STUDIES WITH A WATER CERENKOV DETECTOR PROTOTYPE FOR THE PIERRE AUGERCOSMIC-RAY OBSERVATORY

We present here results obtained from a small scale water Cerenkov det ector prototype for the Auger Observatory [1]. We describe in detail t he experimental setup and data acquisition system employed. We operate d the detector continuously during four months (March-June 1997) to de monstrate that polyethylene is a suitable material to construct the ab out 3200 Auger detectors. Using the charge distribution of the photomu ltiplier (PMT) pulse produced by the Cerenkov signal from muons crossi ng the detector vertically, we inferred the decay length of the Cerenk ov light in the tank and a lower limit on the number of photoelectrons produced by the PMT. We found that the bacterial population in the ta nk decreased while the decay length and number of photoelectrons incre ased over the four-month period of monitoring these quantities. We the refore conclude that the initial water purity is not as critical an is sue as previously thought.