Trigger time acquisition system for ground based air shower experiments

For observations of cosmic gamma rays of several TeV or more, many experimental groups conduct ground-based air shower observation experiments. New projects such as Andes Large area PArticle detector for Cosmic ray physics and Astronomy (ALPACA) are also underway. In these experiments, an accurate trigger time measurement system is required when sudden cosmic phenomena are observed. To record such sudden astronomical phenomena, we have developed an event-trigger time recording system that accurately measures the arrival times of air showers. The system consists of a commercial global navigation satellite system module, a high-precision clock, a multi-hit type time-to-digital converter with a reset function for accurate time measurement, and a network-time-protocol server installed on a computer. It was confirmed that the time accuracy compared to UTC was approximately 1μs, and the time deviation was approximately 11.4 ns with one standard deviation. The developed system is versatile and can also be used for the ALPACA experiment. It is expected that the time accuracy of this system can be improved to approximately ±40 ns.