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Advanced search. Skip to main content Thank you for visiting nature. As an example, Figure 4 shows the fraction of surviving muons after traversing a given thickness of volcano rock, modelled by a realistic chemical composition of the lava from Etna, mainly including SiO 2 , Al 2 O 3 , FeO, MgO, and CaO. Transmission factor of cosmic muons as a function of the rock thickness traversed, as extracted from GEANT simulations for a realistic lava scenario from Mt.
Such quantity is sometimes called the opacity. A muon tracking detector, able to measure the number of muons arriving to it from any given direction, provides an experimental measurement of the opacity along different directions. The knowledge of the path length L for any muon direction provides a density map, i.
A density map—once the actual traversed thickness is known and inserted for any specific orientation—may then reveal differences in the density evaluated along different directions. This is the basic principle of the muon absorption tomography.
Although the muon absorption tomography may only provide two-dimensional density maps, in principle the combined use of several detectors, pointing to the object from different orientations may produce a 3D map of the object. The construction and use of a set of identical detectors, placed in different locations and working with comparable performance is not a trivial task and the real use of this opportunity is still to be exploited.
A standard setup for muon absorption experiments requires a muon tracking detector telescope , usually employed in transmission mode i. The reconstruction of a large number of tracks in the telescope allows for a 2D tomographic map, with a resolution which depends on the telescope tracking performance and on experimental disturbances such as multiple scattering effects in the material surrounding the object to be explored, as well as in the air.
Many other aspects of the detector performance, such as its overall detection efficiency, response uniformity, sensitive area, alignment properties, duty cycle, cost and transportability, … influence the real capability of the instrument.
Considering the possibilities offered by muon absorption tomography, several applications have been proposed, with many experimental results obtained so far. Here a brief review of these fields is given. A large interest in absorption muon tomography is related to the possibility of exploring the hidden part of mountains, especially active or potentially active volcanoes, by means of cosmic muons traversing part of their solid structure and being partially absorbed with respect to those coming from the open sky Figure 5.
This idea, exploited for the first time by Nagamine et al. Important contributions to the field have been given by the Japanese collaboration leaded by H. Tanaka [ 8 , 9 , 10 , 11 , 12 ], which has employed a muon telescope made by several detection planes with scintillators with PMTs separated by Lead plates, by the Diaphane Collaboration [ 13 , 14 , 15 , 16 ], which carried out various measurement campaigns in several locations of the world in France, Italy and Philippines with scintillator-based muon telescopes, by the TOMUVOL Collaboration [ 17 ], employing resistive plate chambers detectors, and by the MU-RAY Project [ 18 , 19 ], which has employed a muon telescope based on scintillator strips with SiPM photosensors for the exploration of Mt.
Vesuvius in Italy. A simple geometrical sketch showing the principle of muon tomography applied to the study of mountain structures. A tracking telescope is placed downstream of the structure being explored, reconstructing muon tracks which ideally have traversed a thickness of solid rock. A comparison with the tracks coming from the open sky or from the backside is used to provide a 2D density map of the structure.
A nonnegligible background however may originate from muons which are scattered either from the solid rock or from the air. A recent project has been developed also by our group in Catania, devoted to the study of the top craters of Mt. Etna, the highest active volcano in Europe, with a telescope equipped with three 1 m 2 segmented planes of scintillator strips with multianode PMT readout, already installed since last year close to the top of the mountain.
Preliminary tomographic images of such craters have been already obtained by a comparison between the map produced by muons originating from the front side and the corresponding map produced by the muons coming from the back side.
It must be remembered that other Projects [ 20 , 21 ] are exploiting the possibility to employ the Cerenkov light produced by the muons in the air after traversing the large thickness of the rock by a Cerenkov detector prototype ASTRI , originally devised for astrophysical investigation in view of the large Cerenkov Telescope Array CTA Project.
The interest in this field is twofold: from one side methods based on muon tomography may complement and sometimes even surpass the potential offered by traditional methods in the understanding the inner part of these structures, revealing empty spaces, or different density profiles inside the mountain.
On the other side there is the hope to reach the resolution and capability to monitor in real time the time evolution of the subsurface structures, in order to control potential activities giving rise to explosions and lava eruptions. This last possibility at the moment is still far from being fully reached, while static investigations have offered beautiful pictures of the interior of mountains and volcanoes in several parts of the world. An important aspect of the technique, which in some cases offers a better figure of merit in comparison to geological and geophysics methods, is the spatial resolution, which can be expressed as.
The use of absorption muon tomography is of course not only limited to the study of large mountain structures, but proves much more useful for the investigation of smaller geological locations, underground cavities, caverns, mines and tunnels, due to the reduced thickness to be explored, hence to the large flux being measured.
There are several examples of the use of this technique for these applications [ 21 , 22 , 23 , 24 , 25 ]. As a recent example, in one of these investigations [ 22 ], carried out to explore underground cavities in the Naples area, a muon detector similar to that employed for volcano muography was employed, with size 1 m x 1 m, and segmented into 32 scintillator strips. The vertical rock thickness above the detector was in the previous case about 40 m, which did not reduce too much the muon count rate, allowing for a significant result to be obtained in less than one month of data taking.
Actually, for many of these applications, the range of rock thickness usually amounts to a few tens metres, which is a value much less than the values of interest for large volcanic structures. The possibility to install the detector in places which are not so prohibitive as for volcanic explorations gives larger opportunities to use this technique, which will likely be employed more and more in the near future to investigate underground environments.
As recalled at the beginning of this Chapter, one of the first examples of muon absorption tomography is represented by the well-known work by Alvarez and collaborators [ 2 ], who employed a muon detector inside an Egyptian pyramid to search for possible hidden chambers. Such void was first explored by nuclear emulsions and then confirmed by measurements carried out with scintillation hodoscopes and gas detectors; hence, it represents a beautiful example of interrelations between different observation techniques pointing to the same body of evidence.
Additional examples of the use of the muon absorption technique for archaeological studies have been reported over the last years [ 27 , 28 , 29 ], among which is a study of the cavities in the Teotihuacan Pyramid of the Sun [ 27 ]. The first investigation concerned with the use of the muon scattering process to obtain a radiography of the hidden content in a volume dates back to the work by Borozdin et al.
This technique proved to be very promising for several reasons: it does not introduce any additional radiation, as it is for instance for X-rays; moreover, most of the scattered muons contribute to build the image, contrary to absorption, where a large fraction of muons is absorbed by the material itself.
In the scattering mode, the muon tomography technique makes use of this process, which strongly depends on the properties of the material, especially its atomic number Z, thus allowing to discriminate between low- and medium-Z elements with respect to high-Z elements. The projected scattering angle distribution follows in a first approximation a Gaussian shape, with a width given by:.
Due to the possibility that illicit fissile elements Uranium or Plutonium could be transported inside containers, this technique was suggested as a viable alternative to other traditional methods to inspect and scan a large volume.
A muontomograph employing the scattering process basically requires two good muon tracking detectors, one placed above and the other placed below the volume to be inspected. Reconstruction of the muon track above and below the volume allows to evaluate the amount of scattering suffered by the muon, and in the simplest approach where a single scattering centre is assumed , the so-called POCA Point of Closest Approach algorithm determines the 3D coordinates of the scattering centre Figure 6.
A sufficiently large number of individual tracks, traversing from any direction of the volume, allow to build 2D and 3D tomographic images. The performance of a muontomograph may be evaluated in terms of its spatial and angular resolution depending on the tracking detectors , of the overall detection efficiency which is the result of the detection efficiency of each tracking plane , which in turn determines the required scan time, of the capability to identify high-Z elements and discriminate them with respect to lighter elements, of the sensitivity to false-positive events, which would require an alarm and the opening of the container for a detailed control.
An artist view of the experimental setup being employed for muon tomography applications of cargo containers. Two muon tracking detectors, one placed above and the other placed below the container, are used to reconstruct muon tracks before and after traversing the container content. Several applications were oriented to the problem of scanning the content of a cargo container, searching for hidden high-Z materials, which is an important aspect of homeland security.
At present, only a small fraction of them are checked, while laws under discussion might require more detailed procedures to be followed over the world. Following this approach, several small-scale prototypes have been built over the last years, employing a variety of detection technologies, from gas chambers to segmented strip scintillators, Resistive Plate Chambers and GEM.
Several contributions in this field have been reported by various groups [ 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 ], who have designed small scale as well as large and full-size scale detectors for muon tomography. For many decades, an impressive amount of used fuel has been produced, and most of this material is stored either in spent fuel pools or in dry storage casks.
Such containers are sealed after filling them with the spent fuel, with no possibility to open and visually check their content. As negatively-charged muons pass through a volume, they interact with the negatively-charged electrons in the material and are deflected. Researchers can analyse their angles of deflection before and after passing through a volume to gather information about the mass they are inspecting. Click the links below to read about different ways researchers are using muon tomography and CMS technology to safeguard our cities, preserve our environment, and protect human health.
Muon Tomography Every second of every day, we are bombarded with thousands of particles that pass through our bodies without us noticing.
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