Muography

Muography, or muon radiography, is a non-invasive imaging technique that uses naturally occurring cosmic-ray muons to probe the interior of large, dense objects. By measuring the attenuation or deflection of muons as they pass through matter, muography can reveal the object's internal density distribution.

Muons are produced when cosmic rays strike the upper atmosphere and reach the Earth’s surface with a broad range of energies. Their flux is directional and energy dependent; denser regions reduce the number of muons detected from that direction. In absorption (muon tomography), detectors are placed on opposite sides or around the target; by comparing the observed muon flux from many angles, a two-dimensional or three-dimensional map of integrated density is reconstructed. Scattering muography (muon scattering tomography) uses the small deflections muons undergo in matter to estimate density variations inside the object.

Detector technologies include arrays of scintillators, gas-filled drift tubes, and resistive plate chambers. Data collection can require weeks to months, depending on the size of the target and the desired resolution, and the reconstruction hinges on sophisticated algorithms.

Applications span several fields. In volcanology, muography is used to image magma chambers and conduit systems within volcanoes. In archaeology, muon imaging has been employed to search for hidden chambers, notably in the Great Pyramid of Giza under the ScanPyramids project, which reported detected voids. In civil engineering and security, muography can inspect the interiors of large structures and critical infrastructure without disassembly.

Limitations include long data acquisition times, dependence on detector geometry and background, and limited spatial resolution. As detectors and reconstruction methods advance, muography continues to complement traditional imaging approaches for studying the interiors of large, dense objects.