A High Resolution Method for Measuring Cosmic Ray Composition beyond 10 TeV

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. Previous direct measurements of composition have been limited by experiment collecting power, resulting in marginal statistics above 10 eV, precisely the region where the “knee” of the cosmic-ray energy spectrum is starting to develop. In contrast, indirect measurements using extensive air showers can produce sufficient statistics in this region but generate elemental measurements which have relatively large uncertainties. Here we discuss a technique which has become possible through the use of modern ground-based Čerenkov imaging detectors. We combine a measurement of the Čerenkov 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 hadronic interaction can be separated by an imaging Čerenkov system with sufficient angular and temporal resolution. Monte Carlo simulations indicate an expected charge resolution of ∆Z/Z < 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 10 eV. The identification and rejection of background produced by charged particles in ground based gamma-ray telescopes is also discussed.