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[James H Thomas <jhthomas AT berkeley.edu>]

Dear TPC Experts, I have been working with the most recent TPC survey data to confirm as much information as I can about the TPC before installation, and after.   Here are my preliminary conclusions: I previously reported that the thickness of

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Dear TPC Experts,

I have been working with the most recent TPC survey data to confirm as much information as I can about the TPC before installation, and after.  Here are my preliminary conclusions:

I previously reported that the thickness of the Aluminum end wheels is 4.495 inches (114.18 mm) and this is in very good agreement with the Engineering acceptance criterion when it was manufactured.  The acceptance criteria was that any thickness between 4.48 and 4.50 inches (113.8 to 114.3 mm) was acceptable.   Now I can also report that the thinnest point was 113.6 mm, and the thickest point was 114.5 with a probable point to point survey error of 200 microns.  The distribution of points when binned every 100 microns looks like this:

    1         1          1         5          7          7         11         3          1

113.7   113.8   113.9   114.0   114.1   114.2   114.3   114.4   114.5

The mean is 114.18 and the standard deviation is .18 mm.   So the thickness of the wheel does vary from point to point and probably reflects the cutting tools wearing down as the job progressed.  Once the tools wear to a certain point, they replace the cutting tool and continue.  This is why the acceptable range was 4.48 to 4.50 inches.  

Also as reported before, the TPC is 2.4 mm shorter than engineering design specification.  The field cages are made out of Kapton and so it is not surprising that a less rigid material is harder to control. Thus our drift distance from CM to padplane is most likely 1.2 mm shorter than specification.

Based on what I have seen, point to point survey measurements are accurate to roughly 200 microns in the beamline direction, and roughly 100 microns in the plane transverse to the beam.  Averages of many points can, of course, be better.

Finally, I translated the survey data to R-Phi coordinates and discovered a systematic shift in the rotation of the East and West ends of the TPC.  The East end is rotated (like a clock) by 0.41 mR with respect to the West.  (The angle of rotation is positive in the STAR coordinate frame).  This is in very good agreement with the same angle of rotation measured by Howard Wieman and Bill Love back in 1997.  Wieman and Love measured 0.43 mR (positive).

So with these notable systematic effects, the other TPC dimensions are nearly perfect.  The ends are flat, the ends are parallel, and there are no other twists or bends that I can find.  The precision of this statement is 200-300 microns.  There are a few larger excursions here and there but in general the TPC matches expectations within our ability to survey and measure these things.  This was true when the TPC was installed, and it appears to be true today (30 years later).

Individual sectors are 'out of place' by 200-300 microns and every sector is different in this respect.  Since this is only slightly larger than our point to point measurement error, and we only have high quality data for half the sectors, I think the best way to improve our tracking is to correct for the systematic effects noted above, then assume the sectors are otherwise in their ideal positions and refine the pad alignment using cosmic rays tracks.  That was the conclusion we drew in 1997 and I think it is still true in 2026, even though we have much better survey techniques than before.

Cheers,
Jim

--

Jim Thomas

Email:  jhthomas AT berkeley.edu 

Phone:  USA  (510) 759-4936

Homepage:  https://sites.google.com/berkeley.edu/jhthomas 

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