Improvement of cosmic-ray muography for Earth sciences and civil engineering

Muography is utilizing the cosmic-ray muons to deduce the amount of material across large-sized objects, similarly to X-raying of human body. We specifically developed Multi-Wire Proportional Chamber (MWPC)-based tracking systems with high (> 98 %) efficiency, fair (< 10 mrad) angular resolution and low (< 6 W) power for Earth sciences and civil engineering. A muography observatory is assembled from trackers on a sensitive surface of 5.5 m 2 at 2.8 km distance in South-West from Sakurajima volcano to provide projective density images of the crater regions for future measurements of mass variations occurred during eruptions. An industrial applicability of portable, MWPC-based instruments have been demonstrated with the muographic imaging of an underground iron pillar. We investigated the limits of muography: the Gaisser model is suggested to be modified with an energy exponent of -2.64 and constant scale factor of 0.66 for imaging in near-horizontal directions after large (> 1,000 m.s.r.e.) thicknesses. The multiple scattering of muons across the targeted object is limiting the imaging resolution from 10 mrad to 5 mrad with the increase of thickness between 50 m.s.r.e. and 2,000 m.s.r.e.. The precise measurement of low-energy muon spectra is required to improve muography of small-sized objects. We developed a 5-meter-length, rotatable, MWPC-based spectrometer to precisely measure the energy spectra of muons between 0.5 GeV and 5 GeV from vertical to horizontal directions. It is a consecutive series of thirteen detectors with a positional resolution of approx. 4 mm and lead plates. The spectrometer and the first results are presented