We have developed an accurate simulation model of the large 9 inchphotomultiplier tubes (PMT) used in water-Cherenkov detectors of cosmic-rayinduced extensive air-showers. This work was carried out as part of thedevelopment of the Offline simulation software for the Pierre Auger Observatorysurface array, but our findings may be relevant also for other astrophysicsexperiments that employ similar large PMTs. The implementation is realistic in terms of geometrical dimensions, opticalprocesses at various surfaces, thin-film treatment of the photocathode, andphoton reflections on the inner structure of the PMT. With the quantumefficiency obtained for this advanced model we have calibrated a much simplerand a more rudimentary model of the PMT which is more practical for massivesimulation productions. We show that the quantum efficiency declared bymanufactures of the PMTs is usually determined under conditions substantiallydifferent from those relevant for the particular experiment and thus requirescareful (re)interpretation when applied to the experimental data or when usedin simulations. In principle, the effective quantum efficiency could varydepending on the optical characteristics of individual events.
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