Situated on the flanks of Mount Etna, one of the most active and imminently threatening volcanoes in the world, lies the beginnings of a telescope intended to spy on the highest of high-energy cosmic events. The quarry of the next-generation Imaging Atmospheric Cherenkov Telescope is, namely, the showers of muon particles created as photons of energies up to 100 TeV—around 100 billion times more energetic than a medical X-ray—collide with our upper-atmosphere in hot bursts of Cherenkov radiation. Its creators weren't thinking so much about the geological innards of the ASTRI's volcanic neighbor, but imaging the otherwise unpenetrable guts of Etna might prove to be a bonus application of the telescope, according to a paper posted last week to the arXiv pre-print server.
Muons are intense in every way. They have many of the properties of electrons, such as charge and spin, but they also have around 200 times the mass. When they're created in photon-matter collisions, muons race away at the speed of light. The effect of this is that the newly created particles are initially moving at velocities faster than the local speed of light. The speed of light varies according to the medium it's traveling through, but the muons haven't yet gotten the memo.
This is what the ASTRI telescope and its peers are looking for. As the muons crash into the surrounding atmospheric medium, the result is a shockwave analogous to the "sonic boom" produced when a projectile exceeds the sound barrier. As this shockwave plows through its surroundings, radiation is emitted. Given high enough energies, this radiation can occur in the visible spectrum as a blue glow.
What does this have to do with volcanoes?
Seeing inside of huge objects with help from muons is already a fairly established practice. It allowed archaeologists to find a hidden chamber within the Egyptian pyramid of Chephren at Giza; engineers to discover the depth of "overburden" above a tunnel in Australia; and, lately, border agents to detect the presence of nuclear material in shipping containers.
The aforementioned utilities have been achieved with a technique akin to X-ray imaging. Cosmic ray muons penetrate materials far deeper than X-rays, however, and so it becomes possible to use the scattering of those muons to put together three-dimensional images of really big (or deep) structures. That catch is that this sort of imaging has so far required the use of hodoscopes—instruments consisting of complicated arrays of materials intended to detect particles on an individual basis.
"This technique requires several detection layers and a sufficiently high timing resolution to reduce the level of fake coincidences due to the unavoidable charged particles background," the authors of the Mount Etna paper explain. This presents some significant limitations.
The method proposed for ASTRI and Mount Etna is a bit different. Rather than look for individual particles, it looks for Cherenkov radiation, e.g. the fizz of light created as muons plow through some medium, such as a volcano. "The advantage of using Cherenkov telescopes for muon radiography is due to their imaging capability which results in negligible background and improved spatial resolution compared the traditional particle detectors," the paper continues.
They tested things out using toy-scale versions of the telescope and volcano, finding that not only could they successfully target Mont Etna with the device, they could achieve a 10-fold improvement in sensitivity. In just a few nights of observation, the scheme could detect a 200 meter conduit, perhaps filled with the rising magma of a future eruption.