Development of Air Shower UV Cherenkov Detector to Servay gamma-ray Point Soura

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It is prevailing to utilize atmospheric Cherenkov light from air shower to observe TeV cosmic ray. Since UV light coming from the iniverse is absorbed by the atmosphere, it is expected that detecting UV component of Cherenkov light improves S/N and makes the technique applicable to new regions. Mt. Norikura was adopted as the observation site where Cherenkov light is observed which is less absorbed by the atmosphere than at sea level. We mode observation in 1991-93. We measured Cherenkov light, and also background such as starlight and moonlight using mirrors of diameter 60-80cm, with UV PMTs, visible tubes, an equatorial plate etc. It turned out that background increases with wavelength in the region of wavelength below 450nm. We measured the spectra of Cherenkov light with bandpass filters. The peak of the spectrum appears around 300nm. The lower limit of wavelength Cherenkov light of which arrives at the surface of the earth is 240nm. These results make it clear that higher S/N is achieved at shorter wavelength in Cherenkov detection at the mountain altitude. We developed a liquid filter of solution of NiSO_4 and CoSO_4 which is almost opaque for visible light. It suppressed the light of the full moon by a factor of 1/25. In addition, shower images, that is to say, angular distributions of Cherenkov photons are obtained using a mirror of diameter 1.8m and a UV-IIT of aperture 100mm with CsTe photocathode. Such kind of IIT was developed especially for this research for the first time. We estimated the relationship between the photon number of Cherenkov light of shower image and the primary cosmic ray energy, and the trigger efficiency of our system by combining data acquired by a scintillation array of Osaka City University at the same site and our ones.