[Halld-pid] PMT test results question

[Beni Zihlmann]

Hi Paul,

sorry that I missed the meeting. Mark explained to me the situation and 
I have some
remarks and questions because I think something is completely wrong with 
the ADC
spectrum using the Am source.

1) the Am 241 generates 5.6 MeV alphas. This alphas are mono-energetic 
and should
      produce a lot of light in the scintillator and therefore a very 
large signal in the PMT
      if you put this little scintillator directly in front of the photo 
cathode.
      -> so have a look on the scope how the signals look like. They 
should be huge if you
      use 1750V

2) Because the alphas are mono-energetic the resulting energy spectrum 
in the ADC
      should be a peak. Or maybe two peaks depending of the pureness of 
the Am241
      source and the resolution of the PMT/scintillator system. This 
peak should be rather
      sharp, high above the pedestal and Gaussian. If any tail this tail 
should go to lower
      energy not higher energy. These are the main reasons why I think 
the ADC spectra
      you see are completely off and something in the setup must be off. 
My first thinking
      is that you look a noise and the actual signal is at much much 
higher ADC values.
      -> again that is why looking on the scope at the signals and in 
particular large signals
      should be done.

3) Because I say that the Am241/scintillator source produces so much 
light it might be that
     1750 is far to high Voltage, and for sure you do not need 
amplification.
     Note: 5.6 MeV from the alpha particle is deposited in the first 1mm 
of the scintillator and
     the scintillator in total is not that think so there is hardly any 
attenuation and all the light
     is seen by the photo cathode. => lots and lots of light => huge signal!

4) you trigger on the same signal as you are looking at. That means you 
should never see
     a pedestal. However the data show a pedestal => that does not make 
sense.

5) the peak in the ADC is at a similar location above pedestal as the 
single photo electron peak
     is at the same HV and amplification. Again that does not make sense 
because from the
     alpha source you should have lots and lots of light.


I hope this gives you some ideas of what you can look at in your setup 
to see
what might be going wrong.


cheers,
Beni