Aging characterization on large area photo-multipliers

An accurate study and measurement about the aging effects on two large area photomultipliers (PMT) has been performed for over 3 years. The PMTs were 10 in., 10 stages Hamamatsu R7081, one with standard bialkali and the other one with super-bialkali photocathode. The gain, dark count rate, charge and timing properties have been measured, as well as the fraction of the spurious pulses. During the aging cycles, the anode current of the two photo-multipliers has been monitored and recorded in order to measure the total output anode charge and determine the aging grade. The aging conditions have been set by the use of a 400 nm LED regulated to about 3 photoelectrons (pe) at about 400 kHz. The aging process was stopped when the total charge arrived up to about 2000 C for both the PMTs. Measurements of the parameters of the two PMTs have been performed periodically using a 400 nm pulsed laser in single photoelectron condition. Considering the main results, only the gain showed a variation while all the other parameters remain quite stable. A first phase of up-drift shows an increase of the gain of about 10% and is followed by a final phase of down drift, which shows a faster diminution of the gain of about 30%. The mechanism of the gain drift has been modeled and compared with the results.