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

[David Flay]

Hey Brad,

So I've been investigating the THaScintillator class for how the analyzer
implements the coefficients.

I found in the ApplyCorrections member function:

    if (fLT[i] != 0.) {
       fLT_c[i] = (fLT[i] - fLOff[i])*fTdc2T - TimeWalkCorrection(i,kLeft);
       nlt++;
     }
     if (fRT[i] != 0.) {
       fRT_c[i] = (fRT[i] - fROff[i])*fTdc2T - TimeWalkCorrection(i,kRight);
       nrt++;
     }

Where fTdc2T = resolution of the TDC = 0.1e-9 s/channel.

the member function TimeWalkCorrection() does:

   Double_t tw(0), tw_ref(0.);
   int npar = fNTWalkPar/(2*fNelem);

   Double_t *par = &(fTWalkPar[npar*(side*fNelem+paddle)]);

   tw = par[0]*pow(adc,-.5);
   tw_ref = par[0]*pow(ref,-.5);

   return tw-tw_ref;

now, the ReadDatabase() member function does:

   // timewalk coefficients for tw = coeff*(1./sqrt(ADC-Ped)-1/sqrt(ADCMip))
   for (int i=0; i<fNTWalkPar; i++) fTWalkPar[i]=0;

ADCMip is a very large number, so we don't need to worry about it, as
1/sqrt(ADCMip) ~ 0.

comparing these two member functions, I'm thinking that the correction
would be  to determine the slope of TDC vs. 1/sqrt(ADC) => coeff above. 
Then, the database subtracts this coefficient off from the original TDC
value (shown in ApplyCorrections()).  However, as I showed in my previous
talk, this method (at the moment) is a bit tricky, in that I already have
a <small> effect, and the method hasn't necessarily worked...

I just wanted to confirm how the analyzer utilizes these coefficients,
before I get fully immersed in working on the paddle-to-paddle time
differences concerning beta vs. track-x, as you suggested it's best to
focus on that.

Dave

-------------------------------------------------
David Flay
Physics Department
Temple University
Philadelphia, PA 19122

office: Barton Hall, BA319
phone: (215) 204-1331

e-mail: flay at jlab.org
            flay at temple.edu

website: http://www.jlab.org/~flay
              http://quarks.temple.edu
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