BCAL Threshold -- Input Needed

[Matthew Shepherd <mashephe@indiana.edu>]

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