[Halld-pid] Start Counter Meeting Minutes, April 28, 2015

[Mark Ito]

Folks,

Find the minutes below and at

https://halldweb.jlab.org/wiki/index.php/GlueX_Start_Counter_Meeting,_April_28,_2015

   -- Mark
__________________________________________


GlueX Start Counter Meeting, April 28, 2015
Minutes

Present:

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


News from the Run

  * With a 200 nA beam on a 3×10^-4 radiator, Eric reported see rates
    from 100 to 200 kHz in the individual start counter sectors.
  * When beam started up last Friday, the scalers rate distribution was
    symmetric, but recent log entries show an asymmetry of 10 to 20% in
    the rates. The cause is not understood at present. It is nothing
    like the asymmetry we saw last Fall when the start counter was
    tilted by about 6 degrees. Werner remarked that we should expect
    some level of asymmetry in the raw scalers rates.
  * Werner asked about whether thresholds in the discriminators can be
    set on a channel-by-channel basis. Eric told us that the boards
    accept different thresholds, but our current software can only load
    a single value for all channels on a board.
  * Bias voltages on the SiPMs are currently all set as per Hamamatsu's
    recommendation, as tabulated by Nick and Fernando. Recall the four
    SiPMs for a given sector are all run from a common voltage channel.


Bias Voltage Scan

We discussed a plan to scan bias voltages for the SiPMs. This has never 
been done for the start counters, including during fabrication and 
testing at FIU. The study would tell us if we should pursue gain 
balancing among the sectors. Eric reminded us that the gain should 
change by about one percent per 10 mV change in bias voltage. We decided 
on a plan to vary the voltages in the range of -200 mV to +200 mV from 
nominal over the next few days, changing setting after we accumulate a 
significant chunk of data (a few hours of beam). Thus the gain should 
only change in the range of -20% to 20% during this exercise.

Calibration

Calibration work is still in progress, both the time-walk correction and 
propagation delay correction scheme are evolving. Eric also mentioned 
that there will be some work to incorporate the new calibration schemes 
in the PID library.

Next Meeting

We agreed to go to a two-week schedule, starting after the collaboration 
meeting.

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