Study on the detection efficiency and uniformity of a Water Cherenkov Detector based on Geant4

The Water Cherenkov Detector (WCD) is an important and widely used detector in cosmic ray physics, mainly used to measure the electromagnetic component of extensive air showers. Generally, a full detector simulation based on Geant4 for large arrays would require too much computing time and memory, because it would be a very CPU-intensive job to track all the Extensive Air Shower particles (mainly electrons and photons) hitting a very large area WCD detector. Therefore we developed a code simulating the behavior of muons, gamma-rays and electrons crossing a cell of a full WCD array. The results of this simulation can be collected in look-up table that can be used by fast simulation codes for large arrays. In this note we describe the results obtained simulating a single cell of cubic geometry (5×5×5 m3). The code we have developed for this particular geometry can easily be adapted to other detector designs. Firstly, the parameters in simulation are determined analysing the properties of the WCDs. Then, we calculate the most probable value (MPV) and sigma of the Landau function describing the distribution of the photoelectron numbers detected by the WCD photomultipliers. The results of the simulation are tested by a comparison with experimental results available in the literature: the relative error between their MPVs is only 1.37% for vertical muon- signal. We show that photoelectron distributions do not only depend on particle types and energies, but also on the incidence zenith angles and on the distance between the particle impact position and the center of the cell (i.e. the photomultiplier).