Development of the air shower core detector for the primary cosmic-ray observation at the knee.

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The development of the burst detector was made to measure the energy flow of the air shower core originated by heavy cosmic rays such as iron component in order to resolve the primary composition of the cosmic rays at the knee region (10^<15>eV〜10^<16>eV) which is a key information on the cosmic ray origin. A proto type burst detector consisting of 40cm x 50cm x 3.5 cm lead and scintillator of 1cm thick was setup in 2003. The burst signal is read out by photomultiplier through wavelength shifting fiber. Wide dynamic range is obtained by using two photomultipliers of the different gain. From the air shower simulation study, it was shown that the primary composition can be well studied through the observation of the air shower core by this kind of the detectors placed at Tibet Yangbajing (4300 m above sea level) with 3.75m spacing in a grid. The data analysis is based on artificial neural network to separate the nuclear species (reported on 59^<th> annual conference of the Physical Socie … More ty of Japan 2004 and Autumn conference 2004).Systematic checks and improvements were made on the proto type detector in 2004 for the linearity, the dynamic range and the positional uniformity. Data acquisition system was also developed and 4 detectors were set up in the center of the Tibet air shower array. The new burst detector is under operation since Oct. 2004 with trigger condition of burst size greater than 40 particles. The trigger rate is well within the expected value and the observation is successfully continued until now. The result of an early analysis of the experimental data is in a good agreement with the simulation showing that the instrument is working efficiently for the aimed purpose (reported on 60^<th> annual conference of the Physical Society of Japan 2005).It is ensured that the burst detector developed in this research can be used in the next step to observe the primary composition at the knee energy range. It is expected that the enlarged instrument of 20 x 20 channels in a grid will provide the knee composition with high statistics through a few years observation. Less