MEASUREMENT OF THE ENERGY SPECTRUM OF COSMIC RAYS ABOVE 3X10^17 EV USING THE INFILL ARRAY OF THE PIERRE AUGER OBSERVATORY

The Pierre Auger Observatory, in Argentina, combines a 3000 km^2 surface array of water Cherenkov detectors with fluorescence telescopes to measure extensive air showers initiated by ultra-high energy cosmic rays. This ``hybrid'' observatory (in operation since 2004, and completed in 2008) is fully efficient for cosmic rays energies above 10^18 eV, that is, from just below the ``ankle'' of the energy spectrum up to the highest energies. After the completion of the main observatory, the Auger collaboration has started to deploy new instruments to extend the energy range down to about 0.1 EeV. The planned extensions include two infill surface arrays with 750 and 433 m spacing, with muon detection capabilities, and three additional fluorescence telescopes with a more elevated field of view. The 750 m infill array (covering about 24 km^2) and the new telescopes are now operational. Their aim is the measurement of cosmic rays from below the second knee of the spectrum up to the ankle, where data from the extensions overlap those from the main observatory. The study of the evolution of the spectrum through the second knee and the ankle, together with the primary mass composition, are crucial to the understanding of the transition from a galactic cosmic ray origin to an extragalactic one. This thesis makes use of data from the 750 m infill array: the objective is the measurement of the cosmic ray energy spectrum in the energy region above 3x10^17 V, where the array is fully efficient. To get to the energy spectrum, several steps are needed, from the reconstruction of events, through the precise determination of the exposure of the array, up to the determination of the primary energy. The thesis deals with these aspects, before reaching the final result. The first chapter gives a general introduction to cosmic ray physics and detectors. It also summarizes experimental results above the first knee of the spectrum with particular emphasis on those obtained above 10^17 eV. The next two chapters describe the Pierre Auger Observatory and the infill array, respectively. In chapter 2, the main Auger results are summarized too, after a schematic description of the different components of the observatory. Chapter 3 sets the stage for the following chapters. It presents a more detailed description of the characteristics of the infill array, in particular the trigger definitions, event selection and reconstruction. In chapter 4 the performance of the reconstruction of the lateral distribution of observed showers is studied in detail. This is particularly important for the energy spectrum, since the signal at a fixed distance from the shower axis is used as the energy estimator of the event. This signal is estimated by means of the measured lateral distribution of the shower. Chapter 5 presents a comparison between the event reconstruction of the infill and main arrays. Using the set of showers detected by both instruments, the derived geometry and energy estimation are compared, showing a good agreement. In chapter 6, the energy threshold of the array, and hence the set of events to be used, is defined. The methods to obtain the exposure of the array are discussed, as well as related systematic uncertainties. Finally, in chapter 7, the technique to derive the primary energy for each detected shower is presented. The derived energy spectrum is discussed, and the flux is shown to be consistent with that measured by other instruments in the overlapping energy regions.