Abstract
To obtain direct measurements of the muon content of extensive air showers with energy above 1016.5 eV, the Pierre Auger Observatory is currently being equipped with an underground muon detector (UMD), consisting of 219 10 m2-modules, each segmented into 64 scintillators coupled to silicon photomultipliers (SiPMs). Direct access to the shower muon content allows for the study of both of the composition of primary cosmic rays and of high-energy hadronic interactions in the forward direction. As the muon density can vary between tens of muons per m2 close to the intersection of the shower axis with the ground to much less than one per m2 when far away, the necessary broad dynamic range is achieved by the simultaneous implementation of two acquisition modes in the read-out electronics: the binary mode, tuned to count single muons, and the ADC mode, suited to measure a high number of them. In this work, we present the end-to-end calibration of the muon detector modules: first, the SiPMs are calibrated by means of the binary channel, and then, the ADC channel is calibrated using atmospheric muons, detected in parallel to the shower data acquisition. The laboratory and field measurements performed to develop the implementation of the full calibration chain of both binary and ADC channels are presented and discussed. The calibration procedure is reliable to work with the high amount of channels in the UMD, which will be operated continuously, in changing environmental conditions, for several years.
Highlights
A key characteristic of the scintillator-based detector is its high segmentation: each module is divided into 64 segments, coupled to a set of 64 SiPMs
The aim of the UMD is the direct measurement of the muon content in air showers with energy above 1016.5 eV
The ultimate scope of the calibration is the conversion from raw signals to the number of muons
Summary
The UMD electronics consist, basically, of: (i) an array of SiPM with its read-out system, (ii) a Field-Programmable Gate Array (FPGA) that houses the acquisition logic and a soft-core for. The binary mode benefits from the detector segmentation: muons can be directly counted as pulses above a certain threshold This mode is very robust since it neither relies on deconvoluting the total number of particles from a single integrated-signal, nor on the precise optical device gain or its fluctuations, and is almost completely independent of the hitting position of the particle on the scintillator strip and the corresponding light attenuation through the fiber. We show the mean time shift calculated with data taken with a module deployed in the field (red square), as an example In this case, as it is not possible to identify the position on the strip where the muons impinge on the scintillator, we assume the average optical-fiber length corresponding to the center of the module. The selected fiber for the UMD is the Saint-Gobain BCF-99-29AMC
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