We noted that the dark countrate of PMT north is about 20x higher than that of PMT middle. 

We went into the ESR cave to look for stray light. 
We decided to put a black cloth over all the PMTs. 
Also we made the tubes of the N2 airflow to the PMTs black (tape around). 
This did not make a significant change. 

We exchanged PMT north (same type 9423B) to see if that made a difference. 
It did not really change either. 

PMT north = 2650 V 
PMT middle = 2700 V 

The first picture below shows the dark rates for these two voltages. 

At about 17:00 we changed the voltage of PMT north to 2700 V. 
This did not change anything. 


Carsten swapped the cables of PMT north and PMt middle (inside ESR, before the amps) at 17:14.
The second picture below shows the result of that. 
Now black and red are exchanged. 


Carsten swapped the cables back again (17:20). 
Carsten and Kristian will look at the PMT signals in the ESR cave.