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The Fly's Eye Observatory (1981-1993)
The Fly's Eye Detectors
A novel idea for the detection of large EAS was proposed in the 1960's by Kenneth Greisen of Cornell University, but it wasn't until the very late 1970's that the technology existed for a workable system. This was the beginning of operation of the Fly's Eye by the University of Utah, at a site in the western desert of that state.
The Fly's Eye observes the passage of EAS through the atmosphere via the excitation of atmospheric nitrogen by the charged particles in the shower. The fluorescence light is very weak and in the blue/near ultraviolet region of the spectrum (300-400nm). The original Fly's Eye (FE I) was completed in 1981 and consists of sixty seven 1.5m diameter mirrors which point in different directions and cover the entire night sky. At the focus of each mirror is a cluster of 12 or 14 sensitive light detectors called photomultipliers. A second eye (FE II) was added in 1986 with 36 mirrors at a distance of 3.4 km from FE I in order to view certain showers stereoscopically allowing high quality shower reconstruction. The two eyes are instrumented with a total of 1400 photomultipliers. Even though the nitrogen fluorescence light is weak the combination of the collecting power of the mirrors and the sensitivity of the photomultipliers means that the system can detect showers over an area exceeding 1000 square kilometers. The analogy sometimes brought up in this context is that a shower initiated by a high energy cosmic ray is as bright as a 5 Watt blue light bulb travelling at the speed of light down through the atmosphere. The Fly's Eye can detect this at distances exceeding 15km.
Results from Fly's Eye
The Fly's Eye technique measures the arrival direction of the cosmic ray, its energy and its chemical composition (whether it was a proton, or iron nucleus for example. The composition issue is not as straightforward as the arrival direction or energy, as there are some uncertainties in the particle physics interactions that occur when the cosmic ray strikes an atmospheric nucleus, given that we are at energies a million times higher than those studied at man-made accelerators). All three areas give clues to the origin of these particles. The distribution of arrival directions is the most obvious indicator of origin, but it also turns out that energy spectrum (the distribution of energies of measured cosmic rays) can be used to determine the history of the particles (e.g. how far they have travelled) and the conditions and acceleration mechanisms at their source. There is particular interest at energies above 1019eV where a feature is expected in the spectrum due to interactions in deep space between cosmic rays and the 3 Kelvin cosmic microwave background radiation, a remnant of the Big Bang. This feature would be expected for cosmic ray protons if they are extragalactic in origin (indeed they would need to travel a distance of 60 million light years for this effect to be noticed). The feature in the spectrum is known as the Greisen- Zatsepin cut-off, named after the two scientists (one American, the other Soviet) who first (independently) proposed its possible existence.
The highest energy particle ever observed was detected by the Fly's Eye in 1991. With an energy of 3.5 x 1020eV (or 56J), the particle, probably a proton or a light nucleus, had 108 times more energy than particles produced in the largest earth-bound accelerators. The origin of the particle is unknown. At such a high energy, and with its assumed charge, the path of this particle through the cosmos would have been relatively unaffected by galactic and intergalactic magnetic fields. Yet no plausible astrophysical source is known along the arrival direction, within the maximum possible source distance imposed by collisions with photons of the cosmic microwave background. This event remains a mystery! It is clear that it existed, but there is no obvious explanation for its source.
A study of the arrival directions has shown that cosmic rays appear to be remarkably isotropic - that is, there doesn't seem to be any preferred direction in space for their arrival direction - not even from the plane of our galaxy. This isotropy is not unexpected for the lower energy particles, those scrambled by the galactic magnetic fields, but it is rather remarkable for the cosmic rays above 1019eV. This may fit with the extragalactic hypothesis of the origin of the latter particles, given that there is an isotropic distribution of directions for other galaxies (if you look far enough!), but again one can't be definitive about this because of the lack of large numbers of events.