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.

[1] Cosmic ray muon radiography can measure the density distribution within a volcano. Unidirectional radiography shows a precise cross-sectional view of a conduit and a magma body through a volcano parallel to the plane of the detector. However, it only resolves the average density distribution along individual muon paths. Precise size and shape of underground structure, such as a conduit or a magma body, provide clear and pervasive information on understanding dynamics of volcanic eruption. Here we show a highly resolved three-dimensional tomographic image of an active volcano Asama in Japan. Specifically, we developed a portable power-effective muon radiography telescope that can be operated stable with a realistically sized solar panel so as to place it around an active volcano where commercial electric power is not available. The resulting image below the crater floor shows that a local low-density region accumulates sufficient gas pressure to cause Vulcanian eruption. The present muon computational axial tomography scan has a resolving power with a resolution of 100 m, allowing it to see great detail in volcanoes.

We have developed a portable assembly type cosmic-ray muon telescope system with power-effective real-time readings to monitor the internal structure of a volcano. Using this system, we have performed measurements at the Satsuma-Iojima volcano and studied the feasibility of using a continuous ux of cosmic-ray muons over the observation period. The system is based on the measurement of time-dependent muon absorption along different, nearly horizontal paths through a solid body. The rationale is that one can deduce the time-dependent changes in the density distribution of muon absorption in the interior of the object where an absorption variation, i.e., a density path variation, becomes an intensity variation since the muon energy spectrum is exponential or, expressed otherwise, it drops rapidly when the energy threshold increases. The muon telescope, which has a surface area of 1 m 2 , was installed at the observation point located 1.2 km from the summit crater of Satsuma-Iojima. Muon tracks within scintillator layers in the telescope were analyzed continuously by real-time three-dimensional image processing to measure the level of absorption along different ray paths through the summit crater region. A typical angular resolution of the muon detector of ±16 mrad corresponds to a spatial resolution of ±20 m at a distance of 1.2 km. Our results show the density structure determined in Satsuma-Iojima volcano, Japan, which is located above sea level. A density structure situated above sea level can be analyzed at a resolution that is signi cantly higher than is possible with conventional geophysical measurements.

The application of muon radiography will be greatly enhanced by the use of two muon sensor modules that save electric power consumption and are easily transportable. Muon sensor modules used for a volcano observation must have a low electric power consumption requirement and be both waterproof and portable. In this article, we discuss two candidate sensor modules: (1) a portable muon sensor module with wavelength-shifting (WLS) fibers and a multi-anode photomultiplier tube (MAPMT), and (2) a regular scintillator telescope with PMT complemented by a low-power Cockcroft-Walton circuit (CWPMT). A realistic telescope system consisting of a muon sensor module with MAPMT has been tested and found to consume 76 W, most of which (72 W) is used by the redundant electronic circuit required for pulse shaping; this could be modified to drastically improve the power consumption. In comparison, a muon telescope system with a CWPMT was found to consume 7.57 W. We also calculated the muon stopping length in SiO 2 by means of a Monte-Carlo simulation. This calculation provided the average density structure along the muon path in rock, where the muon path length was shorter than 1.5 km, with an accuracy of about 5% during a 90-day measurement period by assuming a 1-m 2 muon detector with an angular resolution of 25 mrad.

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