[d2n-analysis-talk] S2m Raw L and R TDC Times -- with a (hopefully useful) primer on thinking about DAQ timing

[Brad Sawatzky]

On Tue, 30 Nov 2010, David Flay wrote:

> The third plot shows the (corrected) R PMT TDC signals, with a binning of
> 0.05ns/bin.

Looks good.

> > Note that the width of the blue peak around bin 1800 should be
> > driven by the time it takes for light to propagate across the bar to
> > the L pmt.  If the LHRS acceptance is uniformly illuminated it
> > should have a flat top (ie. a rectangular 'peak' with a width that
> > is equal to index_refraction*bar_length/c not a Gaussian like in
> > your plots.  Perhaps this is an e-P elastics run?  (The width will
> > also be smeared by the timing resolution of the TDC: 0.5ns/bin for
> > 1877s.)
> 
> the DB lists these TDCs as 1875's with a resolution of 0.05ns/bin.
> The run for those plots was production, 4-pass, p = 0.60 GeV.

Maybe it's fine.  If the bars are only 3' long then you're looking at a
1--2ns propagation time from end to end.  Smear that with the TDC
resolution and what you plot isn't crazy.

> > Assuming that no-hit events show up in bin zero (suppressed on these
> > plots), then I would say the blue spike at ~4100 is due to the
> > left-side PMT carrying the trigger timing.  That should be
> > impossible for a good particle unless there is a hardware cabling
> > error.  If the spike is just random coincidences (ie. junk), then
> > height of the spike suggests a L+R coincidence windows of 100 bins
> > == 50ns.  That seems a little wide to me, but it's hard to judge
> > factors of two off the plot.  (Basically, you assume a flat randoms
> > distribution and ask yourself how many bins' worth of background
> > have piled up in the spike on the right.  Then convert the #bins to
> > ns and that is your coincidence overlap.)
> >
> > If, however, there is a spike at 4100 in the red histo too (hidden
> > behind the blue), then I would guess that those are really the
> > no-hit events (ie.  no hits in that channel within the TDC window
> > setting).  The fact that they show up in a single bin is a
> > consequence of the analyzer assigning some fixed value to such
> > no-hit events (ie. 0?) and the formula used to convert and correct
> > the common-stop TDC data to nanoseconds in your histogram.  This
> > assumption would mean there should not be any hits in bin zero
> > though -- that needs to be checked.
> 
> From this third plot, there are no events in the bin for 0 ns.

But there is a spike at both ends of the right-PMT histograms.  Does the
spike around bin 160 in "s2m_R-cor-indiv...png" correspond to no hit in
that TDC channel? (ie. That spike is an artifact of the analyzer?) Look
at the ADC vs TDC spectrum for the individual right PMTs.  See what the
ADC distribution is for hits in the spike on the right side.

-- 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
The most exciting phrase to hear in science, the one that heralds new
  discoveries, is not "Eureka!" but "That's funny..."   -- Isaac Asimov