[Halld-cal] Fwd: Dark hits in BCAL

[David Lawrence]

Hi All,

  At the BCAL Reconstruction meeting today we discussed the status of the dark hits in the simulation.
Specifically, that the simulation overestimates their amplitude compared to real data. It turns out I
did not check in any change regarding this, but here is the e-mail I sent that gives the details on what
was done in October.

Regards,
-David

Begin forwarded message:

> From: David Lawrence <davidl at jlab.org>
> Subject: [Halld-cal] Dark hits in BCAL
> Date: October 24, 2014 at 1:21:54 AM EDT
> To: Will Levine <wlevine at gmail.com>
> Cc: Hall-D Calorimetry <halld-cal at jlab.org>, Tegan Beattie <beattite at jlab.org>
> 
> 
> Hi Will,
> 
>   I was chatting with Elton today about your presentation at the calorimetry meeting this morning.
> I’m sorry I missed it.  From my discussion with Elton and from what we talked about at the 
> Calibration meeting on Wednesday, it sounds like the simulation of the dark hits is simply
> giving a much larger contribution that what we are seeing in the real detector. This likely due
> to the conversion factors used to convert photo-electrons and energy into mV not being correctl. 
> As was discussed, these factors need to be adjusted in order to bring the simulation into better
> agreement with the real data. 
> 
>  The relevant factors in mcsmear/smear_bcal.cc are the following shown with their current values:
> 
> BCAL_mevPerPE  =  0.668338
> mV_per_MeV     =  0.881599 
> 
>  The value of mV_per_MeV is inversely proportional to BCAL_mevPerPE (the latter being used
> in the calculation of the former). The dark hit amplitude and the signal amplitude are both multipled
> by mV_per_MeV. Only the dark hits though are proportional to BCAL_mevPerPE. What this 
> means is than in order to adjust the relative amplitude between signal and dark hits, without
> changing the signal size in mV, both of the above parameters have to be modified.For example,
> to effectively reduce the dark hit amplitude by a factor of 10 relative to the signal, we divide 
> both BCAL_mevPerPE  and mV_per_MeV by 10 in the code:
> 
> BCAL_mevPerPE /= 10.0    =>  0.0668338
>    mV_per_MeV /= 10.0    =>  0.881599 
> 
> note that the value of mV_per_MeV doesn’t actually change since the value of BCAL_mevPerPE
> is used to calculate it.
> 
> I’ve made a few plots to show a comparison of with and without this change. You can see for the 
> second plot (the logY one) that the signal part of a cell energy (above about 75MeV) is not
> affected much. There is significant reduction in the lower energy part though. The last plot shows
> the reconstructed cluster energy. This indicates a drop of about 10% in the reconstructed
> energy (simulation was all 1GeV photons). I think this is what you were alluding to when you
> said simply removing the dark hits from the cells with signal caused issues since the reconstruction
> has been tuned to put the peak at the right place when dark hits are present. Either those parameters
> will need to be re-tuned, or the value of mV_per_MeV scaled by a smaller factor than BCAL_mevPerPE.
> 
> The scale factor of 10 was arbitrary and the actual factor should probably come from a comparison
> to the real data. I think we have two handles by which to do that. The first is the number of hits (first
> plot). We could reproduce that plot with “no-signal” events and adjust the scaling factor until it matches
> what we see from data taken with the detector using a random trigger. The second would be to convert
> GeV into Pulse Integral fADC units and look at the “dark hits pedestal” width. This would again need
> to be done with no-signal data. Hopefully, the same scale factor will give agreement in both, though
> we may need to tweak the threshold to get that.
> 
> I’m not sure how much time you have to work on this so we can discuss how best to proceed.
> 
> Regards,
> -David
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