Publication in the Physical Review Letters:
Cosmic Radiation Knees Understood
Cosmic particles can cause so-called particle showers in the atmosphere, which can be measured in ground-based experiments such as KASCADE-Grande at the KIT. The data allow to derive mass, energy and direction of origin of the primary particles. With increasing energy, the particle flow decreases. Yet this energy spectrum displays turning points at which the increase changes.
An international cooperation of astrophysicists working at the KASCADE-Grande experiment now for the first time were able to measure the energy spectrum and the elements composing the cosmic particles up to 1018 electron volt and thus provide an explanation for these turning points or knees. "The first "knee" in the spectrum is located at approximately 4*1015 electron volt, because at higher energies the hydrogen atoms simply are missing", explains Dr Andreas Haungs from the KIT, who heads the KASCADE-Grande experiment. At low energies, hydrogen is the most frequently occurring element, also in cosmic radiation, so that the particle flow above 1015 eV displays a significant turning point or bend.
At an even higher level of energy, heavier elements successively disappear from the cosmic radiation spectrum. The next very obvious bend is at 1017 eV, an energy level exactly corresponding to 26 times the hydrogen bend. "This is where the atomic nuclei of iron disappear", explains Haungs. Iron atoms likewise are a frequently occurring end product of fusion processes inside a star, yet because of their 26 times higher nuclear charge they can also be accelerated more by the electromagnetic fields of the cosmic accelerators, that is, exactly to a 26 times higher energy level than hydrogen atoms.
"KASCADE-Grande thus has proven that from our Milky Way only particles with energy levels up to about 1017 electron volt reach Earth." Particles arriving at Earth with an even higher energy level therefore must have been created outside our galaxy, for example in the black holes of other galaxies. Measurements at the Pierre Auger Observatory in Argentina, in the building and evaluation of data of which the KIT likewise is involved, are to provide insights into such highly energetic particles from alien galaxies over the course of the next few years.
The KASCADE-Grande project now definitely will be terminated by the end of this year. It has been operated by an international collaboration including scientists from Mexico, Italy, Poland, Rumania, Brazil and the Netherlands.