Development of MWPC-based tracking system for muography of active volcanoes
Muography is an non-invasive imaging technique which uses cosmic muons created in the atmosphere for the inspection of various sized objects from few meters up to few kilometers – similarly as medical examination is performed with X-rays. The project oriented development of particle detector technologies makes possible their outdoor application. Thanks this technological progress, the research and forecast of natural hazards, such as volcanic activity became one of the main targets of muography (see in Fig. 1). It can provide the possibility of probing of interior of gigantic sized active volcanoes and remote monitoring of their density variation from safety distance (> 1km) with higher resolution (< 10 m) as conventional geophysical methods perform . The application of muography would provide useful information to the decision about eviction of urban area around the active volcano in case of emergency.
The muographic imaging of gigantic sized volcano with the thickness from few hundreds meter to few kilometer is challenging for present days technologies too. The large thickness results in a small number of penetrated muons (~ 10-3 - 10-7 cm-2sr-1s-1), which is comparable with the flux of the not penetrated “background” particles, such as protons, scattered low energy muons. Therefore, the finite flux limits the time resolution of the measurements and it suggests the maximization of detector size. The presence of low energy (few GeV) background particles suggests the application of dense shielding layers, such as lead plates and tracking layers with optimized positional and angular resolution to reject the deflected low energy particles.
The Innovative Gaseous Detector R&D group of Wigner RCP (REGARD) has been developed the new variant of MWPC-based tracking detectors (see in Fig. 2 left) for muography with reduced weight of 10 kg, exceptional operational stability, high efficiency of above 99 % and reasonable spatial resolution of 4 mm . Based on this novel technology and the design of previous scintillator-based MOS, the REGARD and Earthquake Research Institute, The University of Tokyo (ERI, UT) have been developed together the so called MWPC-based Muography Observation System (MMOS), shown in Fig. 2 right panel .
The first large sized MMOS is under development at the Sakurajima volcano in Kyushu, Japan within the collaboration of our group and ERI. The Sakurajima is an excellent target because it is one of the most active volcanoes with hundreds of eruptions in every year. The first tracking system has been installed at the distance of 3 km from the southern peak of Sakurajima in 2017 January. During the first data taking period, the applicability of MMOS has been demonstrated to perform high-definition muographic imaging with low background noise. Figure 3 shows the photographs of volcano and the muographic images which show the ridge of the volcano and demonstrate the excellent angular resolution . At the beginning of 2018, the size of MMOS has been extended to 2 m2. The MMOS provides on-line data which is stored and analyzed on a remote server and visualized on https://mmos.muographers.org/ web server .
In the forthcoming years, the REGARD will continuously build, and install tracking detectors at the Sakurajima volcano. We will extend the size of MMOS to about 20 m2 to allow time-sequential muographic imaging with the time resolution of 2-3 hours and provide useful information to volcanologists.
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 L. Oláh, H. K. M. Tanaka, T. Ohminato, D. Varga, Scientific Reports (2018) 8:3207
 On-line monitoring page of MMOS: https://mmos.muographers.org/
Figure 1: Schematic drawing of muography and the the first time-sequential muographic images of the erupting Satsuma-Iwojima volcano .
Figure 2: The fist MWPC detector in our laboratory in 2015 and the first three modules of the MWPC-based Muographic Observation system at the Sakurajima volcano in 2018 February.
Figure 3 The photographs (A and C) and the first high-definition muographic images (B and D) of Sakurajima volcano.
Author: László Oláh