• Cosmic-ray muon;
  • radiography;
  • degassing

[1] Muon radiography can provide essentially a cross section through the object parallel to the plane of the detector, on which the average density along all the muon paths is projected, somewhat like X-ray radiography. Very recently the use of emulsion films has given us a clue for visualization of the interior of volcanoes. To image a larger volcano in shorter time, we need a larger detector to collect more muon events. However, the time required for imaging will be proportional to the detection area. In order to overcome this problem, we developed a portable assembly type cosmic-ray muon telescope module to image the density distribution of magma in the conduit of Mt. Iwodake volcano, Japan. A muon detector with an area of 1 m2 was set up at the foot of the volcano. We mapped differentially absorbed cosmic-ray muons, which depend on the varying thickness and density beneath the crater floor. We successfully imaged density distribution in the conduit as well as the conduit shape, assuming the density anomaly is localized in the vent area. The observed location of the magma head is consistent with the degassing model of rhyolitic systems proposed by K. Kazahaya et al. in 2002.