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<p>Folks,</p>
<p>Please find the minutes below and at
<a class="moz-txt-link-freetext" href="https://halldweb.jlab.org/wiki/index.php/GlueX_Start_Counter_Meeting,_March_23,_2017#Minutes">https://halldweb.jlab.org/wiki/index.php/GlueX_Start_Counter_Meeting,_March_23,_2017#Minutes</a>
.<br>
</p>
-- Mark<br>
______________________<br>
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<h2><span class="mw-headline" id="Minutes">Minutes</span></h2>
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<p>Present:
</p>
<ul>
<li> <b>FIU</b>: Mahmoud Kamel</li>
<li> <b>JLab</b>: Thomas Britton, Mark Ito (chair),
Simon Taylor</li>
</ul>
<p>There is a <a rel="nofollow" class="external text"
href="https://bluejeans.com/s/7ITwj/">recording of
this meeting</a> on the <a rel="nofollow"
class="external text"
href="https://jlab.bluejeans.com/">BlueJeans site</a>.
</p>
<h3><span class="mw-headline" id="Calibration_Update">Calibration
Update</span></h3>
<p>Mahmoud showed <a rel="nofollow" class="external text"
href="https://halldweb.jlab.org/talks/2017/SC_Meeting_03-23-2017.pdf">the
latest results</a> from his time-walk calibration.
</p>
<ul>
<li> 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.</li>
<li> 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.</li>
</ul>
<h3><span class="mw-headline"
id="Propagation_Time_Correction_Constants">Propagation
Time Correction Constants</span></h3>
<p>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.
</p>
<h3><span class="mw-headline"
id="Time-dependent_geometry_adjustments">Time-dependent
geometry adjustments</span></h3>
<p>We discussed a few issues:
</p>
<ul>
<li> 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.
<ul>
<li> 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.</li>
</ul>
</li>
<li> 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.</li>
<li> 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.</li>
<li> Lately only Richard and Simon have been modifying
the main GlueX geometry files.</li>
</ul>
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<pre class="moz-signature" cols="72">--
<a class="moz-txt-link-abbreviated" href="mailto:marki@jlab.org">marki@jlab.org</a>, (757)269-5295
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