BCAL Dark Hits

[Matthew Shepherd <mashephe@indiana.edu>]

Hi all,

I've just committed code to the repository that generates dark hits on  
the BCAL sensors.

Briefly, the algorithm is:

For each of sensor in the inner layers only ( N_sensors = 2 * N_cells ):

- determine the number of dark pulses from random poisson with mean =  
( dark rate ) * ( ADC integration window )
- generate additional cross talk pulses from random poisson with mean  
=  ( number from above ) * ( cross talk probability )
- convert sum of both of these to an energy based on the photoelectron  
< - > energy conversion

If either of the sensors of a cell already has a hit, then add this  
extra energy to the hit.  Otherwise, create a new hit with this energy  
and a time that is randomly picked uniformly between -25% and +75% of  
the ADC integration window.

A few notes:

The relevant parameter to adjust the dark rate is  
BCALRESPONSE:DARK_RATE_GHZ.  The nominal value is 0.041 (41 Mhz), i.e.  
to get back to the rosy response do -PBCALRESPONSE:DARK_RATE_GHZ=0.

Hits for upstream and downstream sensors are handled independently.   
The reconstruction code requires a hit in both ends in order for a  
cell to be used in reconstruction -- this requirement provides some  
suppression for spurious cells since one must have a dark pulse above  
threshold in both the upstream and downstream readouts.

The algorithm for determining the threshold depends on dark rate (see  
the GlueX-doc 1231).  This means that as the dark rate is turned up,  
the relative number of cells with spurious hits should stay constant  
at the fraction set by:  BCALRESPONSE:OCCUPANCY_FRACTION_LIMIT.   
However, the energy spectrum of these spurious hits (because the now  
contain more dark pulses) is higher.

Attached is a plot of "hybrid" pi0s -- one photon in BCAL and one in  
FCAL taken from a small sample of 10k PYTHIA events.  There are no  
cuts on this sample.  You can see what the peak looks like for three  
different dark rates.  Clearly this is only the tip of the iceberg.   
There is energy dependence to study as well as how can the  
reconstruction be tuned (minimum cell energy cut) etc. to accommodate  
various dark rates.  My goal was to get some working code into the  
repository so we can begin to explore this.

-Matt

dark_rate_compare.pdf