Stability and calibration of a water Cerenkov detector prototype

We present results of studies made with a reduced-scale water Cerenkov detector (WCD) prototype for the Pierre Auger Observatory. This detector is made of high-density polyethylene and was operated continuously for four months. We studied time variations in the amplitude and shape of Cerenkov pulses due to cosmic ray muons and studied the correlation of these parameters with the bacterial population of the water. We also developed and tested a new technique to calibrate and monitor WCDs remotely, based on muons stopping and decaying inside the tank. We conclude that high-density polyethylene tanks fulfill the requirements for use in the water Cerenkov detectors of the Pierre Auger Observatory