Study of the performances of the AMIGA muon detectors of the Pierre Auger Observatory

The evolution of the spectrum through the second knee and ankle, and the corresponding predicted changes in composition, are crucial to the understanding of the end of Galactic confinement and the effects of propagation on the lower energy portion of the extragalactic flux. The latter is strongly related to the cosmological distribution of sources and to the composition of the injected spectrum. To study this energy region, the Pierre Auger Collaboration has extended the energy range of its Observatory in Argentina from 3 × 10^18 down to 10^17 eV by means of new detectors including three additional fluorescence telescopes with a more elevated field of view (HEAT) and a nested surface array with 750 m spacing and additional muon detection capabilities (AMIGA). With these enhancements, the Pierre Auger Observatory will provide full efficiency detection of cosmic rays down to 10^17 eV, a direct measurement of the muonic component of the extensive air showers (EAS), and the possibility to study the transition between the galactic and the extragalactic component of high energy cosmic rays. The number of muons in extensive air showers reflects the composition of the primary cosmic rays which interact at the top of the atmosphere; together with the optical detection of the depth of maximum development of the showers it can be used to disentangle the composition information from the different hadronic models scenarios. The muon detectors of AMIGA allow to perform a direct measure of the muonic component of the showers, exploiting the fact that muons lose a small fraction of their energy in the atmosphere and can penetrate several meters underground, while the lower energy electromagnetic component is on the contrary almost completely absorbed. The main aim of this work is that of evaluating the performances of the muon counters of AMIGA. They have been studied during the costruction phase of six modules in the mechanical workshop of INFN-Torino (Italy) and by developing a simulation code to study the propagation of the particles through air and soil down to the scintillator detectors, in order to evaluate their response.