Observing muon decays in water Cherenkov detectors at the Pierre Auger   Observatory
  Muons decaying in the water volume of a Cherenkov detector of the Pierre Auger Observatory provide a useful calibration point at low energy. Using the digitized waveform continuously recorded by the electronics of each tank, we have devised a simple method to extract the charge spectrum of the Michel electrons, whose typical signal is about 1/8 of a crossing vertical muon. This procedure, moreover, allows continuous monitoring of the detector operation and of its water level. We have checked the procedure with high statistics on a test tank at the Observatory base and applied with success on the whole array. 
P. Allison, F. Arneodo, X. Bertou, N.G. Busca, P.L. Ghia, C. Medina,   G. Navarra, L. Nellen, H. Salazar Ibarguen, S. Rancho...