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This meeting was held at the moment the first phase experiments have ended and the enlargement of the Tibet Air Shower Array has just been completed. At present, the array is substantially improved, and according to Professor J.W.Cronin, a Nobel prize winner, the 'Figure of Merit' of the array has increased from 12 to 32 now, taking the first place in the world.
Among the 15 talks in the meeting, about one half cover physics work based on the first phase data and the techniques used in the second phase experiment. Another one half are the reviews on Tibet AS $\gamma$ experiments and the development tendency of the related research fields. From the talks and discussions have been reached some common recongnizations:
(1).In the first phase experiment, we've obtained the flux data (upper limit) of ~ 10TeV $\gamma$-ray emission for more than 50 objects in the northern sky (including the Crab Nebula and Mrk421), the first such measurements in the world and also the unique data so far for most of those objects. Recently, some air Cherenkov devices have entered this energy region, but their observation are limited to a few of the objects. In the near future, our enlarged array will still be the unique fulltime and wide angle device for ~10TeV $\gamma$-ray astronomy, but for a positive detection, we'd better reduce its detecting energy threshold further more. In our new plan, a dense detector area inside the current array will be made to lower the energy threshold about 3 times;
(2).The displacement observation of 10 TeV sun shadow caused by cosmic rays is a new topic of cosmic ray research and can only be done with Yingbajing Air Shower Array so far. The improved Yangbajing array has an effective area 8 times large as before, being able to monitor the monthly variation of the interplanetary magnetic field and solar activities.
With the help of the improved array and other methods, we may investigate the correlation of some terrestrial and atmospheric physics effects through the observation of solar activities and solar wind variations. Recently, a suggestion has been made to set up a big meutron monitor in Yangbajing. And a preliminary agreement has been reached between IHEP, China and the Institute of Physical and Chemical Research of Japan for a 10-year cooperation in this field;
(3).Yangbajing site is also advantageous to the measurement of the "knee" structure in cosmic ray energy spectrum around $10^{15}-10^{16}$eV. The air showers induced by cosmic rays with such energies just reach their development maximum near the atmosphere depth of Yangbajing (~600g/cm^{2}), causing the smallest fluctuation and the primary composition dependence of the shower size. Based on the more improved statistics, the enlarged array will provide a more precise and accurate cosmic ray all particle spectrum covering the "knee". With the installation of the emulsion chamber and burst detectors this autumn, more attentions can be paid to the investigation on the nuclear composition of cosmic rays around the "knee" region;
(4).Some speakers reminded that all good results obtained in Tibet experiments so far are closely related to the high elevation of Yangbajing site. However, limited by the scale and single sort of detector, the geographical advantages of Yangbajing have not been brought into full play at present. Aimed to a breakthrough in the ground-based $\gamma$-ray astronomy and in the "knee physics", some powerful upgrade and/or new plans are carrying out worldwide, such as MILAGRO (at Los Alamos), HEGRA (at La Palma Island) and GRACE (at Mt.Abu, India). Meanwhile, researches in this field have shifted to the energy region <100GeV, a "Virgin Soil" observation window, it is where that Yangbajing can make its particular contribution. In the meeting, the possibility of adding some Cherenkov light image telescopes to Yangbajing and working in 200-500 GeV has been discussed. The possibility of setting up a new image Cherenkov telescope array and a water Cherenkov detector array (2kmx 2km) in Yangbajing and working in <100 GeV region is also be considered with great interest.
All participants were inspired by the results obtained during the first phase of Tibet experiments and by the predicted bright prospects of the second phase.
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June 1996 BEPC News