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Update on the Princeton RPC Remediation Test Chambers and

Update on the Princeton RPC Remediation Test Chambers and Preliminary Test Results on Humidity Level in the RPC Chamber. Changguo Lu Princeton University. SLAC IFR Meeting, June 23, 2001, SLAC. Update on the Princeton RPC remediation test chambers. Update on the test chamber #1.

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Update on the Princeton RPC Remediation Test Chambers and

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  1. Update on the Princeton RPC Remediation Test Chambers and Preliminary Test Results on Humidity Level in the RPC Chamber Changguo Lu Princeton University SLAC IFR Meeting, June 23, 2001, SLAC

  2. Update on the Princeton RPC remediation test chambers Update on the test chamber #1 • For the BaBar FWT#49 RPC chamber after the oxygen treatment its efficiency and dark current show big improvement. A big question is: will the improvement be sustainable? • At 9th day: no degradation; • At 29th day: the upper half of the chamber (x=3,4) still maintains good efficiency, but the lower half of the chamber (x=1,2) deteriorates. • At 42nd day: the upper half of the chamber starts to show deterioration. FWT#49 installed in BaBar detector vertically as shown .  There might be several reasons to cause the degradation. The oxygen treatment certainly can improve the damaged chambers, but it can not solve all problems. We need to find out the other culprits and search for their suitable remediation methods.

  3. Efficiency summary table

  4. X = 1 areas: efficiency decreased X = 2 areas: efficiency decreased

  5. X = 3 areas: efficiency somewhat decreased X = 4 areas: efficiency somewhat decreased

  6. Questions need to be answered • If the dark current reduction and efficiency improvement after the oxygen treatment is due to polymerization of the uncured Linseed oil, why without heating up the chamber the efficiency of the chamber degrades again? Does this behavior have same origin as BaBar RPC’s symptom - persistent efficiency degradation since installation? • While efficiency degrades the dark current does not increase, why? Not the same origin? • Why right after the oxygen treatment the efficiency always is the best, then degrades by 2~3% within few hours? What is the mechanism behind this behavior? • Can several oxygen treatment cycles finally stop the efficiency degradation?

  7. Test results on the second damaged RPC chamber History of the chamber(Aron Soha): First temperature cycle (06/29/2000 to 09/22/2000) 29°C (1.3days) => 32°C (2.7days) => 36.7°C (11.3days) => 29°C (18days) => 24.5°C (52days) After first temperature cycle Efficiency was ~65% Dark current was ~35A

  8. History of the chamber… Most recent cycle (early 2001): 22°C => 34°C (1 day) => 38°C (6 days) => 22°C After this cycle Dark current > 150A Efficiency ~ 10%

  9. History of the chamber… The chamber was pulled out of SLAC test-stand and disconnected from RPC gas system, left the gas tubing opened to the air on 3/6/2001. Flow RPC gas again at Princeton on 5/18/2001.  The chamber was in poor air flow condition for 73 days (3/6/01 to 5/18/01). Y 1,1 1,4 4,1 4,4 X

  10. Efficiency test results

  11. Dark current variation During oxygen treatment, –1000V was applied to the chamber to monitor the dark current variation: Dark current was dropping even after the temperature correction.

  12. In RPC gas mixture –8000V was used. Dark current increases with the time: at the end of the test (after 17 hours), Idark reached 36A, the initial dark current value (before any oxygen treatment): 39A .

  13. Summary on the second test chamber • After 73 days letting gas tubing opened to air the second test chamber seems to regain most of its efficiency: 77%. The first round of oxygen treatment didn’t further improve its efficiency (76%). After second round of oxygen treatment it gains little bit efficiency (80%). • At the first few hours after turn on HV the chamber shows very good efficiency. For the region of X=4, Y=1, in first hour efficiency  94.5±1.0%; 12 hours later it dropped to 87.9%. Dark current also increases after flowing RPC gas and HV on. Initial dark current was 39A, 16 hours later dark current increased to 36A. • Singles rate varies in 1.5 ~ 3.5kHz, no big variation during the entire test period, except in first few minutes after turn on HV the singles rate was fact of 2 higher, then decreased steadily to the normal rate. • Both efficiency and dark current deteriorate within few hours after initial good improvement from oxygen treatment. That is different from the first test chamber (FWT#49), the later shows no dark current increase after one month of operating in the RPC gas, but part of the chamber shows the symptom of efficiency degradation.

  14. Humidity measurement of the test chambers Motivation of this study • Experience from Belle RPC chamber: > 1500ppm humidity level caused disastrous efficiency deterioration. Surface etched due to fluoride acid  rough surface absorbs more water, reduce surface resistivity  increase dark current  reduce surface potential  decrease efficiency; • BaBar RPCs have long term deterioration problem, the time scale is ~ month, which also appears in our test chamber #1; • There is another time scale (~ hours): after long shut down, switch on HV, BaBar RPC efficiency always shows few percentage point better before returns to normal. The time scale is hours. Our test chambers have similar behavior; • Search for the physical processes that can cause such time scale effects.

  15. Facts about the humidity Humidity effect on surface resistivity **using linear extrapolation from 0% and 100% data derived this number

  16. Humidity effect on the volume resistivity Bakelite can absorb significant amount of water (more than 0.6% of its own weight). **using linear extrapolation from 0% and 100% data derived this number

  17. Relative surface resistivity (value @ 100% R.H. as 1) Relative volume resistivity (value @ 100% R.H. as 1) Both surface and volume resistivity are having same trend: resistivity decreases when R.H. increases. For the efficiency important thing is the ratio of , so Bakelite RPC shouldn’t be very sensitive to the atmospheric humidity. Dark current should be affected by R.H. But the inner surface could attach more water molecules under the high electric field, therefore / will decrease  loss of efficiency.

  18. An hypothetical explanation of persistent efficiency dropping after turn on HV Without HV the humidity in the chamber will reach a certain equilibrium level, which is ~ 1000ppm. With HV the inner surface has strong electric field. Water molecule is a polarizable molecule, which will be attracted to the surface, and stay there. Water molecule model** ** http://www.sbu.ac.uk/water/molecule.html

  19. According to this scenario, it is not unconceivable to have the surface resistivity reduced by fact of 5, and volume resistivity unchanged. The resistor network model calculation shows the following V distribution: ( = 5.31011/,  = 2.91012cm) Dead space would increase dramatically! Also dark current would increase by fact of 4.

  20. Humidity level of the test chamber A Cermet Hygrometer with two sensors are used for this test. Measurement range: –80oC to 20oC, Accuracy: ±1oC, Resolution: 0.1oC.

  21. Very preliminary test results (might be wrong! I am still working on it) • There is no difference on the outlet humidity (~1000ppm) with or w/o 38 ft long Polyflo tubing in front of chambers  water coming from the Bakelite chamber itself. • Gas flow in the chamber is very slow, it has enough time to reach equilibrium state before leaving the chamber. Polyflo tubing is permeable to the water vapor, the water content in ¼” tubing (Dan Marlow, Belle RPC): Q = 2080 L*R/F ppm L: length in meter, R: relative humidity (1 for 100% R.H.), F: flow rate in cc/min. In our case, R = 100%, F = 50cc/min, L = 11.5m, Q = 480 ppm.

  22. (2) Humidity level of the outgoing gas mixture varies with room temperature Only connect chamber #2 to the system, flow N2/O2: 1700ppm 1440ppm

  23. Summary • Oxygen treated chamber #1 show efficiency and dark current improvement, but after one month of operation efficiency starts deterioration, dark current still holds lower level; • Chamber #2 after 2.5 months without flowing RPC gas, and gas tubing opened to air, it regained most of the lost efficiency, and reduced dark current dramatically. Further oxygen treatment only can increase its efficiency and reduce dark current for half day; • Bakelite plate contains water about ~0.6% of its weight. Its surface and volume resistivity varies with humidity, at 0% and 100% of R.H. the resistivity can be a fact of ~30 different; • Water molecules attached on the inner surface of the Bakelite chamber due to strong electric field may can explain some efficiency and dark current deterioration puzzles, need to get experimental evidence. If it is possible, I would like to get more damaged chambers to investigate the various aspects of RPC chamber.

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