A visual detection and monitoring of volcanic eruptions is the most essential information. In February 2, 2009, Asama volcano, Japan erupted and a large amount of volcanic ash was ejected from the vent. We have observed the activity at Asama since October 12, 2008. For eruption monitoring we used cosmic‐ray muon radiography (muography), a new volcano monitoring system recently developed by Tanaka et al. (2009). We measured a quantitative mass loss inside the crater during the eruption event although no changes were found below the crater. The measured value of 30,780 tons is consistent with a model calculation of volcanic ash flow as observed on February 2, 2009. The obtained radiographic image suggests that a “boiling liquid expanding vapor explosion” occurred and a part of an old lava mound was exploded. This picture is consistent with the analytical result of the volcanic ash ejected on February 2, 2009.

[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 an Electron-Tracking Compton Camera (ETCC) for an all-sky survey at the MeV gamma-ray band. The ETCC consists of a gaseous tracker and a position sensitive scintillation camera to measure the momentum of the Compton-recoil electron and the scattering gamma ray so that we can reconstruct the energy and momentum of the incident gamma ray photon by photon. Also the ETCC has strong background rejection methods using tracking information such as the dE/dx particle identification and the Compton kinematics test. To confirm feasibility of observing celestial objects in space, we performed a balloon experiment to successfully observe the diffuse cosmic and atmospheric gamma rays, which confirmed the effectiveness of the background rejection capability. Based on the first balloon experiment result, we are developing a large ETCC and plan to launch it for the test of the imaging property. The performance of the SMILE-II ETCC is simulated and then it will obtain an effective area of 1.1 cm 2 for 200 keV by improving the electron track reconstruction efficiency by a factor of about 10, which results in the detection of Crab nebula at > 5σ level for several-hour observation in the middle latitude with an altitude of 40 km.

(2011), Correction to "Three-dimensional computational axial tomography scan of a volcano with cosmic ray muon radiography," J. Geophys. Res., 116, B03301, doi:10.1029/2011JB008256.[1] In the paper "Three-dimensional computational axial tomography scan of a volcano with cosmic ray muon radiography"