The Fly's Eye detector - a cosmic record-breaker
11 October 2000
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The night of 15 October, 1991, was dark and clear in the West Desert of Utah - perfect for the Fly's Eye detector, which on that night observed the effects of the highest energy cosmic ray so far seen. The Fly's Eye detected fluorescence from nitrogen in the Earth's atmosphere, caused by the shower of charged particles produced when the cosmic ray - a high-energy particle from outer space - slammed into the atmosphere. The energy of the primary particle is estimated to have been about 320 EeV (1 EeV = 1018 electronvolts, or eV). This is about 50 Joules in energy - more than the kinetic energy of a tennis ball served by a top player.
This image shows an aerial view of the original Fly's Eye detector, on top of Little granite Mountain. The cylindrical structures seen more clearly in the larger view contained mirrors nearly 1.6 m in diameter, which reflected light to an array of phototubes. Together the 67 mirrors formed a "fly's eye view" of the sky. The detector ran from 1981 to 1993, being joined by a second array, 3.4 km away, in 1986. The two "eyes" provided a stereo view of the cosmic-ray showers. The Fly's Eye has now been superceded by the HiRes detector, which continues the study of ultra-high energy cosmic rays, the origin of which remains a mystery.
Credit: University of Utah Physics Department