[d2n-analysis-talk] THaScintillator Class: Calculation of the Timewalk Correction

[Brad Sawatzky]

On Wed, 07 Apr 2010, David Flay wrote:

> On Wed, Apr 7, 2010 at 1:55 PM, Brad Sawatzky <brads at jlab.org> wrote:
> 
[ . . . ]
> > > Where fTdc2T = resolution of the TDC = 0.1e-9 s/channel.
> >
> > This is only true for an 1875.  I don't think we used that model TDC.
> > Make sure the this resolution number corresponds to the TDC you're
> > getting your data from.
> 
> Are you talking about just the resolution, or the calculation as well?

You have to answer this yourself.  As in any calculation, the units
better be consistent throughout.  As with all questions of this type:
do the calculation by hand for few events and convince yourself that the
result makes sense.

If I was writing the method I would be using raw bins for the walk
correction and then scaling the final result by the seconds/bin TDC
resolution.  The code does not seem to be structured that way though.
That means the TimeWalkCorrection() had better be pulling the correct
value of fTdc2T from the db_*.dat file or it's broken.

> I've been searching through my various timestamped directories, and
> they all say 1875.  However, I'm not totally confident in that,
> because I created 'master' db_L.s1.dat, db_L.s2.dat files based off of
> run 20676 (20090313 directory) and copied those to each and every
> other directory (once I figured out that the cratemap was wrong).

I looked back to double check.  We did stick with the 1875 in the
LHRS[*].  As Vince points out, they have a 0.05ns/bin resolution (*not*
the 0.1ns/bin I mentioned earlier).  If the fTdc2T variable you
mentioned in the ApplyCorrections() method is using 0.1ns/bin then it's
wrong.  If fTdc2T is hardcoded in the class, then we need to know that.
It would be better if fTdc2T is pulled from a db_*.dat file, but it
would then mean that the db_*.dat file is wrong.

[*] My memory was fuzzy on this...  At one point we had considered
    replacing the 1875 with an F1 so it wouldn't bottleneck the DAQ
    rate.  In the end we went with the status quo and kept the 1875.

-- Brad

-- 
Brad Sawatzky, PhD <brads at jlab.org>  -<>-  Jefferson Lab / Hall C / C111
Ph: 757-269-5947  -<>-  Fax: 757-269-5235  -<>- Pager: brads-page at jlab.org
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