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High Resolution Fly's Eye (HiRes) Project
The High Resolution Fly's Eye (HiRes) is an international collaboration formed with the aim of discovering the sources and likely acceleration mechanisms for the highest energy cosmic rays, the most energetic particles known in the Universe. These particles must be produced in extreme environments but so far no theory has been able to convincingly explain where, or how, acceleration takes place. Our group at the University of Adelaide is a long-standing member of the collaboration. HiRes began operation in its final configuration in 2000. It is a successor to the pioneering Fly's Eye observatory.
The US$15M HiRes project follows on from the successful Fly's Eye observatory. Adelaide physicists have been associated with the Fly's Eye and HiRes experiments for more than 15 years. HiRes is situated on the same site as the original Fly's Eye, in the western desert of Utah, USA. Together with a collaboration of approximately 25 physicists from the University of Utah, Columbia University, the University of New Mexico, Rutgers University and Montana State University, we have built an operational observatory with unprecedented sensitivity and resolution. Every clear, moonless night the atmosphere over a 3000 square kilometre ground area is monitored from two sites. The passage of a cosmic ray shower causes the atmosphere (actually the nitrogen) to fluoresce, and the faint near-UV light is detected by arrays of large mirrors equipped with fast photomultiplier cameras.
From this information we reconstruct the arrival direction and energy of the original cosmic ray, and we can estimate the likely mass of the particle. With this enormous aperture, HiRes collects approximately 300 events per year with energies above 1019 eV and many more events below this energy. HiRes is a mature, operating observatory with current physics output. Several papers have already been published from the prototype phase of the experiment, including new measurements of the cosmic ray mass composition in the energy range 1017-1018 eV and the energy spectrum of the highest energy particles.
Charged particles in air showers excite fluorescence emission from atmospheric nitrogen molecules. HiRes detectors are designed to observe the band emission between 300 and 400 nm. The emission is rather feeble, with approximately 4 photons being emitted per metre of a charged particle track, but the large number of particles in giant air showers makes the technique feasible for cosmic ray energies above approximately 1017 eV. The fluorescence light is isotropically emitted around the shower axis, in contrast to the very intense Cherenkov light which is beamed in the direction of shower motion. Thus it is possible to observe fluorescence light from air showers landing many kilometres from the detector. HiRes can view a 1020 eV shower, containing 1011 particles at its maximum size, out to distances of 30 km.
Fluorescence detectors employ large spherical mirrors to collect the light and focus it onto photomultiplier cameras. For HiRes we have constructed two sites, HiRes 1 and HiRes 2, which sit on two small mountains in western Utah, with a separation of 13 km. HiRes 1 has 21 three metre diameter mirrors which are arranged to view the sky between elevations of 3 and 16 degrees over the full azimuth range; HiRes 2 has 42 mirrors which image the sky between elevations of 3 and 30 degrees over 360 degrees of azimuth. At the focus of each mirror is a camera composed of 256 40-mm diameter hexagonal photomultiplier tubes, each tube viewing a 1 degree diameter section of the sky.