YBJ International Cosmic Ray Observatory is located at 90°26'E and 30°13'N in Yangbajing (YBJ) valley of Tibetan highland, near the cross point of Qinghai – Tibet and China - Nepal highways, and the Qinghai - Tibet railway (to be completed in 5 years), about 90 km away from the city of Lhasa. YBJ's wide and flat topography, convenient traffic, scarce heavy snow, rich geothermal power source, about 4,000 residents and many neighboring institutions & services, make it the best site of high altitude observatory in the world.
YBJ Observatory was founded in 1990. It has hosted the Tibet ASγ Experiment (Sino-Japanese Cooperation) ever since. After 6 years’ preparation, the ARGO -YBJ Project (Sino-Italian Cooperation) started its detector installation in 2000. Both of them aim at the research of the origin of high energy cosmic rays, the GRB physics in high energy, the correlation between the movement of the Cosmic ray sun shadow and the solar/interplanetary magnetic field and solar activity, etc. Through the observation of air showers (AS) by AS array - a semi-full sky and continuous observation technique. Taking advantage of the YBJ high altitude, by increasing the density of scintillation detector, the ASγ Array has successfully observed γ rays from Crab Nebula and Mrk 501 (during its flaring period in 1997); by developing the traditional sampling AS array as a "carpet", the ARGO-YBJ full coverage array is intended for exploring the approximately 100 GeV uncultivated land and measuring the antiproton/proton ratio by cosmic ray moon shadow.
Beside the AS array, neutron monitor and neutron telescope have been available for solar and hiliosphere study. Along with the further development of the observatory, the existing detectors will be upgraded and more new type detectors adopted.