Review of Muography Hiroyuki Tanaka

Particles for Earth Studies The investigation into the basic properties of the earth has been a particularly active area of research in the field of solid earth science, and has mainly been conducted by using the "classical probe" such as seismic waves. Proceeding into the 21st century, research may expand to utilize “quantum probes”, such as muons and neutrinos towards solving current questions in solid earth science. We are currently investigating the possibility that these new quantum probes may provide another method for surveys in solid earth science.

m km Mm quantum probes photon muon neutrino (x-ray) photography muogrphay neutrinography Topic of this talk Scale Since its original discovery by Rontgen in 1895, the x-ray has played a prominent role in anatomical studies in the medical field. m km Mm Despite of the great motivation to survey the earth’s interior, we now know that x-rays are not sufficiently penetrative to successfully target geophysical-scale objects. Our current knowledge about the cross sections of the muon and the neutrino with matter solves the problem of how to study the interior of objects beyond the inspectable size limit of x-rays.

Utilizing muons that arrive at a near-horizontal angle, muography can be applied to kilometer-sized objects located at elevations above where the detector is placed. Position Sensitive Plane (PSP) … Fe Pb Fe muon … N1 ・ Ny N1 ・・・ Nx What are muons? Primary cosmic rays reacting with the nitrogen and oxygen nuclei at the top of the earth’s atmosphere produce muons Fe + Pb

The reactions of muons with matter fall completely within the framework of the standard model of particle physics. Thickness (density) of the rock determine the amount of muons that successfully pass through the rock and reach the detector. The penetrating flux refers to the number of muons that have enough energy to continue traversing through a given thickness of rock. By calculating the muon path length multiplied by the average density along the muon path we find the “density length” in units of kmwe

The objective of muography is to detect a muon signal efficiently in order to take a radiographic image of the target object. Detectors designed for muogaphy measurements utilize nuclear emulsion, gaseous and scintillation technologies. The goal is to figure out how existing technologies can be adapted to design a device that can survive in a variety of environmental restrictions …but each detector has cons and pros…

50 microns The nuclear-emulsion-based muon detector has major advantages of both superior high resolution and an ability to run without any electric power. It is a practical detector to use particularly when the commercial electricity is not available. The nuclear emulsion needs to be developed like a regular photographic film. Nuclear emulsions are designed to create a single image within an observation time

The scintillation detector has the potential to take time-dependent multiple images. The detector consists of two or more PSPs. Each PSP consists of Nx and Ny adjacent scintilltor strips which together form a segmented plane with Nx x Ny segments. All the vertex points of the strips that output the signals are considered to reconstruct the muon’s path, but only the vertices along one straight line are exploited. The scintillator is the most essential component of the scintillation detector since it efficiently facilitates the conversion of muon events to photons. Photons are then transferred to the PMT so that the electric current signals can be measured.

In 2006, the Tokyo-Nagoya collaboration team first imaged the internal structure of the peak region of Asama volcano. In 2009, additional muography data from Asama supported conclusions of the 2006 survey Muography captured images both before and after the 2009 Asama eruption. eruption on Feb 2, 2009 330 264 198 132 66 0 156 312 468 624 azimuth angle (mrad) elevation angle (mrad) below 2.20 2.36 above 2.52 density (g/cm3) The image was reconstructed from the muon trajectories recorded in the nuclear emulsion detector. The scintillation detector was accessed from a remote PC The image was interpreted that magma did not flow up the pathway in the 2009 eruption, and instead, high-pressure vapor simply blasted through the old magma deposit that acts as a “plug” of the pathway. The superior spatial resolution capabilities of muography were confirmed. This low-density region has been interpreted as a magma pathway that is plugged by magma deposited on the crater floor. We can expect that if magma ascends and gas is released from the depressurized magma in the pathway of Asama volcano, high gas pressure will cause the plug to explode, rapidly releasing fragments of the old magma deposit.

The actual magma body was also imaged with muography under the Tokyo-KEK-AIST collaboration The Satsuma-Iwojima volcano continuously discharges large amounts of volcanic gasses without significant magma discharge. Muography imaged a large, shallow depth, low-density region existing beneath the crater floor. With the assumption that the conduit has a cylindrical shape, it can be inferred that the density corresponded to liquid water. 1000 800 600 400 200 0 1000 2000 azimuth distance (m) elevation (m) below 1.75 1.86 above 1.95 density (g/cm3) One of the proposed mechanisms of this continuous gas discharge is conduit magma convection (Stevenson and Blake, 1998). However, liquid water is not likely to exist below the hot crater floor, where hot fumaroles are observed with the maximum temperature > 800oC. In this hypothesis, a magma conduit is connected to a deep magma chamber and a "degassing" phenomenon propels convection. Muography measurements have also revealed a low-density region at the uppermost point of the magma conduit, substantiating the convection model’s prediction of degassing magma being present at this location. A continuous supply of non-degassed magma from the magma chamber ensures that there is compensation for the degassed magma and the cycle continues.

Various lava domes were also muographically imaged. 18 16 14 elevation angle (deg) 0 5 10 15 20 25 azimuth angle (deg) 40 30 20 10 10 20 30 40 50 60 below 0.40 1.50 above 2.60 density (g/cm3) below 1.40 1.80 above 2.20 400 230 60 elevation angle (mrad) 0 312 624 936 azimuth angle (mrad) below 1.90 2.40 above 2.90 Various lava domes were also muographically imaged. DIAPHANE collaborators targeting the lava dome of La Soufurier, measured the region that may indicate a high density rock layer generated in between the hydrothermal region and deeper regions. We would expect the region to be over pressured when the geothermal energy flux increases, functioning similarly to the plug observed in Asama volcano. TOMUVOL experiment has created a muographic image of Puy de Dome. As a result, the shallow structure of the dome was clearly imaged revealing two independent higher density regions indicated near the top of the dome. The image of Showa-shinzan lava dome taken by the Tokyo-Nagoya-Hokkaido team in 2006 is shown for comparison. The magma pathway is plugged with magma. The structure beneath the Puy de Dome is unclear mainly due to low statistics. However, if the higher density region indicated beneath the dome is the magma pathway of Puy de Dome, it corresponds to the structure of Showa-shinzan.

Three factors intrinsically affect eruption prediction: starting time, location and the magnitude of the eruption (which is directly related to how long the eruption will last). The first factor, starting time, is increasingly easier to predict based on recent progress of technologies in monitoring volcanic tremors and land deformation. However it is still very difficult to accurately forecast the latter two factors. An important first step towards addressing these factors has been achieved with the previously mentioned muographic images, as measured before and after the 2009 Asama eruption We anticipate that as muography develops, it will become capable of estimating the eruption sequence with increasing precision. Visualization of the internal structure of a volcano with muography provides unique information. By accumulating information about worldwide volcanic eruption events into muography databases, we will increase the reliability of this method towards this application.