Re: BCAL Dark Hits

[Matthew Shepherd <mashephe@indiana.edu>]

Just in the main trunk -- I checked it in only 2 days ago.

-Matt

On Apr 3, 2009, at 1:30 PM, Blake Leverington wrote:

> Hi Matt,
>
> Has this been included in the latest release, release-2009-02-24 or  
> in the main trunk only?
>
> Cheers,
> -Blake
>
> Matthew Shepherd wrote:
>>
>> 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
>>
>>
>>
>>