Development and performance test of a prototype air shower array for search for gamma ray point sources in the very high energy region

Abstract An air shower array consisting of 16 fast timing detectors and 8 particle density detectors was constructed at Mt Norikura (2780 m above sea level) to see the basic performance and to acquire technical knowledge for an enlarged operation at Tibet in search for extraterrestial point sources emitting very high energy gamma rays (> several times 10 13 eV). In particular, we focus on the angular resolution in determining the arrival direction of air showers. When a lead plate of 5 mm thickness (which is close to the optimum one) is placed on each scintillator, the apparent particle number is increased by a factor of ∼1.9 and the angular resolution is improved by ∼1.5 as compared to the no lead plate case. We also performed a detailed Monte Carlo simulation and critically examined the so-called even-odd method which many authors have been using for estimating the angular resolution. It is pointed out that selecting showers in which the core falls inside the array is crucial for getting the high-precision direction. For such contained showers, the present array gives a median angular resolution of 0.80° for proton-induced showers whose median energy is 200 TeV. In the coming Tibet experiment, a median angular resolution better than 0.65° (E γ /100( TeV )) − 1 2 is expected for gamma rays.