Cosmic Rays
'How do cosmic accelerators work and what are they accelerating?'
Cosmic rays are the constant shower penetrating the Earth's atmosphere, consisting of particles and photons. The story of the discovery of cosmic radiation goes back to the early twentieth century, shortly after the discovery of radiation. Theodor Wulf, a German Jesuit, built an electroscope that could accurately determine the intensity of radiation. He used the instrument to test a theory: the theory that natural background radiation comes from radioactive minerals in the Earth's crust. Measurements in caves in Limburg led to a surprise. Wulf discovered a significant decrease in the intensity. His conclusions were that the radiation could possibly come from above. Unfortunately Wulf's measurements at the top of the Eiffel Tower could not confirm this hypothesis. In 1911 a young Austrian physicist managed to do exactly this though. In a series of balloon flights, with a height of nearly 6 km, Victor Hess measured the radiation intensity with one of Wulf's electrometers. From about 4000 m it showed that the rise of the balloon increased the degree of ionization. He thus proved that the ionizing radiation was coming from above. Twenty-five years later Hess received the Nobel Prize for his findings.
Although almost one hundred years research has been done, the mystery surrounding the phenomenon of cosmic rays has only partially been lifted ...
Particle shower
- Schematic representation of the development of a particle avalanche ('cosmic ray air shower '). The cosmic particle is an iron nucleus.
In a collision of high energetic cosmic particles with a nucleus in the atmosphere, new particles are created. These new particles move in the same direction as the primary particle, and collide again to create new particles. This leads to a chain reaction in which a cascade of secondary and a large amount of derived particles. As the multiplicity is increased, a pancake which gradually increases in size is formed: the avalanche ('shower'). All particles that are coming down in such an shower are moving at near light speed.
The figure illustrates the emergence of a shower. Hadronic particles (such as protons and neutrons) interact and produce new hadronic particles, including pions. Some of the newly formed particles are unstable and spontaneously decays into lighter particles after some time. This creates a large number of muons from pion decay. Some of these muons decay into electrons. Photons and electrons / positrons can regenerate in an electromagnetic cascade.
As long as the energy of the particles is high enough to make new ones, the shower continues to grow by inelastic collisions.What remains are only elastic scattering processes and the number of primary particles in the atmosphere reduces.
Most of the energy of the primary particle is absorbed in the atmosphere. A small portion reaches the Earth's surface. These are mostly muons!