[Halld-pid] ADC

[Beni Zihlmann]

Hi Sasha,
I have some questions about this.
to point
1) the Landau peak position represents the value with the
     highest probability. What we need however is the mean
    of the distribution which is above the peak. Only for a Gaussian
    is the mean and the peak position the same. So what is the mean?

2) I do not quite understand why a 50/50 splitter
introduces a factor of 4. I would have expected a factor of 2 when
a 50 Ohm termination is put to the unused output and a factor
of 1 when no termination is present.

4) 2138 photo electrons is huge. that would represent a relative
     error 1/sqrt(2138) of 2%. That is too good to be true. Are you sure
     you run the PMT at 1750V and not at 2000?

cheers,
Beni



> Beni,
>
> OK, here is an estimate for the number of
> photoelectrons from a minimum ionizing peak
> for a hit in the middle of a TOF paddle:
>
> 1) Peak of Landau distribution is at 890 ADC counts,
> pedestal is at 350 counts. With 0.25 pC per ADC count,
> the integrated charge is (890-350)*0.25 pC = 135 pC
>
> 2) There is exactly factor of 4 loss due to a 50-50 splitter
> and 50 Ohm terminator at the PMT. This was verified
> with a pulser. This results in 135 pC * 4=540 pC at
> the anode.
>
> 3) Therefore, the number of electrons at the anode is
> 540e-12 / 1.602176565e-19 = 3370415045
>
> 4) PMT has a gain of 1.576e6 at 1750V. So, the number
> of photoelectrons from a single hit is
>   3370415045 / 1.576e6 = 2138
>
> Sasha
>
> On Friday, September 30, 2011, Beni Zihlmann wrote:
>> Hi Sasha,
>> thanks for the info.
>> the cable length should not matter if the ADC gate is
>> sufficiently long.
>>
>> cheers,
>> Beni
>>> On Friday, September 30, 2011, Beni Zihlmann wrote:
>>>> Hi Sasha,
>>>> in the link
>>>> https://halldweb1.jlab.org/talks/2011-3Q/ADC%20spectra%20and%20Time-walk%20corrections_files/
>>>> you show an ADC spectrum of a paddle PMT: adc_l_par_nottrig.gif.
>>>> what is the HV of the PMT set at and what type of ADC are you using?
>>>>
>>>> cheers,
>>>> Beni
>>>> _______________________________________________
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>>>>
>>>>
>>> Beni,
>>>
>>> This particular plot is done with:
>>>
>>> 1) CAEN ADC v265 in 12-bit mode. This  corresponds to
>>> 0.25 pC per count
>>>
>>> 2) 10-stage PMT with a nominal gain of 1.5*10^6 (at 1750 V).
>>> However, it was used at 2000 V in this particular measurement.
>>> So, its actual gain is unknown in this case.
>>>
>>> 3) What makes things more complications is a signal attenuation
>>> on its way from PMT anode to the ADC due to a 50-50 splitter,
>>> a 50 Ohm terminator right at the PMT,  and 150ft of cable. Right now,
>>> I'm trying to measure what is the correction coefficient I need to apply
>>> to compensate for this attenuation. I should have an estimate of the
>>> number of photoelectrons later today.
>>>
>>> Sasha
>>>
>>>
>>>
>>>
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