1Daya 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

2Daya 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/m2, that is much higher than OPERA’s data: ~17Hz/m2.

• 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.

3Daya 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)

4Daya 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.

5Daya 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 - C2H2F4.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.

6Daya 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.

7Daya 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.

8Daya 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.

9Daya 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.

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.

11Daya Bay Collaboration Meeting, IHEP, Beijing, 8/1-3/2007

HF Production rate in RPC

Test device for the fluoride production: fluoride probe

… 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.

3.86X1019 F-/C — F- adsorption rate in streamer operation.

13Daya Bay Collaboration Meeting, IHEP, Beijing, 8/1-3/2007

Could HF be a problem for Daya Bay ?

If we pipette 90L of 48% concentration HF acid into our test container, after HF drop vaporized, it will produce ~ 1.31021 HF molecules inside the container. The total inner area of the container is 103 cm2,

1.31021/103 cm2 = 1.31018/cm2,

equivalent to a 2m2 RPC operated at 5A 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 0.26A, 1/20 of 5A, 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!