Progress Report on the TIBET AS+MD Project

We plan to build a large (approximately 10,000 m²) water-Cherenkov-type muon detector array under the existing Tibet air shower array at 4,300 m above sea level, to observe 10-1000 TeV gamma rays from cosmic-ray accelerators in our Galaxy with wide field of view at very low background level. The gamma-ray induced air shower has significantly less muons compared with a cosmic-ray induced one. Therefore, we can effectively discriminate between primary gamma rays and cosmic-ray background events by means of counting the number of muons in an air shower by the muon detector array. We will make a progress report on the project, as 5/12 of it is under construction now.