Muon tomography

Muon tomography is a technique that uses cosmic ray muons to generate three-dimensional images of volumes using information contained in the Coulomb scattering of the muons. Since muons are much more deeply penetrating than X-rays, muon tomography can be used to image through much thicker material than x-ray based tomography such as CT scanning. The muon flux at the Earth's surface is such that a single muon passes through an area the size of a human hand per second. Since its development in the 1950s, muon tomography has taken many forms, the most important of which are muon transmission radiography and muon scattering tomography. Muon tomography imagers are under development for the purposes of detecting nuclear material in road transport vehicles and cargo containers for the purposes of non-proliferation. Another application is the usage of muon tomography to monitor potential underground sites used for carbon sequestration.

Cosmic ray muons have been used for decades to radiograph objects such as pyramids and geological structures. The technique of muon transmission imaging was first used in the 1950s by Eric George to measure the depth of the overburden of a tunnel in Australia. In a famous experiment in the 1960s, Luis Alvarez used muon transmission imaging to search for hidden chambers in the Pyramid of Chephren in Giza, although none were found. In both cases the information about the absorption of the muons was used as a measure of the thickness of the material crossed by the cosmic ray particles.

More recently, muons have been used to image magma chambers to predict volcanic eruptions. Nagamine et al. continue active research into the prediction of volcanic eruptions through cosmic ray attenuation radiography. Minato used cosmic ray counts to radiograph a large temple gate. Frlez et al. recently reported using tomographic methods to track the passage of cosmic rays muons through cesium iodide crystals for quality control purposes. All of these studies have been based on finding some part of the imaged material that has a lower density than the rest, indicating a cavity. Muon transmission imaging is the most suitable method for acquiring this type of information.

...
Wikipedia