Muon tomography

One of those topics that occasionally gets a mention is muon tomography, sometimes known as muography. This methodology does sound rather science fiction as it uses cosmic ray muons to create three-dimensional images of volumes in structures, such as the pyramids in Egypt, by processing the data from the Coulomb scattering of the muons. The muons penetrate much more than X-rays can.

In 1936, it was discovered that muons were generated from cosmic rays and were used to measure the areal density of rock overburden. A Geiger counter was used and some results were achieved, but without directional sensitivity subsequent imaging was not possible. More recently, in the 1960s, muon transmission imaging was used to see if there were any hidden chambers in the pyramid of Chephren in Giza, Egypt. On this occasion none were found but later surveys were more successful.

Moving to 1970, the first muogram was produced looking for hidden rooms in the pyramid of Khafre, which involved exposing the apparatus for several months, computer simulations and calculating the muon’s arrival angles. This must have been at the leading edge of technology at this time.

More applications have been considered and, in 2007, film muography, which uses nuclear emulsion, was used to look at the interior of the active Asama volcano and revealed the structure of the magma pathways.
Real-time muography uses muon sensors to convert a muon’s kinetic energy into electrons, enabling the muon occurrences as electronic data rather than chemical changes on film, not that dissimilar to digital radiography.  I have been wondering at the size of the film and electronic plates, especially when looking at structures such as volcanoes and pyramids. There are now micromegas detectors that have a positioning resolution of 0.3 mm, whereas the scintillator-based devices are at 10 mm resolution.

The applications include imaging geological features such as volcanoes, underground water, glaciers and mining. Civil engineering uses include dams and the surrounding areas and more, including one-off applications such as identifying hidden construction shafts, as subsequently located in the Alfreton Old Tunnel, which was constructed in 1862.  In the nuclear sector, it has been used to investigate the condition of the damaged Fukushima nuclear reactors. The technology is also valuable for assessing historical nuclear waste. Other uses include archaeology and large structures, including pyramids, around the world.

The uses and potential uses are still being explored; however, I do not see it being a mainstream non-destructive testing (NDT) technology in widespread use in the near future in the way more conventional NDT methods are.  It will probably become more used in structural health monitoring in civil engineering applications.  A case of science fact not science fiction.

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