Re: BCAL Threshold -- Input Needed

[Blake Leverington <leverinb@uregina.ca>]

Thanks Elton,

George and I had a similar conversation though we hadn't considered the 
beam test pedestals as a place to start. I'll look into to that and see 
what I get.

-Blake

Elton Smith wrote:
> Hi Blake,
>
> I meant to get back to you with a few comments. Your analysis is correct
> as long as electronic noise plays no role. When the dark rate noise is so
> low that it does not impact the width of the pedestal, it is likely that
> electronic noise will be the dominant contribution. The width of the
> pedestal is hard to estimate from first principles, but the beam test data
> can give you an empirical value for it. Since we used vacuum pmts for
> the beam test, the dark rate was presumably negligible in that case
> too. So I suggest you take a look at the pedestal widths from the beam
> test an consider something like 3-4sigma as an estimate for the threshold.
>
> Cheers, Elton.
>
> Elton Smith
> Jefferson Lab MS 12H5
> 12000 Jefferson Ave
> Suite # 16
> Newport News, VA 23606
> elton@jlab.org
> (757) 269-7625
> (757) 269-6331 fax
>
> On Mon, 18 Aug 2008, Blake Leverington wrote:
>
>   
>> Hi all,
>>
>> I'm trying to come up with a number for the the threshold in the case of
>> a 3x3 geometry with FM PMTs as readout.  Since we do not have the dark
>> rate (less than 1 kHz) it seems to me that the threshold for the
>> detector is then only the energy needed to generate one photoelectron or
>> about 0.61 MeV. Does this seem reasonable or is there other noise I am
>> not taking into account?
>>
>> -Blake
>>
>> Matthew Shepherd wrote:
>>     
>>> Hi all,
>>>
>>> Blake and I are now working on retuning the BCAL MC.  As has been
>>> noted many times in the past, the BCAL cell detection threshold is an
>>> *absolutely critical* input to modeling and performance.  It will
>>> "make or break" the the reconstruction in the < 200 MeV range.
>>>
>>> Since it seems to be breaking more than making with our latest round
>>> of revisions I wanted to go through how we determine this once more.
>>> If some of you could give a little thought to this and see if it is
>>> reasonable, that would be great.  There are some assumptions made;
>>> however, we are really just trying to get within +- 30% or so of a
>>> reasonable threshold number.
>>>
>>> Assumption 1:  One will need to do some zero suppression on a per
>>> channel basis on the FADC.  Let's assume that the pedestal alone
>>> cannot cause more than 5% of all BCAL channels to exceed the zero
>>> suppression threshold.
>>>
>>> Assumption 2:  The main contributor to upward fluctuations in the
>>> pedestal is single PE dark pulses from the SiPM.  This assumption is
>>> optimistic since there will undoubtedly also be electronics noise that
>>> will broaden the pedestal also.  The rate of single PE dark pulses is
>>> 40 MHz.
>>>
>>> Assumption 3:  The readout window for the FADC is 100 ns.  This means
>>> SiPM pulses must be fully contained in this window including any
>>> forward backward shifts due to differences in transit time in the
>>> readout chain (hopefully minimized) and transit in the module (about
>>> 25 ns for full length).
>>>
>>> With a 100 ns readout window this implies there will be an average of
>>> 4 single PE pulses in every event.  According to Poisson statistics
>>> this means that 95% of events will have 7 or less pulses.  Therefore
>>> we set the threshold at 8 photoelectrons.
>>>
>>> Now we need to make a careful translation between photoelectrons and
>>> energy deposited in a cell.  For this we use the latest and greatest
>>> GlueX-doc 1069.  The first bullet point at the top of p.9 gives
>>> results for SiPM array.  Scaling for the non-operational cells in the
>>> sensor one estimates 11 photoelectrons per MeV deposited in the fiber,
>>> when attenuation effects are removed.  That means 1 PE = 91 keV
>>> deposited in the fiber.  The sampling fraction for cosmic rays is
>>> about 15% -- therefore 1 PE = 91 keV / 0.15 = 0.61 MeV deposited in
>>> the module.
>>>
>>> The simulation "attenuates" the energy deposited in the cell
>>> (fiber+lead+epoxy) down to each end.  At that stage the hits are only
>>> saved if they exceed a threshold of 8 PE * 0.61 MeV / PE = 4.9 MeV.
>>>
>>> Do we agree that this seems reasonable?  One might hope that signal
>>> processing in the FADC could suppress some of the 1 PE pulses.  This
>>> would effectively lower the 1 PE rate and allow a lower threshold.
>>> However, I'm sure overlooking things like electronics noise in this
>>> estimate, make the estimate somewhat optimistic.  As I said we would
>>> like to get as close as reasonable.
>>>
>>> We are forging ahead with this threshold and will generate new plots
>>> for each of the two segmentations.
>>>
>>> -Matt
>>>
>>>
>>>       
>
>
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