Review of Muography

Hiroyuki Tanaka

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.

Particles for Earth Studies

Scalem km Mm

(x-ray) 　 photography muogrphay neutrinography

photon muon neutrino

Topic of this talk

quantumprobes

Since its original discovery by Rontgen in 1895, the x-ray has played a prominent role in anatomical studies in the medical field.

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.

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Position SensitivePlane (PSP)

muon

Fe

Fe

N1 ・・・ Nx

N1 ・ ・ Ny

What are muons?Primary cosmic rays reacting with the nitrogen and oxygen nuclei at the top of the earth’s atmosphere produce muons

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.

The reactions of muons with matter fall completely within the framework of the standard model of particle physics.

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

Thickness (density) of the rock determine the amount of muons that successfully pass through the rock and reach the detector.

The objective of muography is to detect a muon signal efficiently in order to take a radiographic image of the target object.

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

Detectors designed for muogaphy measurements utilize nuclear emulsion, gaseous and scintillation technologies.

…but each detector has cons and pros…

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

50 microns

The scintillation detector has the potential to take time-dependent multiple images.

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.

The detector The detector consists of two or consists of two or more PSPs. Each more PSPs. Each PSP consists of NPSP consists of Nxx

and Nand Nyy adjacent adjacent

scintilltor strips scintilltor strips which together form which together form a segmented plane a segmented plane with Nwith Nxx x N x Nyy

segments. All the segments. All the vertex points of the vertex points of the strips that output the strips that output the signals are signals are considered to considered to reconstruct the reconstruct the muon’s path, but muon’s path, but only the vertices only the vertices along one straight along one straight line are exploited. line are exploited.

In 2006, the Tokyo-Nagoya collaboration team first imaged the internal structure of the peak region of Asama volcano.

eruptionon Feb 2, 2009

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The image was reconstructed from the muon trajectories recorded in the nuclear emulsion detector.

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.

In 2009, additional muography data from Asama supported conclusions of the 2006 survey

The The scintillation scintillation detector detector was was accessed from a accessed from a remote PCremote PC

Muography captured images both before and after the 2009 Asama eruption.

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 actual magma body was also imaged with muography under the Tokyo-KEK-AIST collaboration

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The Satsuma-Iwojima volcano continuously discharges large amounts of volcanic gasses without significant magma discharge.

One of the proposed mechanisms of this continuous gas discharge is conduit magma convection (Stevenson and Blake, 1998).

In this hypothesis, a magma conduit is connected to a deep magma chamber and a "degassing" phenomenon propels convection.

A continuous supply of non-degassed magma from the magma chamber ensures that there is compensation for the degassed magma and the cycle continues.

With the assumption that the conduit has a cylindrical shape, it can be inferred that the density corresponded to liquid water.

However, liquid water is not likely to exist below the hot crater floor, where hot fumaroles are observed with the maximum temperature > 800oC.

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.

Muography imaged a large, shallow depth, low-density region existing beneath the crater floor.

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below 0.40 1.50 above 2.60 density (g/cm3)

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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.