For the first time a muography of the Stromboli volcano has been created, the result of the collaboration of a group of researchers from the National Institute of Nuclear Physics (INFN) and the National Institute of Geophysics and Volcanology (INGV) in collaboration with Japanese research institutes .
Muography, or muon radiography, is a technique that uses muons, particles that are produced when cosmic rays from space interact with the Earth's atmosphere, to reconstruct an image of the internal structure of an object.
The results of the muon radiography of Stromboli volcano, published today in the international journal Scientific Reports of Nature, have revealed the presence of a low-density zone in the summit area of the volcano. This area corresponds to a collapse structure formed in the area of the craters during the 2007 effusive eruption and subsequently filled by loose pyroclastic material produced by the Strombolian explosive activity. This structure, which influenced the eruptive style of the volcano after the 2007 eruption, has a density that is more than 30% lower than the rest of the bedrock.
"The result obtained will serve to better understand the Strombolian eruptive processes and the dynamics of the slope of the Sciara del Fuoco, which in the past has been repeatedly affected by tsunami-induced landslides", explains Flora Giudicepietro, of the Vesuvius Observatory of Naples (INGV), who contributed to the research.
The muon radiography technique is based on a principle similar to that of radiographs using X-rays, but with respect to this it has the advantage of being able to be used to investigate much larger objects, such as volcanoes, precisely, because muons have much greater penetration into matter than X-rays.
"The muon detector we have designed is based on the technologies developed for the OPERA experiment, which studied the properties of the neutrino beam coming from CERN at the INFN Gran Sasso National Laboratories", explains Giovanni De Lellis of the INFN Section of Naples and the Federico II University, head of the OPERA experiment and one of the creators of the project. “The first challenge we faced was the need to design a compact detector with high angular resolution, which did not require an electrical power supply, which could be transported on the slopes of a volcano and which could withstand bad weather”.
The detector used is made up of 320 nuclear emulsion films, special photographic plates which allow the passage of the particles passing through them to be "photographed" with great precision. The detector surface used is about one square metre. The detector was positioned at the Le Roccette site, at an altitude of 640 metres, and collected the traces of the muons that passed through the volcano for about 5 months.
“The muons produced in the interaction of cosmic rays with the atmosphere penetrate the volcanic rock and can pass through it from side to side. However, depending on the density and thickness of the rock, a part of these is absorbed”, explains Valeri Tioukov of the INFN of Naples, who coordinated the project. “From the number of muons that arrive at our detector from different directions we can therefore understand the density of the material they have passed through.” Periodic radiographs of the summit of the volcano could be used to monitor the evolution of its internal structure.
Image - Difference between the measured muon flux and the one calculated with a Monte Carlo simulation. The blue line indicates the profile of Stromboli volcano as seen by the detector. The low density area in the summit area of the volcano is highlighted in red.
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