Background radiation measurement with water Cherenkov detectors

Abstract

Water Cherenkov Detectors have the nice property of being mostly calorimeters for cosmic ray induced electrons and photons, while providing a clear signal for muons. At large energy deposited in the detector, they observe small extended air showers. This makes them interesting detectors to study the background of cosmic ray secondaries. Using low threshold scaler counters, one can follow the flux of cosmic rays on top of the atmosphere, and/or study atmospheric effects on the cosmic ray shower development. In this paper, background data from the Pierre Auger Observatory are presented. These data are searched for short time-scale variation (one second scale, as expected from Gamma Ray Bursts), and larger time-scale variations, showing modulation effects due to Solar activity (Forbush decreases). Rapid changes in the background flux are also observed during the crossing of storms over the 3000 km2 of the ground array.