We tested the PMTs again, this time with an UV LED that is mounted at a window close to the XUV detector.
PMT south showed a very clear increase in count rate from about 1 to 50 Hz.
PMT mid also showed a small increase by a few Hz.
For PMT north there was no great change between LED off and on, but we think that the count rate also very slightly increased. Note, that PMT north is also the furthest away from the LED.

Also we saw a difference between single- and multi-photon events at PMT south.