FAQs

A muon is a subatomic particle, similar to an electron but around 200 times heavier. Muons are created continuously in the upper atmosphere when cosmic rays from space collide with air molecules, and they rain down over the entire surface of the Earth at all times. Roughly one passes through every square centimetre of the ground each minute. Because they are heavy and weakly interacting, muons travel through hundreds of metres of rock before being absorbed — which is what makes them useful for imaging large solid objects.

Muon tomography — also called muography, muon radiography, cosmic ray muon scanning or passive tomography — images the interior of large structures by measuring naturally occurring cosmic muons as they pass through them. The result is a three-dimensional map of density inside an object far too large or too solid to scan by other means. It requires no artificial radiation source and no access to the interior, because the particles it relies on are already arriving from the sky. In mining it is used wherever what matters is inside a structure that cannot be opened.

Cosmic rays striking the upper atmosphere produce muons, which reach the entire surface of the Earth continuously. Muons lose energy in proportion to the mass they cross, so denser material lets fewer through. A detector placed beneath or beside a structure counts how many muons arrive from each direction, and those counts are reconstructed into a three-dimensional density map of what lies between the sky and the detector.

Muon tomography measures bulk density directly, in place, without extracting samples. Because cosmic muons lose energy in proportion to the mass they cross, counting how many arrive from each direction yields a volumetric density map of the interior — including fluid content and air-filled porosity. Unlike core sampling, which characterizes a point, this characterizes the whole volume, and it can be repeated continuously to track how density changes over time.

The principle is comparable: both build an image from how radiation is absorbed by material of different densities. The difference is the source. A CT scanner emits radiation; muon tomography uses cosmic muons that are already arriving naturally, at all times, everywhere on Earth. That makes it entirely passive, and it works at a scale no X-ray system reaches — hundreds of metres of rock rather than centimetres of tissue.

Muon tomography emits nothing. It is entirely passive: the muons it measures are cosmic particles that already pass through every person and every structure on Earth, continuously, whether or not a detector is present. This means the detectors can be installed anywhere on a site — next to an operating plant, in an occupied area, or underground — with no radiation risk to anyone working nearby.