Re: A proposal for setting a BCAL threshold

[Elton Smith <elton@jlab.org>]

Hi Matt,

Your argument seems reasonable to me as a first guess. Clearly if on
wishes to model additional noise, the thresholds can be increased on any
generated data.

Cheers, Elton.



Elton Smith
Jefferson Lab MS 12H5
12000 Jefferson Ave
Suite # 16
Newport News, VA 23606
elton@jlab.org
(757) 269-7625

On Thu, 7 Jun 2007, 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
>
>