[Halld-pid] Start Counter Meeting Minutes, July 2, 2015

[Mark Ito]

Folks,

Find the minutes below and at

https://halldweb.jlab.org/wiki/index.php/GlueX_Start_Counter_Meeting,_July_2,_2015#Minutes

   -- Mark
_________________________________________


GlueX Start Counter Meeting, July 2, 2015, Minutes

Present:

  * *FIU*: Werner Boeglin
  * *JLab*: Mahmoud Kamel, Mark Ito (chair), Eric Pooser, Simon Taylor,
    Beni Zihlmann


Calibrations

  * There are two methods for measuring the light propagation delay:
     1. Take events with two hits in different start counters and
        measure their time difference and compare it to the difference
        in light propagation distance. Eric has a new fitting procedure:
        first cut the data into slices in z, fit each of those and use
        the results to fit as a function of z. He gets 11 cm/ns in the
        straight section, 16 cm/ns at the bend, and 20 cm/ns in the
        nose. On the bench at FIU he measured 19 cm/ns in the nose
        (method 2, below).
     2. At FIU delays were measured as a function of position along the
        counters at fixed locations. Eric has redone the fitting
        procedure, using a method similar to that of method 1,
        automating several steps which were done by hand before.
  * To compare the two methods calibration constants were generated for
    both. These have been put into a private CCDB and the PID library
    was modified to access the constants. Right now there is a problem
    with the beam-data-generated constants. Eric is looking into this now.
  * When Eric looks at start counter time corrected for light
    propagation (using Simon's 15 cm/ns), for particle time-of-flight,
    and for time of interaction in target he gets widths of around 300
    ps over most of the start counter, consistent with our design goal.
  * We discussed a scheme where we simply measure time delay as a
    function of position directly to get a correction function or
    correction table to be interpolated and applied, without explicit
    reference to nose, bend, or straight. Although this is different way
    of parametrizing the correction, Eric pointed out that the framework
    for analyzing the data is the same and is already developed.
  *  From the FIU measurements there were measurable differences in the
    propagation between paddles. At present we do not have enough
    statistics to do a paddle-by-paddle calibration using beam data.
  * Time-walk corrections have been revisited using the fitting
    procedure he described for the propagation delay. The parameters he
    gets are stable and uniform counter-to-counter. The constants are in
    CCDB now.
  * Attenuation length measurement suffers from low statistics, not
    enough even to do an all-counters-together measurement. For now we
    will use data from the bench data measured at FIU.


Plots to Show at Reviews

We discussed some possibilities for plots illustrative of the status of 
the calibration. We settled on a representative histogram of start 
counter time vs. RF time, corrected for time-of-flight, light 
propagation in scintillator, and the position of the interaction in the 
target. Also a plot showing the resolution, obtained from the 
aforementioned histogram, as a function of position in the start counter.

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