Glass Resistive Plate Chambers

1. Glass Resistive Plate Chambers • BELLE Experiment: • Virginia Tech (barrel) • Tohoku (endcaps) Dan Marlow, Princeton (seminar at Rice), Norm Morgan (Virginia Tech) • Monolith Experiment (proposed at Grand Sasso): Carlo Gustavino • Virginia chambers at Fermilab Valery Makeev

2. RPC Principles of Operation Resistive paint Signal pickup (x) Glass plates 8 kV Signal pickup (y) Resistive paint +++++++++++++++ _ _ _ _ _ _ _ _ _ _ _ +++ +++++ _ _ _ _ _ _ _ Spacers A passing charged particle induces an avalanche, which develops into a spark. The discharge is quenched when all of the locally ( ) available charge is consumed. Before The discharged area recharges slowly through the high-resistivity glass plates. After

3. Plateau Curve 2 mm gap RPCs plateau at a fairly high voltage. Note the slight falloff in efficiency well above the plateau. This effect is real and typical.

4. Principles of Operation: I vs V Curve Glass RPCs have a distinctive and readily understandable current versus voltage relationship. • Low voltage • High voltage

5. Pulse Shape One interesting feature of RPCs is that the signal can be observed both using a pickup electrode and by viewing the light signal using a PMT. The pulses are large (~100 mV into 50 ohms) and fast (FWHM ~ 15ns) There is a very good correlation between the electronic and the light signal.

6. Current, pulse shape, efficiency • Pulse height, currents, plateau depend on: • Gas mixture • Electric field (HV/gap size) • Resistivity of the ink

7. Afterpulsing +++ ++++++++++++++ _ _ _ _ _ _ _ _ _ _ _ _ _ We (I.e. BELLE) observe a significant rate of afterpulsing. Typically the afterpulses are spaced by ~40 ns from the initial pulse (and other afterpulses). We believe that these pulses are caused by photons that escape the primary avalanche and initiate new streamers in a non-depleted region of the chamber. In some cases several afterpulses are observed

8. First result: signal from streamer mode Repond, Lia, (Argonne) • Gas: Freon/Argon/IsoButane at 62:30:8 • High Voltage: 7.5 KV or above • Cosmic ray signal (triggered by 3 layers of scintillator) PED + avalanche PED 1 streamer 2 streamers 3 streamers avalanche

9. Principles of Operation: Rate Capability +++ +++++ _ _ _ _ _ _ _ As noted, each discharge locally deadens the RPC. The recovery time is approximately Numerically this is (MKS units) Assuming each discharge deadens an area of , rates of up to can be handled with 1% deadtime or less. This is well below what is expected in our application.

10. Gas Mixture (Belle) • Traditional Gas Mixture • 64% Argon : 6% Freon 116 30% Isobutane • Constraints • Safety: gas should be non flammable: • mixture ---> 30% Argon : 62% Freon :8% Butane • Environment: Freon 116--> Freon R134A • Cost: Isobutane ---> Butane “silver”

11. Virginia Chambers at Fermilab Valery Makeev Chambers have been stored in an ambient atmosphere for three years Eventually (see next slide) they work with efficiency >90%

12. Recovering efficiency == drying out Chambers stored for a long period of time accumulate water vapor. Water vapor leads to an increased current and reduced efficiency

13. Long term stability of glass RPC’s • BELLE near disaster story (Dan Marlow, Workshop on Aging in Gaseous Detectors, Hamburg, October 2001) • Water damage studies, part I (Norm Morgan, http://www.phys.vt.edu/~morgan) • Water damage and recovery studies, part II (Y. Teramoto, NIM A 484 (153), NIM A 482, hep-ex/0211020)

14. A Major Problem Develops The first signs of trouble showed up shortly after installation and looked something like the plot to the right. The current from a chamber would “suddenly” show a dramatic increase. Given that there is a “pedestal” current resulting from the spacers, the true dark-current increase was in fact substantial.

15. A Major Problem Develops • 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. • High current is a serious problem in glass RPCs (see next slide).

16. A Major Problem Develops . . . High dark currents induce a significant IR voltage drop across the glass plates, which lowers the voltage across the gap, causing the chamber to slide off the efficiency plateau. Increasing the applied voltage doesn’t help since it merely results in increased dark current. The is what I (I.e. Dan Marlow) like to call the classic “RPC Death Spiral”.

17. A Major Problem Develops . . . The correlation between dark current and efficiency loss is readily apparent.

18. A Solution Emerges . . . • After several weeks of study we determined that the problem was due to high levels of water vapor in the gas. • Although we were aware of this problem (even more severe problems occurred in the early days of our R&D when some of our collaborators used urethane gas tubing, which is highly permeable), we had incorrectly assumed that water would not permeate our polyethylene (Polyflow) tubing. • In fact we were susceptible to water contamination for the following reasons: • Low flow rates • Long (5~12 m) runs of plastic tubing. • Hot and humid weather during the Japanese rainy season. • Using published data for polyethylene, we were able to account for the ~2000 ppm concentrations of water vapor that we observed.

19. A Solution Emerges . . . We replaced the long runs of polyethylene with copper (~5 km in all!) and flowed gas at the highest possible rate (~one volume change per shift). Slowly, but surely, the RPCs began to dry out.

20. A Solution Emerges . . . We were greatly relieved to see an accompanying drop in the dark currents!

21. A Solution Emerges . . . Although some residual damage remains, the situation is much improved.

27. Water damage studies in Virginia Short exposure to water vapor leads a reversible loss of efficiency Long exposure to water vapor leads to a irreversible loss of efficiency

28. Autopsy of ‘dead’ chambers Electron Surface Chemical Analysis Virgin glass, side A Good cathode Virgin glass, side B Dead anode Good anode Dead cathode

29. What is all this stuff? Where did some of it go?

30. Y. Teramoto, Osaka

32. Partial recovery Dead

34. Atomic Force Microscope Images Teramoto et al, hep-ex/0211020 • Cathode and anode of RPC operated with freon show deposits: • Conductive?? • Reduce work function?

35. Why deposits matter? Reconfigurable chambers: swap cathodes and anodes, damaged and new Chamber functions properly if it has ‘good’ cathode, anode matters not

36. It is cathode, what matters

37. Clean the electrodes with alcohol? • Scrubbing a cathode helps to bring chamber to a working state • Cleaning an anode does not help very much • In any case, mechanical cleaning is not a practical solution…

38. Bubbling the gas through 28% ammonia • This is a very practical solution • It takes two weeks for a full recovery

39. Cleaned up chambers - like new

40. Summary • Glass RPC chambers are simple to build, reliable detectors, but applicable only for low rate experiments • The only known (so far) damage mechanism to glass RPC’s is related to freon+water vapor+HV combination • Combination of reversible (short term exposure?) and irreversible (long term exposure) effects • Chemistry of deposits on both electrodes is not very well understood • Water vapor proof gas lines are sufficient to avoid the problem • Even in the case of accidental damage a recipe exists for chamber recovery through chemical cleaning of the glass surfaces

41. New idea (Valery Makeev) • Why not pure argon RPC? • Simpler, cheaper gas system • Inert gas • Etc.. Etc.. • Freon and isobutane are necessary to quench UV photons and electrons • Use a honeycomb of thin plastic with cells ~ 3-5 mm instead? • Stay tuned..