed. Ten units of scintillator units can be measured at a time. The test system selects the single muon events of cosmic rays to measure the light output values of the plastic scintillator units. The measurement has an uncertainty of less than 2%. By pretest calibration, the difference between different channels can be eliminated. The calibration was implemented approximately every 3 months, and the test system had been running stably for 28 months. By measuring the ratio of the signals of selected far and near probe events, the changes in the quality of different batches of plastic scintillator units can be demonstrated. The test system realized accurate measurement of the light output, and all batches satisfied the requirements of the experiment.