Cosmic Ray Sources Sought
SCIENTISTS have known since 1938 that the earth is bombarded by particle showers that have energies so high as to defy explanation. On Friday, 19 nations agreed to make a collective effort to find an answer.
The decision was taken at a meeting near San Rafael, Argentina, at which participants selected Millard County, Utah, as a site for observing high-energy cosmic rays hitting the Northern Hemisphere. Last year the consortium chose a site near Mendoza, Argentina, for study of Southern Hemisphere cosmic ray showers.
Cosmic rays consist of photons of gamma rays and of particles of various kinds, including entire atomic nuclei. The highest energy rays are produced by particles traveling at energies greater than 1020  electron-volts.
Dr. James W. Cronin of the University of Chicago, winner of a Nobel Prize in Physics and the main organizer of the two-hemisphere project, said that although particles with ultrahigh energies have been detected, their sources remain a mystery. 'The sources that we discover will have to be objects with unique properties,' he said. 'No known astrophysical objects have the properties required to produce cosmic rays of such high energies.'
When a particle of immense energy hits the earth's upper atmosphere, it is annihilated and its energy is transmitted to a cascade of secondary particles streaming earthward. These, in turn, collide with other molecules in the atmosphere to produce more cascades. Such showers reach the ground in the form of blue Cherenkov light, which can be detected by photoelectric detectors.
Each of the two new Auger observatories (named for the French physicist Pierre Auger, who discovered high-energy cosmic rays in 1938) is expected to cost $40 million, and the 150 institutions participating in the project hope to begin observing soon after the year 2000. Each site will be about the size of Rhode Island, within which huge water tanks spaced six-tenths of a mile apart will serve as detectors.
The enormous size of the detector arrays will make it possible to track cosmic ray particles accurately enough to pinpoint the direction from which they came.