# 05/10/2011

continue...the same set up as before, the distances are the same:

First I change the power supply for HAMAMATSU PMT to the new 2 channel one C.A.E.N N471

The powers are:

PS1 and PS2:  -1000 V (the old 2-channel TENNELEC TC952)
n's: -1100 V (the new 2-channel C.A.E.N N471)


The thresholds are:

PS1 and PS2:  -80 mV
n's: -60 mV


Let see what we have. First he CF-243 source is far far away (sealed in the safe):

counts/min (1 min test) counts/min (1 min test) counts/min (5 min test) counts/min (5 min test) avg counts/min
PS1 187 160 166 160.8 168.4
PS2 187 170 180.4 184.4 180.4
n's 205 204 201.2 202.4 203.2

Interesting. The same power for HAMAMATSU PMT, but different counts result (see my test on 05/09/2011). At this point I do not really know what the voltage I am supplying. Assume the new power supply C.A.E.N N471 is the good one.

Now I increased the power supply for HAMAMATSU PMT (-1500 V as specified in the data sheet [1]) and adjusted the threshold (-500 mV). All other parameters are the same.

First he CF-243 source is far far away (sealed in the safe):

counts/min (1 min test) counts/min (1 min test) counts/min (1 min test) counts/min (1 min test) avg counts/min
PS1 158 168 151 175 163
PS2 194 166 162 176 174.5
n's 224 213 197 201 208.7

Now the CF-243 source is on the top of PS1:

counts/min (1 min test) counts/min (1 min test) counts/min (1 min test) counts/min (1 min test) avg counts/min
PS1 378,893 378,612 377,909 378,665 378,519.7
PS2 22,729 22,819 22,572 22,530 22,662.5
PS1+PS2 1044 1021 1023 975 1015.7
n's 200 210 228 219 214.2

The same results as before:

PS detectors probably detect the cosmic rays and very sensitive to the radioactive source.
The count rates for n's detector is basically the same with and without the source CF243. So what we are really detect? Cosmic? Then why the rate does not depend from source. Internal noise of PMT?