Studies with a water Cerenkov detector prototype for the Pierre Auger cosmic ray observatory

Abstract

We present here results obtained from a small scale water Cerenkov detector prototype for the Auger Observatory [1]. We describe in detail the experimental setup and data acquisition system employed. We operated the detector continuously during four months (March-June 1997) to demonstrate that polyethylene is a suitable material to construct the about 3200 Auger detectors. Using the charge distribution of the photomultiplier (PMT) pulse produced by the Cerenkov signal from muons crossing the detector vertically, we inferred the decay length of the Cerenkov 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 tank decreased while the decay length and number of photoelectrons increased over the four-month period of monitoring these quantities. We therefore conclude that the initial water purity is not as critical an issue as previously thought.