PMT performance can degrade over time. A friend asked mine recently ask for suggestions on how to check PMT performance. In a prior post, I mentioned that the Hamamatsu PMT Handbook talks a bit about the proper way to make calibrated measurements of PMT sensitivity.
If you don’t need calibrated numbers, and just want to compare PMTs to each other and over time with an inexpensive solution, then a simple approach can work:
I was just in CSHL to give a lecture for the imaging course. There, Michael Orger was putting together a black box with an LED and a PMT in it to teach the students about PMT gain and noise. Something like that might be handy for measuring PMT performance over time.
1. A light-tight box with a mount for the PMT.
2. An LED (with a ton of optical attenuation– using ND filters, for example)
3. A power meter to make sure that the LED is stable over time (this is your secondary standard, measure the LED at the same supply power, but without the optical attenuation used for PMT measurements)
You could have two compartments in the black box. Compartment 1 has the LED and the power meter sensor. Compartment 2 has the PMT. In between the two compartments is an almost opaque wall, which just lets a small flux of light through. This way, you can monitor the LED output with the power meter while making PMT measurements– to make sure there is no drift in LED brightness during measurements (as well as between measurement sessions). I’d want to do measurements at several different PMT voltages (ie, gain settings), to see if it changes.