I have replaced the PMT south (looks at the gas target). 
in the hope that this will produce less dark counts. 
Thus far, the dark countrate is low (few Hz). 

brand: ET enterprises 
type:  2" photomultiplier, 9423B 
High voltage: -2.45 kV  (up to -3.5 kV) 
spectral range: 110 - 230 nm 

The power comes from an additional 2.5 kV supply. 
(I have changed the cable: was CH0 before)