High resolution charge measurements of UH cosmic ray nuclei using a direct imaging Cherenkov ground-based observatory

The accurate determination of the elemental composition of cosmic rays at high energies is expected to provide crucial clues on the origin of these particles. Here we discuss a technique that has become possible through the use of modern ground-based Cherenkov imaging detectors. We combine a measurement of the Cherenkov light produced by the incoming cosmic-ray nucleus in the upper atmosphere with an estimate of the total nucleus energy produced by the extensive air shower initiated when the particle interacts deeper in the atmosphere. The emission regions prior to and after the first nuclear interaction can be separated by an imaging Cherenkov system with sufficient angular and temporal resolution. Monte Carlo simulations indicate a widely space array of 10m diameter imaging Cherenkov detectors should have charge resolution of AZIZ <5% for incident iron nuclei in the region of the "knee" of the cosmic-ray energy spectrum. This technique also has the intriguing possibility to unambiguously discover nuclei heavier than iron at energies above 1014 eV. We describe a strawman detector design for a future observatory dedicated to high resolution cosmic ray measurements. This observatory can also serve as a wide field of view TeV gamma-ray survey instrument.