Cosmic-ray Muon Radiography of a Volcano

The origin and structure of the Earth's crust is still a major question. Traditionally the nearby crust of the Earth has been explored in great detail with echo-sounding or gravimetric techniques. Current measurements are based largely on these techniques, which has intrinsic uncertainties. In the present talk, a novel method with cosmic-ray muons to create a direct snapshot of the density profile in a volcano will be discussed as a complementary technique. We call this technique cosmic-ray muon radiography. The principal novelty of this technique makes possible for us to observe from a distance using the unique properties of the high-energy cosmic-ray muons passing through a mountain. This technique is in principle, based upon muon absorption radiography. Muon absorption radiography, using the absorption of muons in matter, is in some sense similar to x-ray radiography of a human body. By measuring the muon absorption along the different paths through a mountain, one can deduce the density profile in the object. This technique gives a horizontally integrated density whereas the gravimetric method gives a vertically integrated dens ity. The major feature of this technique, therefore makes possible for us to perform a tomographic measurement by placing two or more cosmic ray detection systems around the object. Another strong point of this technique realizes a fulltime monitoring since incessantly arriving muons are the most numerous energetic charged particles at sea level. The focus of the present talk will be on the cosmic-ray muon radiography as a novel sensing method of a volcano. Lately, much progress has been made in volcanic remote sensing, and further substantive developments are to be expected over the next few years. A strong interdisciplinary collaboration with the remote sensing community and the land-based observatories will facilitate understandings of volcanic dynamics. Cosmic-ray muons, illuminating every substance on the Earth have an intensity of ~1muon /(cm min) with a mean energy of a few GeV (10 eV). Such high-energy muons have been used to explore the internal structure of large-scale substances. One distinguished example is the work, which studied the inside structure of the Egyptian pyramid with vertically arriving muons in order to find any other hidden chambers. It is also known that horizontally arriving cosmic-ray muons have a strong intensity in the high energy region, and thus the size of the detection system becomes realistic. For the purpose of probing the internal structure of volcanoes, these high energy cosmic-ray muons can be used. We developed a simple segmented detection system comprising of plastic scintillation counters, which is expandable to a larger scale. Since, in most cases, the size of the detection system is much smaller than the object, the path of the cosmic-ray muon can be represented by zenith and azimuth angles with reference to a line perpendicular to the detector plane. The two segmented detectors were placed at a distance of 1.5 m in order to achieve ±66-mrad angular resolution. This angular resolution corresponds to a ±200-m spatial resolution at 3000 m apart. The effective viewing angle was 588 mrad. An additional trigger condition to eliminate the cosmicray soft components was also set up. Using with this detection system, we tried to determine the target density of a well known material (iron block). There, the most likely density was determined. The density of iron blocks, as obtained by the cosmic-ray muon data was 7.803 g/cm with an error bar of 3-5%, whereas the density of pure iron was 7.87 g/cm. As the first step application of the detection system, a measurement of the internal structure of the active volcano Mt. Asama was performed (Jan–Apr 2002). A detection system was placed at the foot of Mt. Asama, and remote.monitoring system of the detector was installed. A crater, which is 300m in diameter and 228 m in depth, is located at the top region. There, the crater and the shallow conduit image were clearly obtained by cosmic-ray muon radoiography, and then it was quantitatively compared with GEANT-based Monte-Carlo simulations. As the next-step, we measured the intensity of muons passing through Mt. West Iwate in order to determine the average density in Mt. Kurokura (May 2002May2003). In this period, the obtained density was statistically in agreement with the gravimetric result. The cosmic-ray muon method may give us the different, but complementary information to the gravimetric method when the detection system is enlarged. We performed a time-dependent analysis of the intensity of the cosmic-ray muon passing through Mt. West Iwate. All data from different regions were divided into six two-month data and compared with each other; i.e. the one through nothing, the one through Mt. Kurokura, and the one through Mt. Ubakura. We found that there was a time dependency only in the cosmic-ray muon passing through Mt. Kurokura. The effect of cosmic-ray modulation was cancelled by taking the ratio n(θ,φ) between the forward data and the backward data. In the present talk, the pasibbility of the application of cosmic-ray muon radiography to the other field in Earth sciences will be briefly mentioned: Muon radiography of an active fault, fulltime monitoring of the glacial flow pattern, cosmic-ray muon tomography with portable muon detectors etc. Also, several other muon earth sciences will be introduced: non-destractive testing of the planetary materials by using negative muon capture, muon spin relaxation in Banded iron Formation etc.