Muography

Muography is an imaging technique that produces a projectional image of a target volume by recording elementary particles, called muons, either electronically or chemically with materials that are sensitive to charged particles such as nuclear emulsions. Cosmic rays from outer space generate muons in the Earth’s atmosphere as a result of nuclear reactions between primary cosmic rays and atmospheric nuclei. They are highly penetrative and millions of muons pass through our bodies every day.

Muography utilizes muons by tracking the number of muons that pass through the target volume to determine the density of the inaccessible internal structure. Muography is a technique similar in principle to radiography (imaging with X-rays) but capable of surveying much larger objects. Since muons are less likely to interact, stop and decay in low density matter than high density matter, a larger number of muons will travel through the low density regions of target objects in comparison to higher density regions. The apparatuses record the trajectory of each event to produce a muogram that displays the matrix of the resulting numbers of transmitted muons after they have passed through hectometer to kilometer-sized objects. The internal structure of the object, imaged in terms of density, is displayed by converting muograms to muographs.

Etymology

There are two explanations for the origin of the word “muography”: (A) a combination of the elementary particle “muon” and Greek γραφή (graphé), “drawing” [1] together suggesting the meaning “drawing with muons”; and (B) a shortened combination of “muon” and “radiography”.[2] Although these techniques are related, they differ in that radiography uses x-rays to image the inside of objects in the scale of meters and muography uses muons to image the inside of objects in the scale of hectometers to kilometers.[3]

Invention of muography

Precursor technologies

20 years after Carl David Anderson and Seth Neddermeyer discovered that muons were generated from cosmic rays in 1936,[4] the Australian physicist E.P. George made the first known attempt to measure the areal density of the rock overburden of the Guthega-Munyang tunnel (part of the Snowy Mountains Hydro-Electric Scheme) with cosmic ray muons.[5] He used a Geiger counter. Although he succeeded in measuring the areal density of rock overburden placed above the detector, and even successfully matched the result from core samples, due to the lack of directional sensitivity in the Geiger counter, imaging was impossible.

First muogram

The first muogram was a matrix of the number of muon events produced in 1970 by the American physicist Luis Walter Alvarez.[6] Alvarez installed his apparatus in the Belzoni Chamber to search for hidden chambers of the Pyramid of Chephren. He recorded the number of muons after they had passed through the Pyramid. With an invention of this particle tracking technique, he worked out the methods to generate the muogram as a function of muon's arriving angles. The generated muogram was compared with the results of the computer simulations, and he concluded that there were no hidden chambers in the Pyramid of Chephren after the apparatus was exposed to the Pyramid for several months.

Film muography

Tanaka and Niwa’s pioneering work created film muography, which utilizes nuclear emulsion. Exposures of nuclear emulsions were taken in the direction of the volcano and then analyzed with a newly invented scanning microscope, custom built for the purpose of identifying particle tracks more efficiently.[7] Film muography enabled them to obtain the first interior imaging of an active volcano in 2007,[8] revealing the structure of the magma pathway of Asama volcano.

Real-time muography

In 1968, the group of Alvarez used spark chambers with a digital read out for their Pyramid experiment. Tracking data from the apparatus was onto magnetic tape in the Belzoni Chamber, then the data were analyzed by the IBM 1130 computer, and later by the CDC 6600 computer located at Ein Shams University and Lawrence Radiation Laboratory, respectively.[6] Strictly speaking these were not real time measurements.

Real-time muography requires muon sensors to convert the muon's kinetic energy into a number of electrons in order to process muon events as electronic data rather than as chemical changes on film. Electronic tracking data can be processed almost instantly with an adequate computer processor; in contrast, film muography data have to be developed before the muon tracks can be observed. Real-time tracking of muon trajectories produce real-time muograms that would be difficult or impossible to obtain with film muography.

High-resolution muography

The MicroMegas detector has a positioning resolution of 0.3 mm, an order of magnitude higher than that of the scintillator-based apparatus (10 mm),[9][10] and thus has a capability to create better angular resolution for muograms.

Fields of study

Vesuvius

The Mu-Ray project [11] has been using muography to image Vesuvious, famous for its eruption of 79 AD, which destroyed local settlements including Pompeii and Herculaneum.

Etna

The ASTRI SST-2M Project is using muography to generate the internal images of the magma pathways of Etna volcano.[12] The last major eruption of 1669 caused widespread damage and the death of approximately 20,000 people. Monitoring the magma flows with muography may help to predict the direction from which lava from future eruptions may emit.

Stromboli

The apparatuses use nuclear emulsions to collect data near Stromboli volcano. Recent emulsion scanning improvements developed during the course of the Oscillation Project with Emulsion tRaking Apparatus (OPERA experiment) led to film muography. Unlike other muography particle trackers, nuclear emulsion can acquire high angular resolution without electricity. An emulsion-based tracker has been collecting data at Stromboli since December 2011.[13]

Puy de Dôme

Since 2010, a muographic imaging survey has been conducted at the dormant volcano, Puy de Dôme, in France.[14] It has been utilizing the existing closed building structures located directly underneath the southern and eastern sides of the volcano for equipment testing and experiments. Preliminary muographs have revealed previously unknown density features at the top of Puy de Dôme that have been confirmed with gravimetric imaging.[15]

Underground water monitoring

Muography has been applied to groundwater and saturation level monitoring for bedrock in a landslide area as a response to major rainfall events. The measurement results were compared with borehole groundwater level measurements and rock resistivity.[16]

Glacier tomography

Muography allowed to determine the bedrock geometry beneath active glaciers in the steep alpine environment of the Jungfrau region in Switzerland. The methodology provided important information on subglacial mechanisms of bedrock erosion.[17][18]

Egyptian Pyramids

In 2015, 35 years after Alvarez’s experiment, the ScanPyramids Project, which is composed of an international team of scientists from Egypt, France, Canada, and Japan, has started utilizing muography and thermography imaging to survey the Giza pyramid complex.[19]

In 2017, scientists involved in the project discovered a large cavity, named ScanPyramids Big Void, above the Grand Gallery of the Great Pyramid of Giza.[20][21]

Mexican Pyramids

The 3rd largest pyramid in the world, the Pyramid of the Sun, situated near Mexico City in the ancient city of Teotihuacan was surveyed with muography. One of the motivations of the team was to discover if inaccessible chambers inside the Pyramid might hold the tomb of a Teotihuacan ruler. The apparatus was transported in components and then reassembled inside a small tunnel leading to an underground chamber directly underneath the pyramid. A low density region approximately 60 meters wide was reported as a preliminary result, which has led some researchers to suggest that the structure of the pyramid might have been weakened and it is in danger of collapse.[3]

Mars

Muography may potentially be implemented to image extraterrestrial objects such as the geology of Mars. Cosmic rays are numerous and omnipresent in outer space. Therefore, it is predicted that the interaction of the cosmic rays in the Earth’s atmosphere to generate pions/mesons and subsequently to decay into muons also occurs in the atmosphere of other planets.[22] It has been calculated that the atmosphere of Mars is sufficient to produce a horizontal muon flux for practical muography, roughly equivalent to the Earth’s muon flux.[23] In the future, it may be viable to include a high-resolution muography apparatus in a future space mission to Mars, for instance inside a Mars rover.[23] Getting accurate images of the density of Martian structures could be used for surveying sources of ice or water.

Small Solar System Bodies

The “NASA Innovative Advanced Concepts (NIAC) program” is now in the process of assessing whether muography may be used for imaging the density structures of small solar system bodies (SSBs).[24] While the SSBs tend to generate lower muon flux than the Earth's atmosphere, some are sufficient to allow for muography of objects ranging from 1 km or less in diameter. The program includes calculating the muon flux for each potential target, creating imaging simulations and considering the engineering challenges of building a more lightweight, compact apparatus appropriate for such a mission.

Industry use

Industrial muography is a technique that produces muograms/muographs of industrial objects in order to internally inspect these objects.[8]

Recently, industrial muography has found an application in reactor inspection.[25] It was used to locate the nuclear fuel in the Fukushima Daiichi nuclear power plant, which was damaged by the 2011 Tōhoku earthquake and tsunami.

Advantages

There are several advantages that muography has over traditional geophysical surveys. First, muons are naturally abundant and travel from the atmosphere towards the Earth’s surface.[26] This abundant muon flux is nearly constant, therefore muography can be utilized worldwide. Second, because of the high-contrast resolution of muography, a small void of less than 0.001% of the entire volume can be distinguished.[6] Finally, the apparatus has much lower power requirements than other imaging techniques since they use natural probes, rather than relying on artificially generated signals.[23]

Process

In the field of muography, the transmission coefficient is defined as the ratio of the transmission through the object over the incident muon flux. By applying the muon's range through matter [27] to the open-sky muon energy spectrum,[26] the value of the fraction of incident muon flux that is transmitted through the object can be analytically derived. A muon with a different energy has a different range, which is defined as a distance that the incident muon can traverse in matter before it stops. For example, 1 TeV energy muons have a continuous slowing down approximation range (CSDA range) of 2500 m water equivalent (m.w.e.) in silica dioxide whereas the range is reduced to 400 m.w.e. for 100 GeV muons.[28] This range varies if the material is different, e.g., 1 TeV muons have a CSDA range of 1500 m.w.e. in lead.[28]

The numbers (or later represented by color) comprising a muogram are displayed in terms of the transmitted number of muon events. Each pixel in the muogram is a two dimensional unit based on the angular resolution of the apparatus. The phenomenon that muography cannot differentiate density variations is called the "Volume Effects." Volume Effects happen when a large amount of low density materials and a thin layer of high density materials cause the same attenuation in muon flux. Therefore, in order to avoid false data arising from Volume Effects, the exterior shape of the volume has to be accurately determined and used for analyzing the data.

Technical aspects

The apparatus is a muon-tracking device that consists of muon sensors and recording media. There are several different kinds of muon sensors used in muography apparatuses: plastic scintillators,[29] nuclear emulsions,[13] or gaseous ionization detectors.[2][9] The recording medium is the film itself, digital magnetic or electronic memory. The apparatus is directed towards the target volume, exposing the muon sensor until the muon events required in order to form a statistically sufficient muogram are recorded, after which, (post processing) a muograph displaying the average density along each muon path is created.

References
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