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

Tue Nov 30 13:14:13 EST 2010

On Tue, Nov 23, 2010 at 6:14 PM, Brad Sawatzky <brads at jlab.org> wrote:

> On Fri, 19 Nov 2010, David Flay wrote:
>
> > I've gone back to looking at the raw S2m TDCs, and upon taking a
> > closer look at these on log scale, it's clear that there is a shoulder
> > structure to the immediate right of each self-timing peak in the right
> > TDCs (right = red, left = blue).  The same may be said of the left
> > TDCs.  The main peak of the left TDCs also have a significant width to
> > them.
>
> Interesting plots.
>
> You need to cut out the EDTM pulses -- I don't see that in your cutlist.

> A tail/shoulder at longer times on the L pmts could be driven by
> time-walk.  Note that _all_ structure outside the self-timing peak
> around bin 1950 in the S2m R pmt spectra must either:
>  1) be noise and/or secondary particles -- ie. that paddle can not have
>     generated the trigger, or
>  2) if you know that paddle did generate the trigger (say, because none
>     of the other paddles have any hits), then the wrong hit from the
>     multi-hit is TDC being stored in L.s2.rt[], or
>  3) this is not really a T3 trigger and something else is generating
>     the trigger timing (ie. DL.evtypebits is being constructed
>     incorrectly).
>
> The nice sharp spikes around bin 1950 in the red histos are the events
> where that paddle (R pmt) carried the trigger timing.
>
> Those are the peaks that should be aligned to a common bin in the
> corrected histos.  Note that you need to align the single-PMT spike, not
> the average ([R+L]/2) time for S2m.  The aligned Right-PMT_S2m time
> "L.s2.rt_c[x]", where x is the paddle that generated the trigger, should
> be your reference time when you align s1 later, not the average paddle
> time.
>

I've aligned these guys (the right PMTs only) -- see attached histos.

These (corrected) times were derived from the raw TDC histograms.  I chose a
paddle as a reference (index 7, paddle 8) and aligned the R PMT times to
that value.

The results are as follows (in ns):

S2mRMean[0]: 61.3362    S2mRSigma[0]: 0.1129
S2mRMean[1]: 61.3055    S2mRSigma[1]: 0.0946
S2mRMean[2]: 61.3105    S2mRSigma[2]: 0.1139
S2mRMean[3]: 61.3396    S2mRSigma[3]: 0.0956
S2mRMean[4]: 61.3093    S2mRSigma[4]: 0.1061
S2mRMean[5]: 61.3222    S2mRSigma[5]: 0.0954
S2mRMean[6]: 61.3022    S2mRSigma[6]: 0.1038
S2mRMean[7]: 61.3138    S2mRSigma[7]: 0.1029
S2mRMean[8]: 61.3174    S2mRSigma[8]: 0.0978
S2mRMean[9]: 61.3111    S2mRSigma[9]: 0.0977
S2mRMean[10]: 61.3187    S2mRSigma[10]: 0.1086
S2mRMean[11]: 61.3201    S2mRSigma[11]: 0.0996
S2mRMean[12]: 61.3155    S2mRSigma[12]: 0.1105
S2mRMean[13]: 61.3603    S2mRSigma[13]: 0.1037
S2mRMean[14]: 61.3638    S2mRSigma[14]: 0.1083
S2mRMean[15]: 61.2838    S2mRSigma[15]: 0.0991

These results come from a Gaussian fit centered on 61 +/- 1 ns.

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

> 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.

>

Note that the averaged paddle time "(R+L)/2" does _not_ remove the
> propagation time broadening in S2m since the R PMT is the one that
> generates the start time at the TDC.  In principle, you could apply a
> propagation time correction to both the right and left PMT timing based
> on where the tracking says the particle intersected with the bar and
> remove the propagation time effect.  In practice, at least for us, it's
> not worth the effort.
>
> Because the position-correlated time offsets are not removed from
> S2m_ave[] (and can not be removed without using tracking information),
> you are stuck with them in S1 as well.  Because of this, I recommend you
> ignore all timing information from the Left PMTs in both S1 and S2m.
> You should still require that both PMTs see a hit within a reasonable
> time window (this requires that both PMTs on the bar see light within a
> physically relevant time window event, which will suppress noise).
>
> I sketched out some math on the timing calculations.  If you haven't
> seen this kind of thing before, it really helps when you're trying to
> make sense of what you're seeing.  See the attached scan and drop me a
> line if you have questions.  When I write b:{0,t}, I mean that b can
> vary between 0 and t nanoseconds, depending on where the particle hits
> the bar.

Thanks, this makes things clear & easier to understand

> 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.

-- 
-----------------------------------------------------------
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://www.jlab.org/%7Eflay>
             http://quarks.temple.edu
-----------------------------------------------------------
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