| The High Resolution Fly's Eye (HiRes) detector is an air fluorescent light detector for the study of ultra high energy (>1017 eV) cosmic rays. HiRes comprises two detectors operated in a stereo mode in the dry air of the desert near Dugway, Utah. Between 1993 and 1996 one site was operated as a prototype to study the flux and average mass composition of cosmic rays between 1017 and 1019 eV. In this thesis, I study the energy spectrum and the composition of cosmic rays in this energy region. For this analysis, detector properties like the aperture and the resolution are investigated with an extensive detector simulation. In addition, I study the question whether cosmic rays at the highest energies are gamma rays, as predicted by theories explaining cosmic rays as decay products of topological defects. Several methods are proposed to discriminate gamma showers from protons and heavier nuclei. An application on HiRes monocular data shows no significant result due to lack of statistics, but in the future, the full stereo HiRes detector will unambiguously discriminate between the particle types. |
