Re: A proposal for setting a BCAL threshold

[David Lawrence <davidl@jlab.org>]

Hi Matt,

    It seems the method here is to use a DAQ limit of 5% on the BCAL 
occupancy to imply the threshold we would need to set on the cell 
energy. I'm guessing the DAQ limitation would have to come from network 
bandwidth or some self-imposed disk space limitation. (The modules 
themselves would still have to analyze every channel to to determine 
sparsification so that bottleneck is a function of trigger rate, not 
BCAL rate). Your premise that the fADC works like an integrating ADC and 
that the pedestal width is all due to dark rate means the  7 
photoelectrons you calculate is an upper limit for a fixed occupancy and 
dark rate. Since presumably a real fADC would be able to reject many of 
the dark-rate-only events, it would fold in an acceptance probability 
function to your Poisson to give the probability distribution of 
accepted dark rate events. Integrating the tail of that distribution 
*in* from infinity until you get a total probability of 5% would lead to 
fewer photoelectrons. This, of course, assumes the dark rate really 
dominates the BCAL rate compared to real hits. I guess in short, I think 
your assumption about the fADC operation means the 1.5MeV number should 
be considered an upper limit using this methodology.

    Now, if I understood Richard's comments at the calorimetry workshop, 
we can expect the sampling fluctuations to translate to at least 10MeV 
for even the smallest showers we expect to reconstruct (100MeV?). This 
was the original motivation for setting the threshold here. If that is 
the case, then adding information from these 2-3MeV cells cannot improve 
your energy resolution. They can, however, affect the rates if we choose 
to set the BCAL threshold lower.

    OK, now that I've said all of that, I think the 1.5MeV would be OK, 
but I would vote for setting the threshold down even lower to something 
like 1MeV provided it doesn't blow up the data file too much (which I 
don't think it will). This will allow you to play with the threshold 
outside of hdgeant. We could even make plots of the BCAL rate as a 
function of threshold in MeV which could be useful when writing up more 
detailed specifications for the DAQ.

Regards,
-David



Matthew Shepherd wrote:
>
> Hi All,
>
> Here's a proposal for setting a BCAL threshold so we can start to 
> refine the reconstruction a little bit.
>
> - take dark rate at 32 MHz (design goal from GlueX-doc-795) and assume 
> this is only single PE rate
> - for a 100 ns window this means an average of 3.2 pulses per window
> - assume the fADC processing just generates a pedestal subtracted mean 
> and that dark rate (not electronics noise) dominates the pedestal
> - let's assume the DAQ can handle 5% occupancy in the BCAL
> - if average is 3.2 dark pulses, the probability of having 7 or more 
> pulses in a window is 0.04
>
> --->> set threshold at 7 photoelectrons
>
> 7 photoelectrons * ( 26 keV_fiber / pe ) / 12% = 1.5 MeV energy 
> deposited in cell
>
> I propose we adjust the threshold to 1.5 MeV (down from 10 MeV) and 
> work from there.  Of course this needs further study, and validation 
> through whatever bench studies, beam test, etc. etc..  My main goal is 
> to get around the right order of magnitude so we can make another pass 
> at the reconstruction algorithm which will behave very differently 
> with this much lower threshold.  Does anyone see a serious flaw with 
> this?
>
> -Matt
>

-- 

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