Preliminary results of the AMIGA engineering array at the Pierre Auger Observatory

Abstract: The Auger Muons and Infill for the Ground Array (AMIGA) aims to both extend the detection range of the Pierre Auger Observatory down to energies $\sim 10^{16.5}~\mathrm{eV}$ and to measure the muon content of extensive air showers. To accomplish these goals, its detection system is composed of an array of coupled water-Cherenkov and scintillation detectors deployed in a graded triangular grid of 433 and 750\,m spacings. At each position, the scintillation detector is buried $2.3~\mathrm{m}$ deep so as to shield it from the air shower electromagnetic component and thus only measure the muon component. These muon detectors have $30~\mathrm{m^2}$ area split into modules, each of them highly segmented in 64 plastic-scintillator strips with an embedded wavelength-shifter optical fiber to transport light to an optical sensor located at the center of the module. During the engineering array phase (finished in November 2017) two module areas ($5~\mathrm{m^2}$ and $10~\mathrm{m^2}$) and two optical sensors (photo-multiplier tubes and silicon-photomultipliers) were tested. In this work, we present the final performance of the muon detectors equipped with silicon-photomultipliers which were thereafter selected as the baseline design for the AMIGA production phase. Analyses and results are based both on laboratory and field measurements.