1. 1 Daya Bay Collaboration Meeting, IHEP, Beijing, 8/1-3/2007 RPC aging concern Changguo Lu Princeton University Daya Bay Collaboration Meeting IHEP, Beijing, Aug. 1-3, 2007

2. 2 Daya Bay Collaboration Meeting, IHEP, Beijing, 8/1-3/2007 Why we should concern RPC’s aging? The consensus of Daya Bay RPC single’s rate is ~500 – 1000Hz/m 2, that is much higher than OPERA’s data: ~17Hz/m 2. In cosmic ray background environment serious aging did happen for Belle’s glass RPC. Since IHEP RPC does not use Linseed oil coating, so BaBar-type of aging (due to uncured Linseed oil coating) won’t happen. The special feature of IHEP’s RPC is w/o Linseeed oil coating, that is a double-blade sword: it can make the production easier and surface quality more controllable, but also lose the protection of the Linseed oil film.

3. 3 Daya Bay Collaboration Meeting, IHEP, Beijing, 8/1-3/2007 Aging of Belle’s RPC The first signs of Belle RPC trouble showed up shortly after installation and looked something like the plot below. The dark current from a chamber would “suddenly” show a dramatic increase. The problem started to show up almost immediately upon installation in June of 1998. The number of high-current RPCs increased steadily over the summer. The failure rate increased dramatically in late August, forcing us to shut down the system pending a better understanding. The culprit of this aging was found to be due to water vapor, after a few months intensive investigation Belle solved the problem. (Dan Marlow, Princeton University Seminar at Rice University, July 9, 1999)

4. 4 Daya Bay Collaboration Meeting, IHEP, Beijing, 8/1-3/2007 Does Daya Bay Bakelite RPC have aging risk? Daya Bay RPC use IHEP Bakelite, not glass, do we need to concern aging? YES! The main aging source is HF, by keeping the gas mixture very dry, glass RPC can avoid the attack from HF, but Bakelite RPC can not avoid the water vapor. The resistance to HF attacking: IHEP Bakelite is better than glass, but is not very much. Italian RPC inner surface is coated with Linseed oil, which is proved to be a good defense line to HF, IHEP RPC doesn’t use Linseed oil to coat the inner surface. The background rate of IHEP’s RPC is not so low. So far there is no long term running experience for large IHEP-type RPC system.

5. 5 Daya Bay Collaboration Meeting, IHEP, Beijing, 8/1-3/2007 HF corrosive effect on RPC electrode surface HF acid Testing samples Cover plate with 4 round windows that let the testing samples exposed to HF vapor. The major component in RPC gas mix is R134A - C 2 H 2 F 4. In the electrical discharge it can produce significant fluoride radicals, and further form HF. HF is notoriously chemical reactive, it can attack many different materials.

6. 6 Daya Bay Collaboration Meeting, IHEP, Beijing, 8/1-3/2007 Effect on bare BaBar Bakelite surface Marble side of BaBar Bakelite plate, the marble- pattern is completely disappeared, also discolored. Brown side of Bakelite plate shows slightly discolored mark.

7. 7 Daya Bay Collaboration Meeting, IHEP, Beijing, 8/1-3/2007 Effect on Linseed oil coated Bakelite The Linseed oil coated Bakelite surface is much better protected from HF vapor attack. After 24 hours of exposure there is no discolored area can be seen.

8. 8 Daya Bay Collaboration Meeting, IHEP, Beijing, 8/1-3/2007 Effect on BESIII Bakelite surface Surface resistivity drops very fast in first hour of exposure. Surface has been badly attacked by HF vapor.

9. 9 Daya Bay Collaboration Meeting, IHEP, Beijing, 8/1-3/2007 Effect on various other materials Belle’s RPC glass surface After exposed to HF vapor for ~24 hours, the surface looks powdery fluffy. After water rinses the surface, the fluffy “skin” is removed, the glass surface looks cracky.

10. Summary of the HF tests Surface resistivity change after exposed to HF vapor We can see that the surface resistivity reduction for IHEP Bakelite samples is ~10 -7, for Linseed oil coated BaBar Bakelite samples is ~ 10 -4, 1000 times better than IHEP Bakelite, the glass surface is 10 -8, 10 times worse than IHEP Bakelite.

11. 11 Daya Bay Collaboration Meeting, IHEP, Beijing, 8/1-3/2007 HF Production rate in RPC Test device for the fluoride production: fluoride probe

12. … HF production rate in RPC This amount of F - would be accumulated inside the chamber and attack the electrode!! Finally the sampling solution reaches the ultimate 1.1ppm F - concentration.  (1.1-0.45=) 0.65ppm in 40CC solution of fluoride being trapped inside the chamber after accumulating Q= 22.5mC during the streamer operation.  3.86X10 19 F-/C — F- adsorption rate in streamer operation.

13. 13 Daya Bay Collaboration Meeting, IHEP, Beijing, 8/1-3/2007 Could HF be a problem for Daya Bay ? If we pipette 90  L of 48% concentration HF acid into our test container, after HF drop vaporized, it will produce ~ 1.3  10 21 HF molecules inside the container. The total inner area of the container is 10 3 cm 2,  1.3  10 21 /10 3 cm 2 = 1.3  10 18 /cm 2,  equivalent to a 2m 2 RPC operated at 5  A for 8.5 years. The ratio of surface resistivity change (before/after) is ~ 500 to 900 for BESIII bakelite. The glass surface shows similar resistivity change. The RPC operated underground can achieve very low dark current, such as OPERA RPC reaches 0.26  A, 1/20 of 5  A, therefore HF won’t cause serious trouble, but in Daya Bay case the single’s rate would be much higher than this, thus aging could be a problem!