Studies of the optical properties of new hemispherical photomultiplier tubes

The main optical and electrical properties of the new, fast, and low-gain s ix-stage hemispherical photomultiplier tubes (PMTs) 9116A and 9117A from El ectron Tubes Inc. have been studied in detail for their use in the air Cher enkov telescope MAGIC, a 17-m telescope to be installed in 2001 on the Cana ry Island of La Palma. Special emphasis is put on the discussion of the det ection efficiency and the factors contributing to its absolute value and it s uniformity over the photocathode. In particular, the photoelectron collec tion efficiency and its dependence on the voltage between the photocathode and first dynode have been measured. Also, the influence of the first dynod e multiplication uniformity on the overall uniformity of the PMT response i s discussed. The hemispherical cathode together with a specially shaped light concentrat or allows increase of the effective quantum efficiency. This improvement ha s been measured as a function of wavelength. Aging and afterpulsing rate st udies have been carried out at a PMT gain of 2 x 10(4).