Knapik, Robert, authorHarton, John L., advisor2024-03-132024-03-132009https://hdl.handle.net/10217/237822The observed decrease in flux of cosmic rays as the energy increases can be described by power law with an almost constant spectral index for 12 decades of energy. Observing spectral index changes are used to constrain models for the sources of cosmic rays. The Pierre Auger Observatory was built to study the highest energy cosmic rays and combines two complementary techniques, a fluorescence detector and a surface detector. The surface detector is 100% efficient for energies above 3 EeV allowing for a flux measurement with low systematic uncertainties. This thesis describes the techniques developed to measure the flux of cosmic rays below 3 EeV while maintaining low uncertainties. The resulting energy spectrum confirms the previously measured change in spectral index observed by other experiments. Systematic differences in the measured energy spectra between experiments exist. Possible reasons for these differences and the astrophysical implications are discussed.born digitaldoctoral dissertationsengCopyright and other restrictions may apply. User is responsible for compliance with all applicable laws. For information about copyright law, please see https://libguides.colostate.edu/copyright.cosmic raysenergy spectrumPierre Auger ObservatoryastrophysicsTextPer the terms of a contractual agreement, all use of this item is limited to the non-commercial use of Colorado State University and its authorized users.