A photomultiplier tube model for the water Cherenkov detectors of LAGO

The Latin American Giant Observatory (LAGO) is an international experiment covering 10 Latin American countries and Spain. LAGOresearches gamma-ray bursts and spaceweather phenomena using water Cherenkov detectors (WCDs) deployed at different altitudes. Large-area (8-inches) photomultipliers sense Cherenkov radiation produced by secondary particles crossing the WCDs. We present a generic photomultiplier model applied to the Hamamatsu R5912 tube, used in most of LAGO'WCDs. The model depends on the number of dynodes, the bias voltage, the number of incident photons, the photodetection efficiency, and the bias network. Besides, the implementation of the model includes a simulation of the front-end of LAGO's acquisition electronics, allowing the linearity of the system to be evaluated under different conditions. The model was validated with data recorded by the MuTe-Chitaga (Bucaramanga, Colombia) and Nahuelito (Bariloche, Argentina) WCDs. Geant4 simulations estimate the number of Cherenkov photons arriving at the photomultiplier. We contrasted the anode/dynode pulse amplitude ratio between the data and the model prediction. We also compared the estimated and measured vertical equivalent muon charge (pulse area). The vertical-muon-charge estimated by the model (321.6 UADC) differs by 4% from the measured by the MuTe WCD (333 UADC).