A monitoring system of the fast-timing scintillation counters used for detection of air showers

A monitoring system of the time-offset values, the gains and linearity relations of the responses of scintillation counters has been developed to detect the air shower disks precisely. The system consists of a nitrogen-gas pulse laser, an attenuation unit of the light intensity, optical fiber cables delivering the light pulses to each scintillation counter, and of a microcomputer operating the whole system. It is confirmed by the air shower array at Mt Norikura that the time-offset values and their dependences on light intensity can be monitored with an accuracy of ±0.22 ns within one standard deviation (1σ). According to a simulation with this accuracy, the system can give an angular resolution as low as 0.33° in the median value for the case of the smallest array of four detectors with a 15 m span for vertical incidence. The linearity of the responses of each scintillation counter is achieved with an accuracy of ±0.2% in 1σ in the range of three orders of magnitude of the light intensity, and its absolute gain is given with ±2% in 1σ.