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Re: A proposal for setting a BCAL threshold



Hi Richard and Matt:

We had a phone conference yesterday with SensL and they have just tested 
the wafers for the arrays under Phase 1. 

The 820L pixel type has demonstrated the following performance at room 
temperature:

Voltage above breakdown                         Pure PDE (no cross talk 
or after pulsing)                DR/pixel                  DR/Array

           + 
1V                                                                        
7%                                                   800 
Hz                  65 MHz
           + 
2V                                                                      
11%                                                 1700 
Hz                137 MHz
           + 
3V                                                                      
13%                                                 3200 Hz
           + 
4V                                                                      
17%                                                

At a temperature of +5 C  the dark rates are cut by a factor of 2.6, so 
without an elaborate or expensive cooling we can bring this down to 
around 53 MHz at 11% PDE.   These then will be for Phase 1.  For Phase 
2, the new Si process they have just implemented has reduced the DR by a 
factor of just over 2 and they expect to further improve and come down 
to that of the original Si treatment that does not give good 
reproducibility of breakdown voltage.  Trenching will also reduce cross 
talk and keep the DR at the 1 P.E. level.

The bottom line is that Richard is correct that at the present time 32 
MHz at PDE values larger than have not been achieved.   Are such rates 
achievable for production?  I believe they are but aiming at a more 
realistic 50-60 MHz at PDE ~ 10-15% is the wiser route to go.

I hope this helps,

George


Richard Jones wrote:

> Matt,
>
> The design goal of 32MHz may eventually be achieved, but this is not 
> demonstrated.  At the last meeting George agreed that 100MHz is 
> achievable with what he has seen.  I would believe something more like 
> 150MHz @ 22C from what I have actually seen, but I am not working with 
> Sensl modules.  This depends a lot on temperature, but we agreed that 
> we do not want to have to refrigerate the BCal readout very much.
>
> Richard J.
>
> 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
>>
>