Does anybody know of a way to testing PMT's so degradation can be detected? What I have is a PMT, an LED, a scope and software to activate the PMT with the LED to read the gain with the scope. Under normal conditions the PMT's are used under X rays......of course, I can only test without the X rays.(I have to test without X rays to find a balance between those that are under the rays, so I can relate/compare to PMT's not under the exposure of x rays and prepare for breakdowns) Recently, I've pulled a PMT out of a machine as it was getting noisy. I've compared this to a new one and cannot find any differences in the output gain...actually if truth be told the new PMT was worse than the one I took out of service. Any suggestions, procedures or references would be greatly appreciated. Thanks guys.