The Giant Airshower Detector Project

Cosmic Rays are now known to span the energy range from 10**9 to beyond 10**20 eV and to be, predominantly, the nuclei of atoms from hydrogen to iron. Above 10**14 eV the particles are so rare that their detection relies on measurements of the giant cascades or extensive air showers created in the atmosphere which may be observed with arrays of particle and optical detectors at ground level. The flux of particles falls inversely as the square of the energy until above 10**19 eV only about one per km**2 per year is collected. The origin of these particles is unknown and how Nature can accelerate them to energies, which represent the most significant departures from thermal equilibrium found in the Universe, is a major astrophysical puzzle.

There is suggestive evidence from a number of experiments that above 10**19 eV the cosmic rays are protons. If so, an origin within our own galaxy can be excluded as the protons would propagate rectilinearly and sources within our galaxy would be readily identified. By contrast the arrival direction distribution is isotropic, within the limited statistics, even beyond 10**20 eV where only a handful of events have been detected. This result is extremely paradoxical as sources at large distances (beyond about 30 Mpc) are strongly excluded by the existence of such events. The point is that above 4 X 10**19 eV protons and heavier nuclei interact with the primordial 2.7 K microwave background radiation through well-understood reactions of particle and nuclear physics and are rapidly degraded in energy.

The existence of the most energetic event yet detected, 3 X 10**20 eV, implies that it must have originated within 20 Mpc of the earth. However the arrival directions of this event and others of similar energy do not point to any unusually energetic objects within our galaxy or elsewhere and the lack of appropriate electromagnetic acceleration processes has even led to speculation that the particles are created in the collapse of massive cosmic strings which are topological relics of the early universe.

The results obtained so far derive from the independent efforts of groups in four different nations and are based on only a few hundred records above 10**19 eV. To increase this sample significantly requires an international effort in which all the nations historically involved, and others, are eager to participate. We propose to construct a device with an aperture of 5000 km**2 sr per year. Such an instrument would allow 5000 events to be recorded annually above 10**19 eV and, with the capability of accurate energy and mass determinations, provide data to confront conflicting ideas about the origin of the most energetic particles in Nature. Preliminary cost estimates indicate that this will be a $60 - $100 M project which requires a detailed design study to bring to the proposal stage.

Pages maintained by Sergio J. Sciutto

Back to home page: