Radiographic measurements of the internal structure of Mt. West Iwate with near-horizontal cosmic-ray muons and future developments

Cosmic-ray muon radiography has been studied as a potential method for the measurement of the internal structure of the active volcano Mt. West Iwate. We improved a cosmic-ray muon detection system built with a set of two position-sensitive segmented detectors by the multiplicity analysis of a soft-component background after passing through an Fe plate between the detectors. We measured the density of the region located 160 m under the peak of Mt. Kurokura and compared to the gravimetric data. Cosmic-ray muons as a radiographic probe provide useful information that complements the gravimetric data. We analyzed the data to find time-dependent changes in the cosmic-ray muon intensity after passing through Mt. West Iwate, and a lower limit of 0.82% was found in Mt. Kurokura at the 90% CL. We discuss the possibility of cosmic-ray muon tomography with two or more detection systems.