Ultra-high Energy Cosmic Rays: From GeV to ZeV.

Cosmic ray (CR) particles arrive at the top of the Earth's atmosphere at a rate of around 103 per square meter per second. They are mostly ionized nuclei - about 90% protons, 9% alpha particles traces of heavier nuclei and approximately 1% electrons. CRs are characterized by their high energies: most cosmic rays are relativistic, having kinetic energies comparable to or somewhat greater than their rest masses. A very few of them have ultrarelativistic energies extending beyond 1020 eV (tens of joules). [ABSTRACT FROM AUTHOR]