Page "Cosmic ray" Paragraph 73

Measurements of the energy and arrival directions of the ultra-high energy primary cosmic rays by the techniques of " density sampling " and " fast timing " of extensive air showers were first carried out in 1954 by members of the Rossi Cosmic Ray Group at the Massachusetts Institute of Technology Clark et al., Phys.

Rev., 122: 637 ( 1961 ).

The experiment employed eleven scintillation detectors arranged within a circle 460 meters in diameter on the grounds of the Agassiz Station of the Harvard College Observatory.

From that work, and from many other experiments carried out all over the world, the energy spectrum of the primary cosmic rays is now known to extend beyond 10 < sup > 20 </ sup > eV.

A huge air shower experiment called the Auger Project is currently operated at a site on the pampas of Argentina by an international consortium of physicists, led by James Cronin, 1980 Nobel Prize in Physics of the University of Chicago and Alan Watson of the University of Leeds.

Their aim is to explore the properties and arrival directions of the very highest energy primary cosmic rays.

The results are expected to have important implications for particle physics and cosmology, due to a theoretical Greisen – Zatsepin – Kuzmin limit to the energies of cosmic rays from long distances ( about 160 million light years ) which occurs above 10 < sup > 20 </ sup > eV because of interactions with the remnant photons from the big bang origin of the universe.