The Cosmic-Ray Energy Spectrum Observed With The Telescope Array Low-Energy Extension (TALE) Detector

Dr. Tereq AbuZayyad
Department of Physics and Astronomy
University of Utah

I will report on a cosmic-ray energy spectrum measurement by the TALE air fluorescence detector.  This measurement is the first by an air fluorescence detector to reach this low in energy (PeV energies).  The TALE FD is sensitive to both fluorescence light and to Cherenkov light produced by shower particles.  I will describe how we use Cherenkov light to measure the spectrum from a low energy of 2 ~ PeV to an energy greater than 100 ~ PeV.  Above 100 ~ PeV the detection of cosmic rays by their air fluorescence emission becomes more efficient and it allows for the extension of the measurement to energies greater than a few EeV.
The spectrum exhibits two features, the "second knee" in the lower part of the 1017 ~ eV decade, and a hardening in the lower part of the 1016 ~ eV decade. The energies of these features, for the first time, are tied to the energy scale of fluorescence detectors of ultra-high energy cosmic rays.  I will describe these features and a possible interpretation of the spectral features as they relate to the composition of the cosmic rays and the galactic-extragalactic transition.