Performance of the muon detector A under TIBET III array

In order to observe gamma rays in the 100 TeV energy region, the 4500 m(2) underground muon detector array using water Cherenkov technique is constructed, forming the TIBET III+MD hybrid array. Because the showers induced by primary gamma rays contain much fewer muons than those induced by primary hadrons, significant improvement of the gamma ray sensitivity for TIBET III+MD array is expected. In this paper, the design and performance of the MD-A detector with large Tyvek bag is reported.