Muons reveal the interior of volcanoes

The MU-RAY project has the very challenging aim of providing a “muon X-ray” of the Vesuvius volcano (Italy) using a detector that records the muons hitting it after traversing the rock structures of the volcano. This technique was used for the first time in 1971 by the Nobel Prize-winner Louis Alvarez, who was searching for unknown burial chambers in the Chephren pyramid.


The location of the muon detector on the slopes of the Vesuvius volcano.

Like X-ray scans of the human body, muon radiography allows researchers to obtain an image of the internal structures of the upper levels of volcanoes. Although such an image cannot help to predict ‘when’ an eruption might occur, it can, if combined with other observations, help to foresee ‘how’ it could develop and serves as a powerful tool for the study of geological structures.

Muons come from the interaction of cosmic rays with the Earth's atmosphere. They are able to traverse layers of rock as thick as one kilometre or more. During their trip, they are partially absorbed by the material they go through, very much like X-rays are partially absorbed by bones or other internal structures in our body. At the end of the chain, instead of the classic X-ray plate, is the so-called 'muon telescope', a special detector placed on the slopes of the volcano.

As for ordinary X-ray radiography, a larger muon absorption corresponds to a higher density in the volcano. The flux of backward muons can be used for normalization.
[From H.K.M. Tanaka et al., Earth and Planetary Science Letters 263 (2007) 104]

“This technique was pioneered in Japan by Hiroyuki Tanaka from the University of Tokyo and his collaborators, who first used it to look inside the Asama volcano. They have now joined our collaboration,” says Paolo Strolin, spokesperson of the MU-RAY project and a member of the Italian National Institute for Nuclear Physics (INFN) and the University of Naples Federico II.

Reconstructed average density distribution of the summit of Mt. Asama in Japan.
[From H.K.M. Tanaka et al., Earth and Planetary Science Letters 263 (2007) 104]

From a technical point of view, performing muon tomography of Vesuvius is a great challenge, much beyond what has been done so far. “The morphology of the mountain is complex, partly due to the fact that it has grown in the caldera of a larger volcano, of which what is left is now called Monte Somma,” explains Paolo Strolin. Muons have to go through about two kilometres of rock to reach the detector on the opposite side of the volcano, and only muons of very high energy are able to do so. “For a first investigation, we are using the detector already used in Japan, although that volcano was much less thick than Vesuvius,” explains Strolin.

The thicker the layer of rock, the larger the detector area must be, otherwise it would take too long to take the data. “We are working on a prototype of a new detector,” says Paolo Strolin. “The new system will be modular to globally cover surfaces of the order of tens of square metres. The detectors will have good angular resolution and an improved signal-to-background ratio.” The R&D project on the new generation ‘muon telescopes’ is supported by INFN, the Italian National Institute of Geophysics and Volcanology (INGV), the Italian Government and the University of Naples Federico II.

The MU-RAY collaboration involves physicists and volcanologists from the Universities of Florence, Naples, Perugia and Tokyo, as well as from INFN, the Vesuvius Observatory of the INGV, Fermilab and LAL-Orsay. Other volcanoes are being studied with this technique in Japan and in the Lesser Antilles.

With this article, the Bulletin begins a new series providing readers with a look "outside CERN". We will regularly feature scientific activity going on in CERN's Member States. 

by Francesco Poppi