BCAL Timing

[David Lawrence <davidl@jlab.org>]

Hi Blake,

    Elton, Simon, and I spent a little time yesterday and today looking 
at the BCAL timing issue and wanted to relay to you what we have found.

    We made significant discovery when we realized that tphoton was 
being calculated by (t_ref - tcounter) and the trigger, veto, and BCAL 
times were being calculated by (t_ref - tdetector). The value of "t_ref" 
refers to a reference value obtained by using one channel of the TDC 
module to record the trigger time. What this means is that one should 
really *ADD* the value of tphoton to the BCAL times. This threw us off 
for a bit because we were looking at run 2435 which used the veto as the 
only trigger. In that case, because the veto is the source of the 
reference times, both (tphoton+tveto) and (tphoton-tveto) are perfectly 
flat when plotted against Ephoton. However, when for runs such as 2334 
which used a MOR timed trigger, only (tphoton+tveto) is flat vs. 
Ephoton. This is why you have seen things that look like the tagger is 
not calibrated.

So, for all runs in which the veto is active (regardless of whether it 
is the trigger or not) you should see a nice, flat band when you do this:

bcal->Draw("tphoton+tveto:Ephoton","Nphotons==1 && 
abs(tphoton+tveto-500)<50")

If it is not flat, then that indicates the tagger is not calibrated. If 
it is flat, then the tagger must be calibrated.

The veto analysis of a few runs confirms out suspicion that the tagger 
calibration is unchanged throughout the entire beam test. However, when 
we look  at the bcal/tagger correlation, there is still some dependence 
on t-counter id. We have not been able to track the source of this down 
yet but are still looking at it. We will send you more info as it 
becomes available.

Regards,
-David

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  David Lawrence Ph.D.
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