The performance of the TA$\times$4 surface detector array: 4.3 years of the first-half expansion

The Telescope Array (TA) experiment aims to reveal the origin of ultra-high-energy cosmic rays (UHECRs) by observing air showers using surface detectors (SDs), which spread over an area of approximately 700 km$^2$, and fluorescence detectors (FDs) viewing the skies above the SD array.

The TA experiment has been observing UHECRs since 2008, and has reported an indication of clustering in the arrival directions of cosmic-ray events with energy greater than 57 EeV.

To improve the exposure for anisotropy studies of UHECRs, the TA$\times$4 upgrade was designed to expand the observational area by approximately 2,000 km$^2$ with 500 additional SDs.

Half of the planned upgrade, consisting of 257 SDs, was completed, and the newly installed array began operation in 2019.

In addition to the expanded SD array, two FD stations were constructed for the TA$\times$4 experiment.

In this paper, we present a study of the performance of the expanded SD array, including the energy resolution, angular resolution, and effective aperture, over the first 4.3 years of data acquisition.

While the effective aperture varied initially due to changing detector states, it has stabilized since June 2023 with more than 90\% operational SDs.

Furthermore, a new inter-tower trigger system was implemented to connect six new communication towers to form two geographically separated arrays, increasing the effective aperture.

The time variation of this effective aperture, the resulting total exposure of approximately 3,500 km$^2$~sr~yr, and a comparison with the original TA SD array are presented to demonstrate the performance of the expanded array.