Geomagnetic effects on the PMT tubes of the Pierre Auger surface detectors

In this paper we report the geomagnetic field effect on three photomultiplier tube (PMT) candidates for their massive use in the Pierre Auger Observatory surface detector array. The geomagnetic field effect could be manifest in the variation of the response of the PMT after this has been rotated with respect to its own axis on top of the surface detector. We measured the signal given by secondary cosmic rays and by a LED installed in the interior of the tank with one PMT. Data were digitized using a digital oscilloscope and an ADC-CAMAC module. We found no significant variation of the PMT response when the dynode axis direction changes with respect to the geomagnetic field orientation