The hybrid trigger system in the Telescope Array experiment

Abstract

We report the development of a “hybrid trigger” system for the fluorescence detectors (FDs) and the surface detectors (SDs) in the Telescope Array (TA) experiment. It is shown from our Monte‐Carlo studies that geometry determination accuracies of monocular FD events are significantly improved using SD information (position at the ground level, timing of arrival, etc…). On the other hand, in a lower energy event, the trigger efficiency of SD for lower energy showers is smaller than that of FD. Therefore no SD information is available for events with energies below 1018.5 eV taken with the TA original data acquisition system. In order to collect the small SD signals, we developed an FD‐driven SD triggering system: this introduces an external triggering scheme to the TA‐SD using triggering signals from FD. We present the design of the system, installation and the operational stabilities in TA observation from October 2010.