THE SCIENCE BEHIND COSMIC RAY IMAGING

What are they?

COSMIC RAY MUONS

Cosmic ray muons are nature’s x-rays, highly penetrating, travelling to the Earth’s surface and passing through man-made and natural structures.

These useful particles are generated when high energy radiation interacts in the Earth’s upper atmosphere. There are around 10,000 cosmic ray muons passing through one square metre of ground every minute; we use them to image the interior of large engineering infrastructure.

MUON RADIOGRAPHY

As muons pass through dense objects they can be scattered or absorbed similar to traditional x-rays, just to a much smaller degree. This makes them suitable for imaging extremely large or dense structures.

Since the rate of cosmic ray muons is well understood at sea-level, it is possible to extract information on the internal structure of an object by simply placing a muon detector below it, and recording the reduction in muons compared to an open sky measurement. This method is referred to as “muon radiography”.

MUON TELESCOPES

As muons pass through objects they deposit energy in the form of ionisation (charged particles). Muon detectors use this property to measure a muon’s crossing position using specialised electronics. Our “muon telescopes” measure the crossing point of each muon through two or more planes (we call this a pixel), allowing us to precisely reconstruct the original trajectory of the muon, and determine the average reduction in muon rate along many different angular fields of view.

Taking muon exposures with our telescopes from many different positions allows us to combine the data from overlapping fields of view to extract a density map of the object of interest entirely from below. This allows us to characterise the shape and extent of voiding inside structures, and locate density anomalies completely non-invasively.

Whilst the technology to detect individual muons is relatively simple, construction of large area, high efficiency muon telescopes is remarkably more difficult. Small changes in individual pixel sensitivity can introduce large scale uncertainties in the final density map. Thankfully the geoptic team has over 10 years collective experience developing these systems for the high energy physics and nuclear security communities, with our calibration routines ensuring our telescopes remain stable even in harsh outdoor environments.

APPLICATIONS

These two muon imaging techniques are applicable to a wide range of industrial problems. The Geoptic team has most recently demonstrated their suitability in the rail industry, deploying muon tomography detectors inside railway tunnels to search for hidden voids in the rock above the tunnel.

To learn more about the sectors where the Geoptic team is working to apply these techniques please follow the link below.