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

Dear TPC experts, I realized that our 2026 survey contains data that accurately measures the thickness of the end wheels on the TPC.   We have measurements on the outside of the Aluminum wheel, and again we have measurements on the face (of many)

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Dear TPC experts,

I realized that our 2026 survey contains data that accurately measures the thickness of the end wheels on the TPC.  We have measurements on the outside of the Aluminum wheel, and again we have measurements on the face (of many) of the sectors.   The difference in distance along the beamline is the thickness of the wheel.  This is a useful question to ask because the fabrication specs simply say that anything from 4.480 to 4.500 inches (113.8 to 114.3 mm)  is acceptable.  It turns out that our wheels are on average 114.17 mm thick (0.005 inches thinner than the maximum allowed spec).  This is an average thickness but the point-to-point variations are not large and the thickness of the wheel is never larger than the 4.5" upper limit.

Since 114.17 mm is different from what I assumed on the first pass over the data, I have redone the spreadsheet that analyzes the survey data.  For example, the thickness of the end wheel changes the overall length of the TPC. So, I have re-calculated the ideal length of the TPC and I used that number in the spreadsheet, plus a few other small changes. 

Please see version 9 of the spreadsheet on Drupal (updated with a little help from Richard and Flemming).  https://drupal.star.bnl.gov/STAR/event/2026/05/21/TPC-Meeting 

The updated slides emphasize that the end wheel data points are also an excellent way to find the location of the TPC in the IP6 coordinate system.  Each hole in the Aluminum is not terribly precise but there are many holes and they are uniformly distributed.  See the updated slides for exact numbers.

I also made a mistake in interpreting the previous results.  The TPC is SHORTER than specification.  Previously, I said the opposite.   The overall length of the TPC is 2.4 mm shorter than spec, or equivalently the distance from the CM to the ground grid of wires is 1.2 mm shorter than spec.  In reality, the drift distance from the CM to the ground grid is 2091.8 mm (not 2093 mm).

This observation made me scratch my head so I asked AI.   It says the coefficient of humid expansion for kapton is 22 ppm per % relative humidity.  AI also says the annual average relative humidity in Berkeley is 70 to 75%.    At RHIC, the inside of the TPC was bone dry and the outside was never more than 50% RH (or the field cage went crazy).

So 25% x 22 ppm = 550 ppm    or   550ppm of 4420 mm = 2.31 mm.

We measured 2.4 mm shrinkage.  Perhaps the TPC shrank over the years because it was surveyed once in Berkeley when it was wet and again (30 years later) at BNL when it was dry?

A reference for the Berkeley data is documented in this figure which is captioned: "Figure from Howard Wieman, May-6-97, with dimensions updated by actual survey by Russ Wells."

https://www.star.bnl.gov/public/tpc/hard/tpcrings/dimensions.GIF

That figure appears on page 4 of this document:  https://www.star.bnl.gov/public/tpc/hard/tpcrings/index.html

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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