Muon tomography

Muon tomography or muography is a technique that uses cosmic ray muons to generate two or three-dimensional images of volumes using information contained in the Coulomb scattering of the muons. Since muons are much more deeply penetrating than X-rays, muon tomography can be used to image through much thicker material than x-ray based tomography such as CT scanning. The muon flux at the Earth's surface is such that a single muon passes through an area the size of a human hand per second. Since its development in the 1950s, muon tomography has taken many forms, the most important of which are muon transmission radiography and muon scattering tomography. Since 2010s researchers are also exploring and attempting to use artificially generated muons—created by conventional accelerators or laser-plasma systems—for muon tomography. Muography uses 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 apparatus records 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 objects up to multiple kilometers in thickness. The internal structure of the object, imaged in terms of density, is displayed by converting muograms to muographic images. Muon tomography imagers are under development for the purposes of detecting nuclear material in road transport vehicles and cargo containers for the purposes of non-proliferation. Another application is the usage of muon tomography to monitor potential underground sites used for carbon sequestration.

Etymology and use The term muon tomography is based on the word "tomography", a word produced by combining Ancient Greek tomos "cut" and graphe "drawing." The technique produces cross-sectional images (not projection images) of large-scaled objects that cannot be imaged with conventional radiography. Some authors hence see this modality as a subset of muography. Muography was named by Hiroyuki K. M. Tanaka. There are two explanations for the origin of the word "muography": (A) a combination of the elementary particle muon and Greek γραφή (graphé) "drawing," together suggesting the meaning "drawing with muons"; and (B) a shortened combination of "muon" and "radiography." Although these techniques are related, they differ in that radiography uses X-rays to image the inside of objects on the scale of meters, while muography uses muons to image the inside of objects on the scale of hectometers to kilometers.

Invention of muography

Precursor technologies Twenty years after Carl David Anderson and Seth Neddermeyer discovered that muons were generated from cosmic rays in 1936, 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. 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. In a famous experiment in the 1960s, American physicist Luis Walter Alvarez used muon transmission imaging to search for hidden chambers in the Pyramid of Chephren in Giza, although none were found at the time; a later effort discovered a previously unknown void in the Great Pyramid. In all cases the information about the absorption of the muons was used as a measure of the thickness of the material crossed by the cosmic ray particles.

First muogram The first muogram was produced in 1970 by a team led by Luis Walter Alvarez, who installed detection apparatus in the Belzoni Chamber of the Pyramid of Khafre to search for hidden rooms within the structure. 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 uses nuclear emulsion. Film muography enabled them to obtain the first interior imaging of an active volcano in 2007, revealing the structure of the magma pathway of Asama volcano. 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.

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. 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), and thus has a capability to create better angular resolution for muograms.

Applications

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