Search for hidden chambers in the pyramids

Toward using cosmic rays to image cultural heritage objects

Muons are elementary particles abundantly and freely produced in cosmic-ray interactions and can be used for nondestructive imaging of internal structures. This novel technique, nicknamed "muography", has been applied in use cases ranging from geophysics to archaeology to nuclear safety, but it has been so far underexplored for a vast category of cultural heritage objects that are relatively dense (stone, metals, etc.) and with size ranging from decimeters to human size. In this paper, we review recent development of portable muon detectors and explore the strengths and limitations of the technique. Despite the limited particle flux available, muography is found to be an intriguing option, particularly in cases where the items to be analyzed are not transportable or set up in a confined environment.

Medical imaging applied to heritage

The use of imaging has transformed the study of cultural heritage artefacts in the same way that medical imaging has transformed medicine. X-ray-based techniques are common in both medical and heritage imaging. Optical imaging, including scientific photography and spectral imaging techniques, is also common in both domains. Some common medical imaging methods such as ultrasound and MRI have not yet found routine application in heritage, whereas other methods such as imaging with charged and uncharged particles and 3D surface imaging are more common in heritage. Here, we review the field of heritage imaging from the point of view of medical imaging and include some classic challenges of heritage imaging such as reading the text on burnt scrolls, identifying underdrawings in paintings, and CT scanning of mummies, an ancient calculating device and sealed documents. We show how hyperspectral imaging can offer insight into the drawing techniques of Leonardo da Vinci and explain how laparoscopy has identified the method of construction of a 500-year-old pop-up anatomical text book.

Laser scanner and UAV digital photogrammetry as support tools for cosmic-ray muon radiography applications: an archaeological case study from Italy

The use of light detection and ranging technologies, i.e. terrestrial laser scanner (TLS), airborne laser scanner (ALS) and mobile laser scanner (MLS), together with the unmanned aerial vehicles digital photogrammetry (UAV-DP) and satellite data are proving to be fundamental tools to carry out reliable muographic measurement campaigns. The main purpose of this paper is to propose a workflow to correctly plan and exploit these types of data for muon radiography aims. To this end, a real case study is presented: searching for hidden tombs in the Etruscan necropolis of Palazzone (Umbria, Italy). A high-resolution digital elevation model (DEM) and three-dimensional models of the ground surface/sub-surface of the study area were created by merging data obtained using different survey methods to achieve the most accurate three-dimensional environment. Indeed, the simulated muon flux transmission used to infer relative transmission values, and the estimated density distribution, depends on the reliability of the three-dimensional reconstructed ground surface model. The aim of this study is to provide knowledge on the use of TLS and UAV-DP data and GPS-acquired points within the transmission-based muography process and how these data could improve or worsen the muon imaging results. Moreover, this study confirmed that muography applications require a multidisciplinary approach.

Hidden chamber discovery in the underground Hellenistic necropolis of Neapolis by muography

We report in this paper the muography of an archaeological site located in the highly populated "Sanità" district in the center of Naples, ten meters below the current street level. Several detectors capable of detecting muons - high energy charged particles produced by cosmic rays in the upper layers of atmosphere - were installed underground at the depth of 18 m, to measure the muon flux over several weeks. By measuring the differential flux with our detectors in a wide angular range, we have produced a radiographic image of the upper layers. Despite the architectural complexity of the site, we have clearly observed the known structures as well as a few unknown ones. One of the observed new structures is compatible with the existence of a hidden, currently inaccessible, burial chamber.

Precise characterization of a corridor-shaped structure in Khufu's Pyramid by observation of cosmic-ray muons

Khufu's Pyramid is one of the largest archaeological monument all over the world, which still holds many mysteries. In 2016 and 2017, the ScanPyramids team reported on several discoveries of previously unknown voids by cosmic-ray muon radiography that is a non-destructive technique ideal for the investigation of large-scale structures. Among these discoveries, a corridor-shaped structure has been observed behind the so-called Chevron zone on the North face, with a length of at least 5 meters. A dedicated study of this structure was thus necessary to better understand its function in relation with the enigmatic architectural role of this Chevron. Here we report on new measurements of excellent sensitivity obtained with nuclear emulsion films from Nagoya University and gaseous detectors from CEA, revealing a structure of about 9 m length with a transverse section of about 2.0 m by 2.0 m.

Three-dimensional muon imaging of cavities inside the Temperino mine (Italy)

Muon radiography (muography) is an imaging technique based on atmospheric muon absorption in matter that allows to obtain two and three-dimensional images of internal details of hidden objects or structures. The technique relies on atmospheric muon flux measurements performed around and underneath the object under examination. It is a non-invasive and passive technique and thus can be thought of as a valid alternative to common prospecting techniques used in archaeological, geological and civil security fields. This paper describes muon radiography measurements, in the context of archaeological and geological studies carried out at the Temperino mine (LI, Tuscany, Italy), for the search and three-dimensional visualisation of cavities. This mine has been exploited since Etruscan times until recently (1973), and is now an active tourist attraction with public access to the tunnels. Apart from the archaeological interest, the importance of mapping the cavities within this mine lies in identifying the areas where the extraction ores were found and also in the safety issues arising from the tourist presence inside the mine. The three-dimensional imaging is achieved with two different algorithms: one involving a triangulation of two or more measurements at different locations; the other, an innovative technique used here for the first time, is based on the back-projections of reconstructed muon tracks. The latter requires only a single muographic data tacking and is to be preferred in applications where more than one site location can be difficult to access. Finally the quality of the three-dimensional muographic imaging was evaluated by comparing the results with the laser scan profiles obtained for some known cavities within the Temperino mine.

Atmospheric muography for imaging and monitoring tropic cyclones

Large-scale solid bodies on Earth such as volcanoes and man-made pyramids have been visualized with solid earth muography, and the recently invented technique, acqueous muography, has already demonstrated its capability to visualize ocean tides and tsunami. In this work, atmospheric muography, a technique to visualize and monitor the vertical profile of tropic cyclones (TCs) is presented for the first time. The density distribution and time-dependent behavior of several TCs which had approached Kagoshima, Japan, has been investigated with muography. The resultant time-sequential images captured their warm cores, and their movements were consistent with the TC trails and barometric pressure variations observed at meteorological stations. By combining multidirectional muographic images with barometric data, we anticipate that muography will become a useful tool to monitor the three-dimensional density distribution of a targeted mesoscale convective system.

Periodic sea-level oscillation in Tokyo Bay detected with the Tokyo-Bay seafloor hyper-kilometric submarine deep detector (TS-HKMSDD)

Meteorological-tsunami-like (or meteotsunami-like) periodic oscillation was muographically detected with the Tokyo-Bay Seafloor Hyper-Kilometric Submarine Deep Detector (TS-HKMSDD) deployed in the underwater highway called the Trans-Tokyo Bay Expressway or Tokyo Bay Aqua-Line (TBAL). It was detected right after the arrival of the 2021 Typhoon-16 that passed through the region 400 km south of the bay. The measured oscillation period and decay time were respectively 3 h and 10 h. These measurements were found to be consistent with previous tide gauge measurements. Meteotsunamis are known to take place in bays and lakes, and the temporal and spatial characteristics of meteotsunamis are similar to seismic tsunamis. However, their generation and propagation mechanisms are not well understood. The current result indicates that a combination of muography and trans-bay or trans-lake underwater tunnels will offer an additional tool to measure meteotsunamis at locations where tide gauges are unavailable.

Muon scattering tomography: review

Cosmic-ray muon scattering tomography has gathered attention in the security and nuclear industries in the last 10 years. Muon scattering tomography is capable of identifying atomic numbers of objects, is highly sensitivity to high-atomic-number materials such as uranium, and is very useful for detecting them in a background of low-atomic-number material. The principle, detectors, and applications of muon tomography are presented, as well as its future aspect.

Principles and Perspectives of Radiographic Imaging with Muons

Radiographic imaging with muons, also called Muography, is based on the measurement of the absorption of muons, generated by the interaction of cosmic rays with the earth's atmosphere, in matter. Muons are elementary particles with high penetrating power, a characteristic that makes them capable of crossing bodies of dimensions of the order of hundreds of meters. The interior of bodies the size of a pyramid or a volcano can be seen directly with the use of this technique, which can rely on highly segmented muon trackers. Since the muon flux is distributed in energy over a wide spectrum that depends on the direction of incidence, the main difference with radiography made with X-rays is in the source. The source of muons is not tunable, neither in energy nor in direction; to improve the signal-to-noise ratio, muography requires large instrumentation, long time data acquisition and high background rejection capacity. Here, we present the principles of the Muography, illustrating how radiographic images can be obtained, starting from the measurement of the attenuation of the muon flux through an object. It will then be discussed how recent technologies regarding artificial intelligence can give an impulse to this methodology in order to improve its results.

First results of undersea muography with the Tokyo-Bay Seafloor Hyper-Kilometric Submarine Deep Detector

Tidal measurements are of great significance since they may provide us with essential data to apply towards protection of coastal communities and sea traffic. Currently, tide gauge stations and laser altimetry are commonly used for these measurements. On the other hand, muography sensors can be located underneath the seafloor inside an undersea tunnel where electric and telecommunication infrastructures are more readily available. In this work, the world's first under-seafloor particle detector array called the Tokyo-bay Seafloor Hyper-Kilometric Submarine Deep Detector (TS-HKMSDD) was deployed underneath the Tokyo-Bay seafloor for conducting submarine muography. The resultant 80-day consecutive time-sequential muographic data were converted to the tidal levels based on the parameters determined from the first-day astronomical tide height (ATH) data. The standard deviation between ATH and muographic results for the rest of a 79-day measurement period was 12.85 cm. We anticipate that if the length of the TS-HKMSDD is extended from 100 m to a full-scale as large as 9.6 km to provide continuous tidal information along the tunnel, this muography application will become an established standard, demonstrating its effectiveness as practical tide monitor for this heavy traffic waterway in Tokyo and in other important sea traffic areas worldwide.

A new cylindrical borehole detector for radiographic imaging with muons

Muon radiography is a methodology which enables measuring the mass distribution within large objects. It exploits the abundant flux of cosmic muons and uses detectors with different technologies depending on the application. As the sensitive surface and geometric acceptance are two fundamental parameters for increasing the collection of muons, the optimization of the detectors is very significant. Here we show a potentially innovative detector of size and shape suitable to be inserted inside a borehole, that optimizes the sensitive area and maximizes the angular acceptance thanks to its cylindrical geometry obtained using plastic arc-shaped scintillators. Good spatial resolution is obtained with a reasonable number of channels. The dimensions of the detector make it ideal for use in 25 cm diameter wells. Detailed simulations based on Monte Carlo methods show great cavity detection capability. The detector has been tested in the laboratory, achieving overall excellent performance.

Muon radiography to visualise individual fuel rods in sealed casks

Cosmic-ray muons can be used for the non-destructive imaging of spent nuclear fuel in sealed dry storage casks. The scattering data of the muons after traversing provides information on the thereby penetrated materials. Based on these properties, we investigate and discuss the theoretical feasibility of detecting single missing fuel rods in a sealed cask for the first time. We perform simulations of a vertically standing generic cask model loaded with fuel assemblies from a pressurized water reactor and muon detectors placed above and below the cask. By analysing the scattering angles and applying a significance ratio based on the Kolmogorov-Smirnov test statistic we conclude that missing rods can be reliably identified in a reasonable measuring time period depending on their position in the assembly and cask, and on the angular acceptance criterion of the primary, incoming muons.

Muometric positioning system (μPS) with cosmic muons as a new underwater and underground positioning technique

Thus far, underwater and underground positioning techniques have been limited to those using classical waves (sound waves, electromagnetic waves or their combination). However, the positioning accuracy is strongly affected by the conditions of media they propagate (temperature, salinity, density, elastic constants, opacity, etc.). In this work, we developed a precise and entirely new three-dimensional positioning technique with cosmic muons. This muonic technique is totally unaffected by the media condition and can be universally implemented anywhere on the globe without a signal transmitter. Results of our laboratory-based experiments and simulations showed that, for example, plate-tectonics-driven seafloor motion and magma-driven seamount deformation can be detected with the μPS.

Development of the muographic tephra deposit monitoring system

Measurements of volcanic tephra fallout deposits provide useful information about the magnitude and intensity of explosive volcanic eruptions and potential for remobilization of deposits as dangerous volcanic flows. However, gathering information in the vicinity of erupting craters is extremely dangerous, and moreover, it is often quite difficult to determine deposit thickness proximal to volcanic craters because the thickness of the deposit is too great to easily measure; thus, airborne remote sensing technologies have generally been utilized during the intermission between eruptions. As an alternative tool, a muographic tephra deposit monitoring system was developed in this work. Here we report the performance of this system by applying the muographic data acquired at Sakurajima volcano, Japan as an example. By assuming the average density of the deposit was 2.0 g cm^-3, the deposit thicknesses measured with muography were in agreement with the airborne results, indicating that volcanic fallout built up within the upper river basin, showed its potential for monitoring the episodic tephra fallouts even during eruptions.

Muographic monitoring of the volcano-tectonic evolution of Mount Etna

At Mount Etna volcano, the focus point of persistent tectonic extension is represented by the Summit Craters. A muographic telescope has been installed at the base of the North-East Crater from August 2017 to October 2019, with the specific aim to find time related variations in the density of volcanic edifice. The results are significant, since the elaborated images show the opening and evolution of different tectonic elements; in 2017, a cavity was detected months before the collapse of the crater floor and in 2018 a set of underground fractures was identified, at the tip of which, in June 2019, a new eruptive vent started its explosive activity, still going on (February, 2020). Although this is the pilot experiment of the project, the results confirm that muography could be a turning point in the comprehension of the plumbing system of the volcano and a fundamental step forward to do mid-term (weeks/months) predictions of eruptions. We are confident that an increment in the number of telescopes could lead to the realization of a monitoring system, which would keep under control the evolution of the internal dynamic of the uppermost section of the feeding system of an active volcano such as Mount Etna.

Stand-off radiation detection techniques

Remote detection of radioactive materials is extremely challenging, yet it is important to realize the technique for safe usage of radioactive materials. Gamma rays are the most far distant penetrating photons that are involved with the radiation decay process. Herein, we overview the gamma-ray detection techniques that are material-based and vacuum tube-based. A muon detector is also reviewed as a radioactive material imager. We overview versatile detectors that are currently being widely used and new concepts that may pave the way for promising remote detectability up to several kilometers.

Monte Carlo characterization of the cosmic ray muon flux in shallow subsurface geological repositories intended for disposal of radioactive materials

Recent challenges in monitoring subsurface geological repositories intended for disposal of radioactive materials such as spent nuclear fuel call for new, innovative concepts that are facility independent, cost-effective, passive, and reliable. Once nuclear material is in place at these facilities, reverifying the inventory may no longer be feasible if continuity of knowledge is lost or unavailable to the inspectors. Using cosmic ray muons may present several potential advantages over conventional photon/neutron signatures, and their use in safeguards applications have only received attention in the past decade. However, there have been limited efforts to explore the integration of cosmic ray muons into repository safeguards and study potential gains, risks, and costs. This paper presents a Monte Carlo-based methodology to characterize the cosmic ray muon flux, including muon angular and energy differential distributions at depths representative of subsurface geological repositories. Since there have been limited measurements at these sites and a measurement made in one site is not always transferable to another site, the objective is to develop an efficient simulation method and useful parametrizations to provide a convenient tool for enabling muon simulations at any geological repository site. It is expected these results will provide a better understanding of how muons can be integrated into an existing geological repository safeguards framework.

Proof-of-Principle of a Cherenkov-Tag Detector Prototype

In a recent paper, the authors discussed the feasibility study of an innovative technique based on the directionality of Cherenkov light produced in a transparent material to improve the signal to noise ratio in muon imaging applications. In particular, the method was proposed to help in the correct identification of incoming muons direction. After the first study by means of Monte Carlo simulations with Geant4, the first reduced scale prototype of such a detector was built and tested at the Department of Physics and Astronomy "E. Majorana" of the University of Catania (Italy). The characterization technique is based on muon tracking by means of the prototype in coincidence with two scintillating tiles. The results of this preliminary test confirm the validity of the technique and stressed the importance to enhance the Cherenkov photons production to get a signal well distinguishable with respect to sensors and electronic noise.

Pilot study of eruption forecasting with muography using convolutional neural network

Muography is a novel method of visualizing the internal structures of active volcanoes by using high-energy near-horizontally arriving cosmic muons. The purpose of this study is to show the feasibility of muography to forecast the eruption event with the aid of the convolutional neural network (CNN). In this study, seven daily consecutive muographic images were fed into the CNN to compute the probability of eruptions on the eighth day, and our CNN model was trained by hyperparameter tuning with the Bayesian optimization algorithm. By using the data acquired in Sakurajima volcano, Japan, as an example, the forecasting performance achieved a value of 0.726 for the area under the receiver operating characteristic curve, showing the reasonable correlation between the muographic images and eruption events. Our result suggests that muography has the potential for eruption forecasting of volcanoes.

Muon Radiography Method for Non-Invasive Probing an Archaeological Site in the Naryn-Kala Citadel

The paper presents the test experiment to investigate one of UNESCO’s (United Nations Educational, Scientific and Cultural Organization) world heritage objects, an archaeological site in the Naryn-Kala citadel (Derbent, Republic of Dagestan, Russian Federation) hidden under the ground’s surface. The function of the site could be revealed by the muon radiography studies. Several nuclear emulsion detectors were exposed for two months inside the site at a depth about 10 m from the modern surface. The use of nuclear emulsions as probing radiation detectors combined with the potential of modern image analysis methods provides for a uniquely high resolution capacity of recording instrumentation and 3D reconstruction of the internal structure of the investigated object. Here we present the experiment and data analysis details and the first results.

Bedrock sculpting under an active alpine glacier revealed from cosmic-ray muon radiography

Mountain glaciers form landscapes with U-shaped valleys, roche moutonées and overdeepenings through bedrock erosion. However, little evidence for active glacial carving has been provided particularly for areas above the Equilibrium Line Altitude (ELA) where glaciers originate. This is mainly due to our lack of information about the shape of the bedrock underneath active glaciers in highly elevated areas. In the past years, the bedrock morphology underneath active glaciers has been studied by geophysical methods in order to infer the subglacial mechanisms of bedrock erosion. However, these comprise surveys on the glaciers' surface, from where it has been difficult to investigate the lateral boundary between the ice and the bedrock with sufficient resolution. Here we perform a muon-radiographic inspection of the Eiger glacier (Switzerland, European Alps) with the aid of cosmic-ray muon attenuation. We find a reach (600 × 300 m) within the accumulation area where strong lateral glacial erosion has cut nearly vertically into the underlying bedrock. This suggests that the Eiger glacier has profoundly sculpted its bedrock in its accumulation area. This also reveals that the cosmic-ray muon radiography is an ideal technology to reconstruct the shape of the bedrock underneath an active glacier.

First muography of Stromboli volcano

Muography consists in observing the differential absorption of muons – elementary particles produced through cosmic-ray interactions in the Earth atmosphere – going through the volcano and can attain a spatial resolution of tens of meters. We present here the first experiment of nuclear emulsion muography at the Stromboli volcano. Muons have been recorded during a period of five months by a detector of 0.96 m² area. The emulsion films were prepared at the Gran Sasso underground laboratory and were analyzed at Napoli, Salerno and Tokyo scanning laboratories. Our results highlight a significant low-density zone at the summit of the volcano with density contrast of 30–40% with respect to bedrock. The structural setting of this part of the volcanic edifice controls the eruptive dynamics and the stability of the “Sciara del Fuoco” slope, which is affected by recurrent tsunamigenic landslides. Periodical imaging of the summit of the Stromboli volcano such as that provided by muography can become a useful method for studying the evolution of the internal structure of the volcanic edifice.

Feasibility Study of a New Cherenkov Detector for Improving Volcano Muography

Muography is an expanding technique for internal structure investigation of large volume object, such as pyramids, volcanoes and also underground cavities. It is based on the attenuation of muon flux through the target in a way similar to the attenuation of X-ray flux through the human body for standard radiography. Muon imaging have to face with high background level, especially compared with the tiny near horizontal muon flux. In this paper the authors propose an innovative technique based on the measurement of Cherenkov radiation by Silicon photo-multipliers arrays to be integrated in a standard telescope for muography applications. Its feasibility study was accomplished by means of Geant4 simulations for the measurement of the directionality of cosmic-ray muons. This technique could be particularly useful for the suppression of background noise due to back-scattered particles whose incoming direction is likely to be wrongly reconstructed. The results obtained during the validation study of the technique principle confirm the ability to distinguish the arrival direction of muons with an efficiency higher than 98% above 1 GeV. In addition, a preliminary study on the tracking performance of the presented technique was introduced.

3D Muography for the Search of Hidden Cavities

Muography (or muon radiography) is a technique that exploits the penetration capability of muons, elementary particles similar to electrons but with a mass about 200 times larger. High energy muons are naturally produced in the interactions of cosmic rays with the Earth atmosphere. The measurement of their absorption in matter allows the imaging of the inner structure of large bodies. The technological developments in the detection of elementary particles have opened the way to its application in various fields, such as archaeology, studies of geological structures, civil engineering and security issues. We have developed a new approach to the three-dimensional muography of underground structures, capable of directly localising hidden cavities and of reconstructing their shape in space. Our measurements at Mt. Echia, the site of the earliest settlement of the city of Naples in the 8th century BC, have led us to the discovery of a hidden underground cavity, whose existence was not evident with the usual two-dimensional muography graphs. We demonstrate here that our original approach definitely enhances muography discovery potential, especially in case of complex underground systems.

Muon Radiography of Ancient Mines: The San Silvestro Archaeo-Mining Park (Campiglia Marittima, Tuscany)

Muon absorption radiography is an imaging technique based on the measurement of the absorption of cosmic ray muons. This technique has recently been used successfully to investigate the presence of unknown cavities in the Bourbon Gallery in Naples and in the Chephren Pyramid at Cairo. The MIMA detector (Muon Imaging for Mining and Archaeology) is a prototype muon tracker for muon radiography for application in the fields of archaelogy and mining. It is made of three pairs of X-Y planes each consisting of 21 scintillator bars with a silicon photomultiplier readout. The detector is compact, robust, easily transportable, and has a low power consumption: all of which makes the detector ideal for measurements in confined and isolated environments. With this detector, a measurement from inside the Temperino mine in the San Silvestro archaeo-mining park in Tuscany was performed. The park includes about 25 km of mining tunnels arranged on several levels that have been exploited from the Etruscan time. The measured muon absorption was compared to the simulated one, obtained from the information provided by 3D laser scanner measurements and cartographic maps of the mountain above the mine, in order to obtain information about the average density of the rock. This allowed one to confirm the presence of a partially accessible exploitation opening and provided some hints regarding the presence of a high-density body within the rock.

Unknown structure of the Chephren pyramid

The Cheops and Chephren pyramid have been in Giza for about 3500 years. Their view is enjoyed by many people around the world who ask questions: how and why were the pyramids built? Cheops and Chephren pyramids are very similar on the outside. According to what we know, however, they are diametrically different in their internal structure. In the Chephren pyramid the internal structure was not discovered while the Cheops pyramid has many internal chambers, like: The King's Chamber, the Queen's Chamber. However, according to current knowledge, the Mummies of Pharaoh and his wife were never buried there. So, why were the internal structures of this pyramid built? Were the pyramids supposed to be a visible sign of Pharaoh's power? Are there any undiscovered chambers in both pyramids, which could give us answers to many questions?

Applications of muon absorption radiography to the fields of archaeology and civil engineering

Muon radiography, also known as muography, is an imaging technique that provides information on the mass density distribution inside large objects. Muons are naturally produced in the interactions of cosmic rays in the Earth's atmosphere. The physical process exploited by muography is the attenuation of the muon flux, that depends on the thickness and density of matter that muons cross in the course of their trajectory. A particle detector with tracking capability allows the measurement of the muons flux as a function of the muon direction. The comparison of the measured muon flux with the expected one gives information on the distribution of the density of matter, in particular, on the presence of cavities. In this article, the measurement performed at Mt. Echia in Naples (Saracino 2017 Sci. Rep. 7, 1181. (doi:10.1038/s41598-017-01277-3)), will be discussed as a practical example of the possible application of muography in archaeology and civil engineering.This article is part of the Theo Murphy meeting issue 'Cosmic-ray muography'.

Overview of muographers

By observing the growth of a research community, particularly the factors of size and strength of involvement, it is possible to evaluate the vitality of a new field. The emerging technology of muography, a new visualization technique to look through gigantic objects with an elementary particle called the muon, is the focus of this study. Recently, the initial applications of muography to the study of structures such as pyramids and volcanoes have been expanding to include more commercial interests such as inspecting social infrastructures and energy and mineral resources. Evolutions of the research networks of muography will be discussed with particular attention to the chronological and lateral structures that could be an indicator of the development of this new technology.This article is part of the Theo Murphy meeting issue 'Cosmic-ray muography'.

Investigation of the limits of high-definition muography for observation of Mt Sakurajima

A multi-wire proportional chamber-based muo- graphy observatory is under development for the monitoring of the internal structure of Mt Sakurajima in Kyushu, Japan. We investigated the limits of large-scale and high-definition muography. We adjusted the parameters of a modified Gaisser model and found that the spectral index of γ =  - 2.64 and normalization factor of C = 0.66 reproduce more accurately the measured fluxes than the original parameters at large thickness. A thickness and zenith angle-dependent correction is suggested to the measured muon flux due to the energy cut which is introduced to suppress the background particles. The multiple scattering of muons was simulated across the standard rock and sea-level atmosphere up to the distance of 5 km. We found that multiple scattering decreases from 10 mrad to 4 mrad across the rock due to the decrease in the steepness of muon spectra. The multiple scattering falls down to about 2 mrad after the object in the atmosphere due to the increase in observed arrival zenith angles. The 2 m2 sized multi-wire proportional chamber-based Muographic Observation System (MMOS) was operating between February and June 2018. Three tracking systems operated reliably with tracking efficiencies of above 95%. The muon flux has been measured correctly down to 10-3 m-2 sr-1 s-1 The average density map of Mt Sakurajima has been measured with angular resolution of 12 mrad × 12 mrad (spatial resolution of 34 m × 34 m from the distance of 2.8 km). The average density values were found between 1.4 and 2 g cm-3, except at the crater regions where lower densities were observed.This article is part of the Theo Murphy meeting issue 'Cosmic-ray muography'.

Japanese volcanoes visualized with muography

High-energy muons that are generated via the reaction between primary cosmic rays and the Earth's atmosphere can be used to map out the density distribution in shallow parts of a volcano's interior. This new subterranean imaging technique called muography has been applied to three different kinds of volcano dynamics in Japan: lava dome formation, vulcanian explosions and magma convection. Taking all of the observational data together, it appears that muography can serve as a new and alternative volcano observation technique, providing a fresh approach to understanding eruption mechanism. This review describes observational studies in which muography has been used to explore the volcano's interior.This article is part of the Theo Murphy meeting issue 'Cosmic-ray muography'.

Muography of different structures using muon scattering and absorption algorithms

In recent decades, muon imaging has found a plethora of applications in many fields. This technique succeeds to infer the density distribution of big inaccessible structures where conventional techniques cannot be used. The requirements of different applications demand specific implementations of image reconstruction algorithms for either multiple scattering or absorption-transmission data analysis, as well as noise-suppression filters and muon momentum estimators. This paper presents successful results of image reconstruction techniques applied to simulated data of some representative applications. In addition to well-known reconstruction methods, a novel approach, the so-called μCT, is proposed for the inspection of spent nuclear fuel canisters. Results obtained based on both μCT and the maximum-likelihood expectation maximization reconstruction algorithms are presented.This article is part of the Theo Murphy meeting issue 'Cosmic-ray muography'.

Muography applications developed by IFIN-HH

Cosmic-ray muons have been studied at IFIN-HH for more than 20 years. Starting as fundamental physics research, the muon flux measurements bring new directions of study regarding muography. Two new directions have been recently developed: underground muon scanning of old mining sites in order to detect the possible presence of unknown cavities and underwater scanning of ships in commercial harbours in order to prevent the illegal traffic of radioactive materials. The main goal of the first direction of study is to improve the security of underground civilian and industrial infrastructures, by starting the development of a new, innovative detection system that can be used to identify potentially dangerous conditions using a non-invasive, totally safe method. The method proposed uses information provided by a device placed underground that measures directional cosmic muon flux and identifies anomalies produced by irregularities in the geological layers above. For the second direction of study, the method proposed is based on the detection and analysis of the cosmic muon flux. The high-density materials (uranium, lead-used for radiation shielding, etc.) cause a decrease in the directional muon flux. The detection system will be submerged underneath the ship that will be scanned, being able to locate illegal radioactive materials without exposing any personnel to radiation or contamination. Correlated with simulations based on the known configuration of the ship scanned, the data provided by the detection system will provide the location and dimensions of the undeclared material transported.This article is part of the Theo Murphy meeting issue 'Cosmic-ray muography'.

A portable muon telescope based on small and gas-tight resistive plate chambers

We report on the first steps in the development of a small-size muon telescope based on glass resistive plate chambers with small active area (16 × 16 cm²). The long-term goal of this project is to focus on applications of muography where the telescope may have to be operated underground and/or inside small rooms, and in challenging logistic situations. Driving principles in our design are therefore compact size, light weight, gas tightness and robustness. The first data-taking experiences have been encouraging, and we elaborate on the lessons learnt and future directions for development.This article is part of the Theo Murphy meeting issue 'Cosmic-ray muography'.

Muon geotomography: selected case studies

Muon attenuation in matter can be used to infer the average material density along the path length of muons underground. By mapping the intensity of cosmic ray muons with an underground sensor, a radiographic image of the overburden above the sensor can be derived. Multiple such images can be combined to reconstruct a three-dimensional density model of the subsurface. This article summarizes selected case studies in applying muon tomography to mineral exploration, which we call muon geotomography.This article is part of the Theo Murphy meeting issue 'Cosmic-ray muography'.

Imaging the dome of Santa Maria del Fiore using cosmic rays

The dome of Santa Maria del Fiore, Florence Cathedral, was built between 1420 and 1436 by architect Filippo Brunelleschi and it is now cracking under its own weight. Engineering efforts are under way to model the dome's structure and reinforce it against further deterioration. According to some scholars, Brunelleschi might have built reinforcement structures into the dome itself; however, the only known reinforcement is a wood chain 7.75 m above the springing of the Cupola. Multiple scattering muon radiography is a non-destructive imaging method that can be used to image the interior of the dome's wall and therefore ascertain the layout and status of any iron substructure in it. A demonstration measurement was performed at the Los Alamos National Laboratory on a mock-up wall to show the feasibility of the work proposed, and a lightweight and modular imaging system is currently under construction. We will discuss here the results of the demonstration measurement and the potential of the proposed technique, describe the imaging system under construction and outline the plans for the measurement.This article is part of the Theo Murphy meeting issue 'Cosmic-ray muography'.

The MURAVES project and other parallel activities on muon absorption radiography

The MURAVES (MUon RAdiography of VESuvius) project is a joint activity participated by INGV, INFN and the Universities of Naples “Federico II” and Florence. The collaboration, following the experience gained within the previous INFN R&D project Mu-Ray, is currently completing the production of a robust four square meter low power consumption detector to be installed on the flank of Mount Vesuvius, an active volcano located on the western coast of Italy. The detector is supposed to collect data for at least one year, thus allowing performing a scan of the structure of the Vesuvius volcanic cone. In this work the status of the project and some parallel activities on muon radiography are presented.

Discovery of a big void in Khufu's Pyramid by observation of cosmic-ray muons

The Great Pyramid, or Khufu's Pyramid, was built on the Giza plateau in Egypt during the fourth dynasty by the pharaoh Khufu (Cheops), who reigned from 2509 bc to 2483 bc. Despite being one of the oldest and largest monuments on Earth, there is no consensus about how it was built. To understand its internal structure better, we imaged the pyramid using muons, which are by-products of cosmic rays that are only partially absorbed by stone. The resulting cosmic-ray muon radiography allows us to visualize the known and any unknown voids in the pyramid in a non-invasive way. Here we report the discovery of a large void (with a cross-section similar to that of the Grand Gallery and a minimum length of 30 metres) situated above the Grand Gallery. This constitutes the first major inner structure found in the Great Pyramid since the nineteenth century. The void, named ScanPyramids' Big Void, was first observed with nuclear emulsion films installed in the Queen's chamber, then confirmed with scintillator hodoscopes set up in the same chamber and finally re-confirmed with gas detectors outside the pyramid. This large void has therefore been detected with high confidence by three different muon detection technologies and three independent analyses. These results constitute a breakthrough for the understanding of the internal structure of Khufu's Pyramid. Although there is currently no information about the intended purpose of this void, these findings show how modern particle physics can shed new light on the world's archaeological heritage.