by Gloria Nobile
Since last June, the Sos Enattos mine has been hosting two muon detectors, which add to the many physical and geological measuring instruments already present in the area around the mine, a candidate site to host the future Einstein Telescope gravitational wave observatory. The new detectors were installed by French researchers Jacques Marteau and Jean Christophe Ianigro from the Institut de Physique des 2 Infinis (IP2I) in Lyon, and Hungarian colleagues László Oláh, Gergely Surányi, and Dezső Varga from the HUN-REN Wigner Research Centre for Physics, with the support of the SAR-GRAV research laboratory, which is located in the Sos Enattos area.
Currently in the calibration phase, these instruments – which will be placed in different locations – will collect measurements to perform a muon tomography of the rock mass above their position within the mine. This technique uses the flow of muons, the “heavy siblings” (over 200 times heavier) of electrons, to reconstruct a three-dimensional image of the internal structure of large objects from the outside.
In nature, the main source of muons comes from the interaction of cosmic rays – energetic particles that travel almost at the speed of light and constantly bombard the Earth from all directions – with the Earth’s atmosphere. Muon telescopes record the charged particles passing through them. Since they are positioned underground, those that release a trace within are mostly muons. Highly penetrating, they can indeed easily reach the Earth’s surface and be absorbed after passing through thick layers of matter, up to hundreds or even thousands of meters of soil or rock.
The potential of muon tomography (or muography) has been confirmed in several fields of application. One of these concerns the study of volcanic structures: examples are the MURAVES (MUon RAdiography of VESuvius) and MEV (Muography of Etna Volcano) projects, which use the properties of muons to measure the mass distribution within the Vesuvius and Etna volcanoes, respectively. This method is useful to expand the understanding of a volcano’s hydraulic system and its eruptive dynamics. Additionally, using the same technique, the discovery of a large cavity called the “Great Void” inside the Pyramid of Cheops in Giza, Egypt, was announced by the international ScanPyramids project in early November 2017.
By measuring the muon flux at multiple points, it is possible to reconstruct the image of large structures. And this is exactly what will happen at Sos Enattos. «Muons have a much greater ability to penetrate matter than X-rays, but the technique is very similar to that of radiography», explains Domenico D’Urso, scientific head of the SAR-GRAV laboratory and coordinator of ET Italy. «X-rays do not pass through the patient’s bones, so what is seen on the X-rays – where the ray has not arrived – is white, that is, the bone. Similarly, where muons do not reach or fewer reach, it means there is material, that is, a rock mass of higher density».
To make sure that there is a uniform coverage of the rock mass “overhead,” or at least as expected, the detectors take measurements from various positions. It is necessary to collect and cross a certain number of data in order to build a map and achieve enough resolution to tell if there are voids (with a certain tolerance) and then monitor the amount of material to detect any water flow.
«We have a unique asset in the mine, which also allows us to make underground measurements», D’Urso concludes. «In that position, there should only be rock above our head. With muon detectors, we want to verify that this is indeed the case».
Dezső Varga, László Oláh e Gergely Surányi (HUN-REN Wigner Research Centre for Physics) at Sos Enattos.