Successful implementation of fast preamplifiers in a positron lifetime spectrometer

A method to improve the long-term stability of fast coincidence apparatuses, like a positron lifetime spectrometer, is reported. Ageing of the photomultiplier tubes (PMT), i.e., nonreversible degradation of the gain, can be slowed down by lowering the supply voltages over the PMTs and by compensating the lower gain with fast preamplifiers set at the anodes. The timing characteristics of the PMTs can be preserved by using voltage dividers with which the voltage in the input optics of the PMTs remains high enough for good photoelectron collection efficiency (for XP2020 above 300 V). With this setup, the anode current and the rate of gain degradation can be reduced at least by a factor of 20 with no loss in the time resolution of the spectrometer.