I have had some PMTs keep moderately noisey outputs due to a number of reasons for a week after being sealed up!! It is important to give them a settledown time of a few days before doing any important low level work. There are many issues which can influence noise in these tubes. I once had to move a Gamma source I had stored in the lab to an out building to quite the thing down. It was not messing with the scintillator as much as doing a compton scattering number in the tube elements themselves!
Richard Hull
Created on Monday, June 18, 2001 10:19 AM EDT by Richard Hull