The AMIGA Muon Detector of the Pierre Auger Observatory

B. WUNDHEILER

Encuentro; Pierre Auger Internal GAP Note Nº 2014-067; 2014

Cosmic rays hit the Earth with an energy spectrum covering several orders of magnitude. Its flow follows a power law, ranging from a few hundred hits per m 2 per second at low energies (E ∼ 10^9 eV) to a few per km2 per century at extreme energies (E ∼ 60 EeV). While from E ∼ 10^11 eV the flow decays with an spectral index close to -3, at highest energies the spectrum has areas where variations of this behavior have been measured: the "knee" (∼ 4 × 10^15 eV), the still vague ?second knee? (0,05 < E < 0,5 EeV), the "ankle" (∼ 3 EeV), and the "GZK cut" (∼ 40 EeV). It is assumed that up to energies of about 10 15 eV, cosmic-ray sources are of galactic origin. Galactic accelerators theoretically become inefficient between 10^15 and 10^18 eV. It is believed that extragalactic sources begin to contribute to the flow in this region of the spectrum and its contribution could produce changes in it. Composition studies in the range of the second knee and the ankle are vital to understanding the transition from galactic to extragalactic sources. The physical parameters most relevant to composition studies are longitudinal profiles and the number of muons present in cosmic showers. The Pierre Auger Observatory was originally designed to observe cosmic rays above 10 18 eV, has the ability to record data in hybrid form, using surface detectors and fluorescence simultaneously. After completion in 2008, a second phase began with the commissioning of HEAT (High Elevation Auger Telescopes), and the construction of AMIGA (Auger Muons and Infill for the Ground Array), among others. These developments point to both improve the quality of the observations, as to extend the energy range of detection to include the region of the ankle and the second knee. The AMIGA design consists of detector pairs formed by a surface detector, as those installed in the Auger Observatory, plus a 30 m 2 muon counter buried in its vicinity at 2,25 m deep. It will have a total of 85 pairs of detectors distributed in two triangular arrays, separated by 433 and 750 m. This work focuses on the AMIGA Muon Detector. Its main purpose is to provide information about the muonic content of secondary-particle cascades that follow the impact of the primary particle in the atmosphere. A detailed study of the Detector design and its experimental characterization will be presented. The first data set from the Observatory will be analyzed, as the first cosmic rays showers recorded jointly by the Fluorescence, Surface and Muon Detectors. A phenomenological model that enables a full simulation of the Muon Detector will be presented. Counting strategies that allow the Detector to be used as a Muon Counter will developed and analyzed.