Study of water Cherenkov detector to determine air shower arrival directions with accuracy

Several spread TeV gamma-ray sources have been observed in the galaxy by several experimental groups. Experiments with high angular resolution that can detect gamma rays in the 100 TeV region will help study acceleration mechanisms of those gamma-ray sources. In recent years, three groups using extensive air shower arrays have reported detecting gamma rays in the 100 TeV region from astronomical objects in the galaxy. In extensive air shower experiments, an arrival direction of a cosmic ray is determined by estimating a shape of a front surface of an air shower based on a detected secondary particle density distribution and detection time. The density of secondary gamma rays of an air shower is several times that of secondary electrons and positrons in an air shower and the difference increases as the distance from the shower axis increases. Therefore, secondary gamma-ray measurements are key to determining the arrival direction of the shower. We report the results of investigating the effect of secondary gamma rays on the time determination accuracy of an air shower front using a Monte Carlo simulation. We also report the results of examining different detector structures to improve the time resolution using a water Cherenkov detector, which has high detection efficiency for secondary gamma rays.