Cosmic rays are high-energy particles generated by astronomical sources, such as the sun, and they enter the Earth’s atmosphere at nearly the speed of light. Then, in the upper atmosphere, the particles interact with components of the air, such as oxygen and nitrogen, which triggers the creation of other particles – including muons.
Although most of these secondary particles are stopped within the atmosphere, muons are able to travel down to the Earth’s surface. Muons are 200 times heavier than electrons, this makes them highly penetrating, unlike other forms of radiation (i.e., electromagnetic sources) they are capable of penetrating dense materials, such as brick and concrete. They can easily pass through man-made and natural structures.
At sea level, there are approximately 10,000 cosmic ray muons passing through each square metre of ground, every minute. The combination of their highlypenetrating nature and abundance makes muons ideal to help probe, and make images of, the interiors of large-scale structures and otherwise difficult to access objects, and also check underground.
The muons pass more or less through the large structures and underground settings in different degrees, depending on local density. Recording these differences spatially allows images to be created, in the same way that X-rays are able to provide density information within the medical sector. exploits the use of muons to detect density variations in large and/or dense structures.
In a process known as muon tomography, instrumentation, known as muon detectors or sensors, is deployed either below and/or to the side of the object of interest to measure the muon flux (the free-flux of particles is downward, from the atmosphere). Muon tomography, also referred to as muon radiography or muography, therefore exploits naturally-occurring cosmic radiation as a means of performing non-invasive and non-destructive imaging.
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By GlobalDataUsing muon tomography to image structures dates back to the 1950s, when it was employed to measure the overburden of a tunnel in the Snowy Mountain Guthega hydro-electric project, in New South Wales, Australia (George, 1955). Later, in the 1960s, Nobel Prize winner Luis Alvarez and colleagues employed muon tomography to search for hidden chambers in the second pyramid of Chephren in Giza, Egypt (Alvarez, 1970).
Modern methodology typically involves creating a ‘digital twin’ of a structure and then computer simulations of muons penetrating these structures – with and without assumed voids – are performed. The simulated results are compared with collected field data.
Since the research by Alvarez and his team, a number of pyramids have been analysed using muon tomography. In 2017, teams of Japanese and French scientists famously reported the discovery of a large void with a minimum length of 30m in the Great Pyramid of Giza (Morishima, 2017). In total, three independent teams of scientists used different types of muon detectors, and their own simulations and data analysis techniques, to ensure that there could be no misinterpretation of the data. All three teams saw increases in the muon rate in their detectors in a particular direction, which could be interpreted as a hitherto unknown large void above the Queen’s chamber.
In another novel application of muon tomography, in 2001, Professor Hiroyuki Tanaka from the University of Tokyo led a team that used the technique to image the active volcano of Mount Asama, in Japan (Tanaka, 2001). A muon detector system, placed at the base of the volcano, gathered data on the muon flux passing through the volcano, their trajectory being determined using two segmented detectors, positioned a short distance (~1.5m) apart. Subsequently, in 2013, the team performed the first ever visualisation of an erupting volcano using muon tomography, providing information on the movement of the top of a magma column before and after two eruptions (Tanaka, 2013).
Geoptic specialises in civil infrastructure investigations via the novel and unique imaging technique known as muon tomography.
One application of muon tomography, more recently, and pioneered by Geoptic, is its use in identifying and characterising hidden voids in the overburden of railway tunnels. The same instrumentation can also be employed to perform long-term monitoring of the overburden and, in particular, is able to correlate any density changes with possible water ingress from rainfall.
MUON TOMOGRAPHY IN UK RAIL SECTOR
The Victorian era saw a boom in railway construction. Many of the UK’s railway tunnels, bridges, and viaducts were built from the mid-1800s to the early 1900s. Vertical shafts were used in tunnel construction, helping remove spoil and transport materials down to the workers below.
It was not uncommon for several such vertical shafts to be used during a tunnel project but their subsequent fate was rather arbitrary, it seems – some were left open to the surface, others infilled (to some extent), while yet others were capped off at the tunnel lining and the surface but otherwise left empty as so-called ‘hidden shafts’. Even partially-filled shafts can undergo additional degradation of the shaft lining and/or differential settlement over the extended periods.
All of this leads to a need for as complete an inventory as possible of all such hidden shafts on the UK rail network, not least for regular maintenance checks but, where necessary, for corrective actions to take place.
In the absence of muon tomography, approaches to void identification include rather crude methods, such as drilling into the tunnel wall to assess the integrity of the material behind the lining. This carries a number of health and safety risks as invasive drilling could aggravate any structural instability around a hidden void. There is, therefore, clear appeal and merit to having a fully non-invasive and non-destructive technique (NDT) for investigation beyond rail tunnel walls, such as muon tomography.
In 2018, Geoptic was offered the opportunity of performing the first demonstration of application of muon tomography for imaging of railway tunnels. Specifically, the company was asked to perform a portal-to-portal survey of a disused railway tunnel – the 700m-long Alfreton Old Tunnel, between Alfreton and Langley Mill in Nottinghamshire, England.
The tunnel was built from the portals as well as using at least three full-height vertical shafts, which are visible both from within the Alfreton tunnel and from the surface (see Figure 1).
In a data-taking campaign that lasted 12 days, a detailed portal-to-portal scan of the tunnel was carried out. Instrumentation capable of detecting and recording both the passage of a muon and its approximate direction was deployed in a van, which travelled to along the tunnel, stopping at more than 150 predetermined muon scanning points, located at intervals of approx. 10m (reduced to approx. 5m around objects of interest). Data recording at each location lasted typically 20-30 minutes (depending on the depth of overburden, as measurements were made looking upward to catch muons penetrating downward through the ground).
The results from this data-taking campaign (Thompson, 2020) are depicted in Figure 2, which shows the rate of muons detected – ‘measured rate’ – at each point along the tunnel as well as the ‘expected rate’ and ‘inferred rate’, respectively. The ‘expected rate’ is the rate expected from pre-existing geological and topological information, undisturbed by such rail tunnel construction activities; the ‘inferred rate’ is calculated using redundant data from the muon detection equipment, sampled from directions other than measuring vertically upwards.
The broad features described by the data are clear and agree well with expectations, e.g., the muon rate drops rapidly as you enter the tunnel and rises as you leave the tunnel. In areas where there are clear open shafts to the surface the detected muon rate rises dramatically (with typically 10 standard deviations of significance).
The survey also identified a further area of interest at a location approximately 80m from the Langley Mill portal. Close inspection of the data around this point (see Figure 3) indicated an increase in the muon rate, measured in a number of data points that describes a profile suggesting a hidden shaft – the presence of which was subsequently confirmed by Network Rail to the Geoptic team. It should be stressed that the team had no awareness of this feature prior to the survey.
Following the identification of this hidden shaft, Geoptic were contracted to return to the tunnel in 2019. At this point, a more detailed survey of the hidden shaft took place. Using muon tomography, the exact location, extent (i.e., ‘footprint’ on the tunnel roof) and effective overburden (opacity) were measured. The shaft was confirmed to be approximately 3m in diameter and almost a full height voided shaft.
OTHER APPLICATIONS
As noted earlier, muon tomography has been used to image high-profile objects, such as pyramids and volcanoes. It is fair to say that, until relatively recently, use of the method has been largely the preserve of academia. However, over the last few years a surge in interest along with a multitude of commercial projects has taken place, including muon tomographic imaging in mining, archaeological buildings and blast furnaces (IAEA TechDoc Series, 2022).
Muon tomographic systems have also been installed on tunnel boring machines (TBMs) for metro line creation. Variations in muon flux can provide warning of oncoming dangers, such as obstructive manmade structures and hidden voids and, consequently, reduce the risk of collapse and damage to existing infrastructure (Chevalier et al., 2019).
The method also has significant potential for application to the nuclear waste sector – to permanently store radioactive material, such as spent nuclear fuel, deep underground in ‘geological repositories’ located in highly competent, suitable rock. In this deployment scenario, long-term monitoring of the radioactive material is essential in two aspects: safety (ensuring the processes around the storage areas are safe); and, safeguarding (ensuring the stored waste remains in place and is not diverted for illegal use). Once moved from interim to long-term storage in the geological repositories, canisters of radioactive material are expected permanently to remain in place.
At depths where final disposal sites are likely to be located, the muon flux is significantly attenuated deeper underground compared to levels of flux at the surface. However, this is mitigated with the possibility of performing long image times – where detectors have longer exposure to the muon flux – hence making muon tomography a potential candidate for the safety and safeguarding applications. One example of a safeguarding application could be confirmation of the emplacement of canisters. With sufficient detector resolution, images of canisters in place can be obtained along with density information to determine whether canisters have been replaced with dummies.
Similarly, during the geological repository construction process, muon tomography can be used to understand the condition of the host rock and identify any structural defects that may be present. Leaving muon detectors in the facility once construction is complete would allow continuous monitoring for structural defects as they develop over time as well as monitoring voids, movement or water ingress.
A programme of work to assess the potential of muon tomography to address a range of safety and safeguarding challenges in the nuclear waste industry is currently under consideration by a consortium of European partners, including Geoptic.
Muon tomography is not limited to the imaging of voids; the same methodology can be used to image large objects with densities significantly different from those of the surrounding environment – where ‘significant’ is largely defined by the length of time available for flux measurement and the sensor detection area able to be practically deployed. This has led to several other applications, such as the imaging of large scale industrial equipment: a blast furnace (Bonechi, 2021); and, a nuclear reactor at the Fukushima Daiichi power plant, in Japan, after a tsunami led to three reactor meltdowns, in 2011, but the high levels of radiation made conventional imaging techniques unstable (Miyadera, 2013).
Over the past decade, the ability of muon tomography to non-invasively discover geological irregularities has led to promising research outcomes for the mining sector. Relying solely on the natural cosmic radiation, muon tomographic surveys have minimal environmental and cost implications when compared to existing investigation methods, such as seismic surveys.
One feasibility study used the method to study a known volcanogenic massive sulphide (VMS) deposit in Canada, located approximately 70m below ground. The system measured muon flux levels passing down through the entire ore structure and the recorded sensor data showed greater and lesser density regions, enabling a 3D density model to be produced which was then found to closely correspond to a similar one produced previously, derived from on-site drill core measurements (Bryman et al., 2014).
Muon tomography is a novel imaging technique that harnesses naturally-occurring highly penetrating radiation to form images of otherwise difficult-toaccess objects in a non- invasive manner. It is being applied to surveying challenges in the rail, mining and nuclear sectors. Further applications are expected as the technique becomes accepted and adopted.
