Continued Study of the Time Stability of a Small Water Cerenkov Detector

This note describes continued studies at Fermilab of the long term stability of a small water Cerenkov tank. The water tank, bag liner, water, photomultiplier tube, and data acquisition software were unchanged for the additional measurements from June to November, 1999. However, some details of the geometry of the trigger counters relative to the tank and of the electronics may have differed. The setup for the 1999 results is described in this note. The same analog-to-digital converter (ADC) was used for both time periods. Its pedestal was quite stable during the 1997-1998 measurements, but sizeable changes were observed in the more recent runs. As a result, dedicated pedestal runs were performed, and a number of additional tests were conducted. A schematic of the experimental setup is shown in Fig. 1. A pair of scintillation counters above and a second pair of scintillators below the tank were used to initiate the readout of the integrated signal from the photomultiplier in the water Cerenkov tank. The goal was to use cosmic ray muons that triggered all four scintillators and that passed approximately vertically through the water in the tank. These muons also passed through lead surrounding the lower pair of counters, so that their energy loss was approximately minimum ionizing. The path of the cosmic ray muons was chosen to be offset from the center of the tank so that most of them would not pass through the photomultiplier, where they might produce Cerenkov light in the glass face of the tube.