A new feature in the energy spectrum of cosmic rays has been revealed by DAMPE (Dark Matter Particle Explorer), a Chinese space mission, carried out with a major contribution from Italy, coordinated by INFN, and which has been active for over ten years on a satellite orbiting at an altitude of 500 kilometres. The result, published today in Nature, shows that galactic charged cosmic rays, such as nuclei of hydrogen, helium, carbon, oxygen and iron, share a common feature in their energy spectrum: in all these cases the intensity of the cosmic radiation decreases more rapidly once a specific energy value is reached, which turns out to be proportional to the electric charge of the nucleus. More precisely, the decrease occurs, for all nuclei, at the same value of magnetic rigidity equal to about 15 teraelectronvolts. Rigidity is the property of an electrically charged particle that measures how difficult it is to bend its trajectory using a magnetic field.
Reaching this measurement was possible because DAMPE has managed to explore energy regions so high that they had never been reached by other experiments in space with the same precision. These are energies of the order of 1 PeV (one million billion electronvolts), reached in some of the most extreme astrophysical objects in our Galaxy and more than one hundred times higher than the energies at which protons can be accelerated in the Large Hadron Collider at CERN.
“This result is a major step forward in helping us to better understand the characteristics of cosmic rays and the mechanisms that lead them to reach such high energies”, explains Ivan De Mitri, professor at the Gran Sasso Science Institute, INFN-LNGS researcher and coordinator of the GSSI group in DAMPE. “To date, in fact, we still do not fully understand the mechanisms of production and propagation of these particles. For this reason, the international community is using various experimental techniques which, alongside the study of charged particles, also include observations of very high-energy photons and neutrinos with detectors operating in space, on the ground on large plateaus, under the ice at the South Pole and in the depths of the Mediterranean Sea”.
“Italian groups have taken part in the mission since the stages of design, construction and commissioning of the experiment”, emphasises Giovanni Ambrosi, researcher at the INFN Perugia division and national coordinator of the experiment. “They coordinated the calibration of the detectors at the CERN accelerators in Geneva, and still contribute significantly to data analysis and to the achievement of scientific results”.
DAMPE is an experiment designed for the study of high-energy cosmic radiation and is the result of an international collaboration led by the Chinese Academy of Sciences and including INFN Italian National Institute for Nuclear Physics, the Gran Sasso Science Institute, and the Universities of Perugia, Bari, Geneva and Salento.
The detector consists of three main subsystems: a calorimeter made of bismuth germanate (BGO) crystals, which allows the energy of cosmic rays to be measured with excellent resolution; a subsystem for measuring the absolute value of the electric charge, placed at the top of the apparatus and built with two layers of plastic scintillators; a tracker composed of silicon strips and tungsten sheets, capable of reconstructing with precision the direction of arrival of cosmic rays. Although the mission was originally planned to last five years, the detector is still in excellent condition and promises to provide interesting data well beyond the ten years of activity just completed.
The video story of the discovery produced by the DAMPE collaboration.
