Development of detector and method for density structure measurement of fault zones using cosmic ray muons

We have developed a slim cylindrical muon detector using a plastic scintillator that can be installed in a borehole and a new muography technique that scans inside the borehole using secondary cosmic ray muon without measuring its zenith angle. The zenith angle distribution of incoming muons can be statistically limited by optimizing the detector acceptance. This new detector and technique can be used for the measurement of subsurface structures, as muography requires that the detector be installed below the measurement target. We performed a feasibility study in a borehole through the Atotsugawa fault zone in Hida City, Gifu Prefecture, Japan. We report the detector design, detector performance, analysis method, and the results of the feasibility study.