GlueX Start Counter Meeting, March 23, 2017

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GlueX Start Counter Meeting
Thursday, March 23, 2017
11:00 am EDT
CEBAF Center, Room F326/327


  1. Announcements
  2. Review minutes from the last meeting
  3. Run Recap
  4. Calibration
  5. Time-dependent geometry adjustments
  6. Mcsmear procedure and parameters for the start counter
  7. NIM/TDR
  8. Action Item Review




Talks can be deposited in the directory /group/halld/www/halldweb/html/talks/2017 on the JLab CUE. This directory is accessible from the web at .



  • FIU: Mahmoud Kamel
  • JLab: Thomas Britton, Mark Ito (chair), Simon Taylor

There is a recording of this meeting on the BlueJeans site.

Calibration Update

Mahmoud showed the latest results from his time-walk calibration.

  • He showed plots of pulse-height (in ADC counts) vs. z-position (as measured by charged tracks) at the point of intersection with the start counter. Clear minimum ionizing peaks are visible with amplitude increasing with increasing z.
  • He then showed the difference in TDC time and ADC time as a function of pulse-height (in ADC counts) and the time-walk correction he derives from these plots. He then showed the same TDC-ADC time vs. pulse-height plot after the correction is applied. The pulse-height dependence is largely removed, as expected. In particular, there does not appear to be a large failure of the correction at large pulse-height.

Propagation Time Correction Constants

Although there was no smoking gun in the time-walk calibration, Mark thought that, at least for now, we should go with the single-linear fit in the nose region to for the propagation time correction until that non-linearity is understood. The current constants being used in the monitoring run are consistent with that.

Time-dependent geometry adjustments

We discussed a few issues:

  • Although the start counter is mounted on the target cart its position is physically tied to that of the target. In the HDDS representation, the two positions are specified independently.
    • Simon reported that that the target position has varied from run-to-run by as much as a centimeter. The cause of such a large shift needs to be understood, mechanically.
  • We discussed whether or not target/start-counter position should be reflected in the base geometry or represented as a correction in the new geometry specification scheme, i. e., what is the philosophy for deciding which shifts are considered part of the base geometry and which are called a correction.
  • Thomas pointed out that if we settle on a philosophy for the parametrization, that philosophy should be applied for all detector groups. It would be messy if different sub-systems viewed the base geometry in different ways.
  • Lately only Richard and Simon have been modifying the main GlueX geometry files.