TECHNICAL | R&D
MUON SIGHT IN TUNNELS
Cosmic ray R&D is bringing muon-based systems to highlight density differences that can reveal voids or marked variations in ground conditions, and also permit non-destructive testing behind tunnel lining, explains Prof Lee Thompson, Technical Director of Geoptic
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 meter of ground, every minute. The combination of their highly- penetrating 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, by exploiting 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.
Above, figure 1: Open shaft at disused rail tunnel near Alfreton, UK 36 | Summer 2023
EARLIER CASE STUDIES Using 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,
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