Muons are an extremely dense (207 times heavier than an electron), negatively charged subatomic particle. They were first discovered in 1936 by Carl D. Anderson and Seth Neddermeyer and are formed when high energy protons in cosmic rays collide with atomic nuclei in the Earth’s upper atmosphere. It has been estimated that around 10,000 reach any one square metre of Earth, every minute. Without going into the theory of special relativity, the near-light, relativistic velocities enable them to reach Earth (despite the microsecond scale decay time) and penetrate through rock, hundreds of metres deep. Furthermore, the fact that muons are ever present in background radiation, means that human operators are not put at significant additional risk unlike the potentially harmful, mutatory and cancerous effects of x-radiation.  These two factors are just some of the reasons that make muographic techniques far superior to x-rays in certain applications.

Muography can help predict potential slope failure, although it must be noted that this is not a widespread technique. As muons can penetrate deep underground, groundwater and saturation levels within bedrock can be measured after intense periods of rainfall. Thus, geologists can get a better insight into the potential likelihood of slips and rotational slumps. When the famous Pyramid of the Sun (Teotihuacan, Mexico) was surveyed, a 60-metre-wide low-density region was discovered, leading to some commentators stating that structural integrity has been lost. Companies such as muonvision are able to make mining operations, “safer and more sustainable.” Time, energy, water and importantly the likelihood of slope failure can be reduced, protecting those inside the mines. They claim that they can save mines up to 600kg CO₂ and 400L of H₂O per tonne of copper [1].

One of the prevailing uses of muons is in archaeology, which in turn links to better understanding of cultures, ideas and beliefs – broadly speaking, human geography. In 2017, the use of muons led to the discovery of a 30m long cavity above the Grand Gallery in The Great Pyramid of Giza – the first discovery since the 19^th Century. Kunihiro Morishima and his colleagues from ScanPyramids used the idea that fewer muons pass through stone and dense structures, thus voids lead to an increase in muon readings. As muons cannot be artificially generated, it is a time-consuming process as only background levels can be used. Thus, accessibility to get into pyramid and easily portable devices that do not damage the UNESCO site are a must. This enables the recording apparatus to be closer and orientated upwards (muons predominantly travel downwards) towards suspected voids, allowing for a statistically sufficient muogram to be produced. Confirmation can then be gained by measuring the lateral movement of muons from outside the pyramid. With other pyramid-type structures across the world it is often difficult to find an access route into the inner edifice and instead deep boreholes are required, increasing the time component of surveyance. Through muographic techniques, Egyptologists may be able to advance their understanding of The Pyramids and broader still, the culture and beliefs of the entire Egyptian Civilisation.

Another rapidly growing application of muons is in geology. This is especially true of volcanology and glaciology. In glaciers, muography helps to penetrate through thick ice and provide an ‘image’ of the underlying bedrock’s geometry. Nishiyama et al. utilised the Jungfrau railway tunnel to get below the Eiger glacier [2]. From measuring flux and attenuation, ice thickness to around a 10-30m accuracy can be measured. As a complementary technique, muography enables glaciologists to better understand subglacial erosional processes. The use of muons has become more widespread in volcanology than in glaciology. Simply put, the thicker the rock, the longer the time required is to collect muons, and the flux loses its ‘brightness’. This is reflected in the resulting negative image, akin to in an x-ray. Due to the penetrating power, it is the best imaging technique to look at the top few-hundred metres of a volcano’s cone. The high precision of measurements provides volcanologists with information on anomalies in rock density caused by lava conduits for example. Tanaka and others successfully imaged the density distribution of magma in the conduit of Mt Iwodake volcano [3]. From this it was concluded that “the apparent position of the magma head is in accord with the degassing model of rhyolitic systems that was advanced in 2002.” The team from the University of Tokyo is hoping that through muography, low-density regions can be incorporated into computer models which predict the characteristics of volcanic eruptions. The most famous example of muon radiography in use is the Mu-Ray project at Mount Vesuvius where the 500 metres wide and 300 metres deep crater is being surveyed through muon telescopes. Due to the sheer size of Vesuvius, the investigation takes a long time. Thus, muograms can also be inferred as a function of time, to see where the relatively more and less dense areas are.

Thus far, we have only been looking at muon radiography, which produces a two-dimensional image by looking at the transmitted flux of muons through the object. Muon tomography, however, enables the creation of 3 dimensional images as incoming muons are individually tracked through a medium. The Coulomb scatterings are measured as they enter and leave the object, and greater deviations are observed when passing through denser objects. This plays a crucial role in protecting the world against rogue states and terrorist attacks.

Los Alamos, New Mexico is infamously known as the place where the Manhattan Project was created, and where it developed its nuclear bombs. Now, however, it has assembled a ‘Mini Muon Tracker’, which several countries currently use at entry and exit ports. Scanners implemented in Freeport, Bahamas for example, help to prevent the spread of nuclear material as they can ‘look inside’ entire shipping containers. Explosives, contraband and importantly, shielded nuclear materials, can all be detected from the three-dimensional images, within a matter of minutes. Muons are able to defeat concealing materials in a quick process that does not expose operators to dangerous levels of radiation. In 2006, a prototype,” successfully sniffed out test objects such as a 10-centimeter cube of lead hidden inside an engine block, something that would have evaded a conventional x-ray scan [4].” In the future, Professor Chris Rhodes FRSA (Director of Fresh-lands Environmental Actions) believes that these detectors could be, “applied in cities, under overpasses, or at entrances to government offices, and other sensitive buildings [3].” A commercial version of the scanner could be 16 feet high, 14 feet wide and 60 feet long, with aluminium detector tubes that can detect muons by looking at the corresponding ionisation trails. Depending on the size and loading of the vehicle, it could take from 20 seconds to a minute in order to get an image. With the technological advancement of machine learning, this time could be further reduced, and innocuous data could be ignored, providing a clearer image, and ultimately making the world a safer place.

This method of tomography for imaging nuclear materials, isn’t just limited to preventing the trade across borders which is illegal when tested or used, as set out by the Comprehensive Nuclear-Test-Ban Treaty. It can be used to uphold the Nuclear Non-proliferation Treaty of which one part is the eventual worldwide disarmament. Inspections can be carried out to see the existence and quantities of nuclear material in warheads. Waste silos in the UK can be imaged to see the condition of the waste, the degree of cooling in the spent fuel and the condition of the storage container, which is usually surrounded by metres of concrete and cooling ponds. This allows for the secure storage and continuous monitoring of spent fuel rods.

The third use of muons in the nuclear industry is for looking inside reactors. Test models were run with concrete slabs and a 0.7m thick piece of lead, to simulate the melted core of the reactor at Three Mile Island. A clear image showing the conical void within the reactor core was produced over 900 hours. It took 10 years after the 1979 partial meltdown before the damage to the reactor could be investigated. With muographic techniques however, in 2015 just four years after the Fukushima Daiichi meltdown, it was confirmed that the fuel in reactor number 1 had undergone a complete meltdown. The images showed that there is no fuel left in the reactor and that little remains in the containment vessel. Thus, the corium’s (melted fuel debris) location can be better ascertained. This will ultimately enable the quicker and safer decommissioning of the reactor site, enabling the repopulation of areas. This could be critical in reducing the pressures caused by large populations elsewhere whilst revitalising local ecosystems.

Moving away from the use of muons in denuclearising the world, it is hoped that muon tomography can aid in sustainability. Carbon capture and storage (CCS) is an idea that has yet to gain worldwide usage. In 2019 there were only 19 large-scale facilities in full operation [5]. The basic premise is that CO₂ is liquefied and pumped underground into old mines or oil and gas fields, effectively scrubbed from the atmosphere. Muography could produce images of the CO₂ to detect movement and confirm that it is not escaping and once more becoming a greenhouse gas. This has started to be tested in the Boulby deep mine underneath the North Sea and is potentially far less expensive than seismic modelling and monitoring. At the moment though costs are high due to the lack of commercialised CCS project muon detectors. Capturing carbon dioxide from power stations yields an extremely high energy penalty of around 30%. Reducing the cost of monitoring via muography, will ultimately help reduce the long-term costs of carbon storage [6]. When quantitative evidence of the benefits of CCS becomes more widely realised and acted upon, muons could play a critical role in ensuring the success of these projects. This will ultimately help to reduce anthropogenic global warming, reducing climatic extremes that both directly and indirectly affect people’s livelihoods.

Since their discovery less than a century ago, muons have started to play a core role in the world today. In the future, they may be able to predict volcanic eruptions, make sure that carbon dioxide is safely sequestered, prevent massive slope failure, denuclearise the world and provide an enlightened insight into the great civilisations that came before us.

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References

[1] https://muonvision.com/ [Accessed 18 October 2020]

[2] Nishiyama, R., Ariga, A., Ariga, T. et al. Bedrock sculpting under an active alpine glacier revealed from cosmic-ray muon radiography. Sci Rep 9, 6970 (2019). https://doi.org/10.1038/s41598-019-43527-6 [Accessed 15 October]

[3] RHODES, C. “Muon Tomography: Looking inside Dangerous Places.” Science Progress (1933-), vol. 98, no. 3, 2015, pp. 291–299. JSTOR, www.jstor.org/stable/26406303 [Accessed 19 Oct. 2020.]

[4] WOLVERTON, M. (2007). Muons for Peace. Scientific American, 297(3), 27. Retrieved October 18, 2020, from http://www.jstor.org/stable/26069483

[5] https://www.globalccsinstitute.com/resources/global-status-report/ [Accessed 18 October 2020]

[6] https://www.bbc.co.uk/news/science-environment-24086950 [Accessed 19 October]

(Featured Image: © Jasper Sodha)