FNWI --- IMAPP Department of Astrophysics
Radboud University > Faculty of Science > Department of Astrophysics


Cosmic rays are charged nuclei of atoms, that travel near to the speed of light through the universe with tremendous energies. When we look at the cosmic rays detected so far, the question is, up till what energy can cosmic rays be found. At the highest energies (above 1020 eV) there are two different types of sources. The first option is that cosmic rays are accelerated to these energies, the question is then still: by what object. The other option is that a very heavy unknown type of particle can decay into a cosmic ray at these energies. These unknown types of particles are used in so-called supersymmetric theories or in theories to explain dark matter. Detection of cosmic rays at these energies can confirm or disprove such theories.

The Greisen-Zatsepin-Kuzmin effect

There is an effect that hinders us from seeing cosmic rays at these high energies. At these high energies the particles can react with photons from the Cosmic Microwave Background and decay in particles with less energies. This is known as the Greisen-Zatsepin-Kuzmin effect. Particles above 5 x 1019 eV can only travel a few tens of Mpc before they lose their energy in this way, and therefore only come from the few galaxies around us, which reduces the change of detection. However, there is a way out. When this reaction occurs a neutrino is created with a similar energy. They can travel a very large distance through the universe and thus still can be detected. Therefore our research aims to detect both the neutrinos and the cosmic rays at these energies.

Detection of Ultra-High Energy Cosmic Rays and Neutrinos

To detect cosmic rays and neutrinos at the highest energies (above 1020 eV) a very large detector is needed as there is less than one particle per square kilometer per century and for ten times more energy this is even 100-1000 times less. This means either many square kilometers or many centuries are needed to detect these particles. One very large object that can be used as a detector is the moon. When an Ultra High Energy neutrino or cosmic ray impinges on the moon, it creates a shower of secondary particles. Because this shower kicks out electrons from the moon a net charge builds up. The acceleration and deceleration of these electrons give rise to radiation. This is known as the Askaryan effect. Our aim is to detect these nanosecond pulses on earth with the LOFAR radio telescope.

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