| I present a detailed investigation of the composition of cosmic rays with energies greater than 1 x 1019 eV with the Pierre Auger Observatory. Initial studies were found to indicate that a model of pure proton primaries was inconsistent with the data collected. A detailed method is developed to use observables from the surface detector as proxies for the depth of shower maximum---the depth in g cm-2 where the number of electrons and positrons is greatest. The depth of shower maximum (known as Xmax) is closely associated with the mass of the primary cosmic ray. The estimates from the surface detector are compared, using the hybrid technique, to the subset of showers containing direct measurements of Xmax by the fluorescence detector. Extending the surface detector measurements to the highest energies shows that primary cosmic rays are trending away from the expectation for a pure proton stream toward an increasingly heavy mixture of elements. |
