Geneva camera films cosmic rays

Developed at the Department of Nuclear and Corpuscular Physics, a high-performance detector makes it possible to study the most energetic cosmic rays that constantly bombard the Earth.

A state-of-the-art camera developed by the team of Teresa Montaruli, professor at the Department of Nuclear and Corpuscular Physics (DPNC, Faculty of Science), and designed to study cosmic rays captured its first light last week. Mounted on the SST-1M telescope at the Ondřejov Observatory near Prague in the Czech Republic, the instrument, developed under the direction of DPNC researcher Matthieu Heller, is sensitive to flashes of blue light that last only a few nanoseconds and are produced by gamma and cosmic rays entering the atmosphere. On its first night of operation, the camera successfully recorded the signatures of several events, including at least one directed directly into the telescope's eye. The SST-1M telescope has already begun a campaign to observe the Crab Supernova, a well-known source of very high energy gamma rays.

The Earth is constantly bombarded by cosmic rays," explains Domenico Della Volpe, senior lecturer at the DPNC, who was involved in the SST-1M project. They are made up of all sorts of particles and ions from outside the Solar System and even from the Milky Way. Their existence was discovered more than a century ago, but they are still not well known, and their origin remains a mystery. To find out more, we are particularly interested in gamma rays, i.e. photons, or grains of light, of very high energy. As they are electrically neutral, their trajectory in space is not affected by the magnetic fields in and between galaxies. They therefore allow us to locate their source, even if it is very far away.

The Geneva camera is designed to detect the most energetic gamma rays produced and accelerated by the most powerful phenomena in the Universe: black holes, supernova remnants, pulsars, etc. When they enter the atmosphere, these gamma rays inevitably collide with the molecules of the air and disintegrate. They then produce a rather complex spray of particles that essentially ends in a shower of electrons and positrons. These exceed the speed of light in air (the physically unbridgeable limit is the speed of light in a vacuum, which is slightly higher). This phenomenon generates blue light, known as Cherenkov radiation, which the camera detects.

Rain of particles

The telescope on which our camera is mounted is not an optical instrument," explains Teresa Montaruli. It is a large mirror that reflects the image of the sky directly onto our detector, which is about 1 metre in diameter and is made up of more than 1200 silicon photomultiplier 'pixels'."

The SST-1M's camera pixels are equipped with innovative silicon sensors for gamma-ray astronomy and are connected to a sophisticated electronic system that allows for full digitisation of the data acquisition process. The device can capture more than 1000 images per second and process them automatically almost live. The measurement is triggered with a chronometric precision of a few tens or hundreds of nanoseconds, enabling each pixel to reconstruct the development over time of the particle shower following the entry of a cosmic gamma ray into the atmosphere.

According to its designers, the technology chosen by the Geneva scientists is probably the most efficient to date in terms of effectiveness, cost and ease of production. It has been evaluated as such by a panel of experts in the major Cherenkov Telescope Array (CTA) project dedicated to the study of cosmic rays. Currently under development, this scientific programme, worth more than 300 million euros, plans to install more than a hundred telescopes at the Cerro Paranal site in Chile and at La Palma in Spain.

The project is the result of collaboration with the Institute of Nuclear Physics of the Polish Academy of Sciences, the Jagiellonian University in Krakow, Poland, the Palacký University in Olomouc, Moravia, and the Institute of Physics of the Academy of Sciences of the Czech Republic. The latter two are responsible for the installation of the SST-1M telescope at the Ondřejov Observatory. In a fortnight' time, a second telescope, located next door, will receive a second camera, which is currently still being tested in the DPNC laboratory.

The SST-1M project has also benefited from the valuable collaboration of the Astronomical Observatory of the Taras Shevchenko National University in Kyiv, whose country is currently the victim of a brutal aggression by the Russian army, strongly condemned by the University of Geneva.