I-V testing procedure for PMTs
Hans-Gerd Berns, UWa, 4/22/96

1) Use the home-made adaptor cable to hook up the high-voltage supply to
   the the desired PMT cable.
   Be sure to have the HV supply switched off, or at least its outputs 
   disabled!  Otherwise you might experience a nice high voltage shock.  
   Although the LeCroy HV supply has a built-in safety switch-off feature 
   that prevents fatal HV accidents, but the pain can still be pretty 
   decent!...

   high voltage                 resistor
   input              hot        1.2M
         HV BNC    @--------o---/\/\/\---o---------0 (+)
         connector |        |            |               
                   |       --- cap.     --- cap.   
                   |       --- 1nF      --- .01uF   PMT cable
                   |        |            |          connector  
                   |        |            >  res.    to PMT
                   |        |            >  300
                   |        |            >  Ohms
                   | shield |            |
                   ---------o------------o---------0 (-)

2) Check if the connectors and cables are isolated from the rest of the 
   environment.

3) Switch on the HV supply, or enable the HV output.

4) Set the HV output initially to 100V, then read the current value and 
   make a note of it.
   If the HV supply trips off (overcurrent) at this voltage level 
   then the PMT is shorted for some reasons, e.g. flooded, etc.
   Maybe try again with a lower initial voltage, e.g. 20V, or stop
   here, i.e. skip the rest, switch off the supply and disconnect the cable.

5) If the initial reading seems to be ok, continue with incremental 100V 
   steps until reaching 1000V or the operation voltage of the tube (tube 
   dependent, 1600-2500V).

6) As an option, keep the last voltage for a minute or so and see whether
   the current is fluctuating or constant.
   Then decrease the voltage with 100V steps until reaching 0V again, making
   notes of the voltage/current values again.

7) Switch off or disable the HV supply again.  Disconnect the PMT cable.

8) In the analysis of the V-I curve(s), you should see a linear behavior
   with the incremental and decremental curve fully overlapping.
   
   A typical resistance value of a "healthy" SuperK PMT is:

       delta Volt
   R = ------------- = approx. 25-26 MOhms.
       delta Current

9) As an additional option, check the capacitance of the PMT with a DVM
   with C measurement capabilities.  With the 180' coax cable, a "healthy" 
   SuperK PMT should have approx. 24-26nF.


PS:  The measured capacitances and resistances of "healthy" SuperK PMTs
come mostly from the long 180' coax cable (4.7nF) and the dynode voltage
divider on the PMT sockets (~26 MOhm / ~20 nF).  I sketched a schematics
from a leftover PMT socket and saved it as a postscript file here
for better understanding of these V-I curves.

- Hans