Stability and calibration of a water Cerenkov detector prototype

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