Development and operation of a muon detection system under extremely high humidity environment for monitoring underground water table

In order to investigate the complex nature of the landslides triggered by rainfall, dynamic muon radiography of the motion of the underground water table is performed in a the drainage tunnel drilled underneath an the estimated fault plane. However, the humidity inside the tunnel is almost 100%. In order to suppress the moisture effect, a scintillation counter with Cockcroft-Wwalton photomultipler tubes (CW-MPT) was developed and tested at the observation site. The counter was stably operated for 38 days without gain degrading. Based on the result, we constructed a muon detection system with CW-PMTs at the same site and started operation runs. In this work, the data from borehole-based measurement of the underground water levels were analyzed and discussed. It was confirmed that the comparison between muon and borehole data would be useful.