Cosmic rays with energies up to at least a few PeV are confined by the magnetic fields in galaxy and therefore, expected to be accelerated within it. While there is currently no direct evidence for it, diffusive shock acceleration in supernova remnants remains the prevailing theory that explains the origin of galactic cosmic rays. Strong magnetic fields close to the shock confine particles to the shock. The particles adiabatically cross the shock and gain energy, with some probability of escaping the acceleration region that is dependent on the ratio of momentum and charge (rigidity). The finite lifetime of supernova remnants implies that particles can only be accelerated to some maximum energy, expected to be ∼ 3Z × 10 ^15–17 eV, where Z is the charge of the particle.

Measuring the composition of cosmic rays accurately at high energies is a unique experimental problem, because flux of all cosmic rays falls steeply with energy. Experiments flown above the Earth's atmosphere achieve elemental and sometimes isotopic charge resolution, but become limited by statistics at the few TeV/amu regime, because of their limited collecting area. Ground-based telescopes can expand the collecting area by using the atmosphere as a calorimeter and estimating the charge from the air shower properties, but have limited charge resolution. By measuring the Cherenkov radiation of the primary particle, the direct Cherenkov method is shown here to measure the flux of cosmic rays with better than 25% charge resolution.

The TrICE experiment was designed to discover direct Cherenkov radiation, by exploiting the inherent timing and angular separation between the direct Cherenkov radiation and the Cherenkov radiation produced in the particle air shower. TrICE was capable of imaging high-resolution showers using a multi-anode photomultiplier camera with angular resolution of 0.086°. While DC light was not observed over the background in TrICE, VERITAS can select for DC events by using stereoscopic techniques. VERITAS achieves a charge resolution of 21.5% and an energy resolution of 16.5%. The flux of iron nuclei are measured from 22 TeV to 141 TeV, and can be described by a power law given by Φ = (5.8±0.84_stat±1.2 _sys) × 10^–7(E/50 TeV)[special characters omitted] TeV^–1 m^–2 s^–1 s^–1. The data agree well with direct measurements from satellite- and balloon-borne experiments, as well as the measurements made by H.E.S.S. using the same technique.