AMIGA, Auger Muons and Infill for the Ground Array

The Pierre Auger Observatory is planned to be upgraded so that the energy spectrum of cosmic rays can be studied down to 0.1 EeV and the muon component of showers can be determined. The former will lead to a spectrum measured by one technique from 0.1 EeV to beyond 100 EeV while the latter will aid identification of the primary particles. These enhancements consist of three high elevation telescopes (HEAT) and an infilled area having both surface detectors and underground muon counters (AMIGA). The surface array of the Auger Observatory will be enhanced over a 23.5 km2 area by 85 detector pairs laid out as a graded array of water-Cherenkov detectors and 30 m2 buried muon scintillator counters. The spacings in the array will be 433 and 750 m. The muon detectors will comprise highly segmented scintillators with optical fibres ending on multi-anode phototubes. The AMIGA complex will be centred 6.0 km away from the fluorescence detector installation at Coihueco and will be overlooked by the HEAT telescopes. We describe the design features of the AMIGA enhancement.