05/10/2011

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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).

Now I increased the power supply for HAMAMATSU PMT (-1500 V) 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

So the same results as before:

PS detectors probably detect the cosmic and very sensitive the radioactive source.

The counts rate 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? But why the count rate are so big?