Development of an air shower array for accurate determination of the arrival direction of cosmic rays in the 10^<15>eV region.

array constructed in 1986 has been updated in 1987 to have The final system used for testexperiment as follows：16 fast timing detectors are placed in a 4x4 matrix with 15 m separation. Each consists of a plasticscintillater of 500x 500x35 size and a fast photomultiplier accomodated in a steinless steal box ofwhcih inner surface is painted white. A lead plate of 0.5 cm thick is placed on the top of the boxhave better time resolution by converting high energy gamma rays into election pairs. A cammacsystem is used for the data taking which is controlled by a micor-computer. the data is recorded inoptical disk with a频率关于1hz . In addition to these，8 detectors dedicated to the electron number measurement are distributed in the center andperipheral part of the array.The two main purposes of the test experiment, i.e.，1) establishing the stability for a long time （especially for heavy weather conditions）和2)acquiring the augular resolution better then 1have been achieved successfully. The angular resolution is estimated and its dependence on Theshower size,the number of detector and the timing accuracy of the detectors has been clarified. the presentsystem give -0.7 for 10 <15>eV cosmic rays where means that the estimated arrival direction fallsin within from the true direction for 50% cases. Increasing the number of detectors and upgradingthe quality of scintillaters will give us better resuolution than the present onebetter than 1 - for 10^<14>eV region showers better than 0.3 - for 10^<15>eV showers. thus we have​reliable gamma-ray astronomy in the 10 eV region can be explored。

The array constructed in 1986 has been updated in 1987 to have the final system used for test experiment as follows: 16 fast timing detectors are placed in a 4x4 matrix with 15 m separation. Each consists of a plastic scintillater of 500x 500x35 size and a fast photomultiplier accomodated in a steinless steal box of whcih inner surface is painted white. A lead plate of 0.5 cm thick is placed on the top of the box to have better time resolution by converting high energy gamma rays into election pairs. A cammac system is used for the data taking which is controlled by a micor-computer. The data is recorded in an optical disk with a frequency of about 1 Hz. In addition to these, 8 detectors dedicated to the electron number measurement are distributed in the center and peripheral part of the array.The two main purposes of the test experiment, i.e., 1) establishing the stability for a long time (especially for heavy weather conditions) and 2) acquiring the augular resolution better then 1゜, have been achieved successfully. The angular resolution is estimated and its dependence on the shower size, the number of detector and the timing accuracy of the detectors has been clarified. The present system give -0.7゜ for 10^<15>eV cosmic rays where means that the estimated arrival direction falls in within from the true direction for 50 % cases. Increasing the number of detectors and upgrading the quality of scintillaters will give us better resuolution than the present one; better than 1゜ for 10^<14>eV region showers better than 0.3゜ for 10^<15>eV showers.Thus we have established the basis for a full scale experiment at Yambatin (4300m) in Tibet and expect that a reliable gamma-ray astronomy in the 10^<15>eV region can be explored.