Fluoride contamination of the RPC working gas and ageing phenomena

1. Fluoride contamination of the RPC working gas and ageing phenomena RPC 2005 Seoul – 12 october 2005 Giulio Aielli in collaboration with Dip. Chimica University of Roma “Tor Vergata” Bio-Electro analytical group (BEAT)

2. Introduction • The RPCs make large use of electronegative gases to control and limit the discharge process. F compound gases such as C2H2F4and SF6 were introduced for their effectiveness and industrial common use. • The decomposition of such a gases under electrical discharge produces a significant concentration of fluoride radicals that can be detected in the RPC exhausted gas. • The F radicals may easyly produce HF. Due to its high chemical reactivity, this represents a possible cause of the inner surface damaging if it is not quickly removed by the gas flow. • HF film on inner bakelite surface increases ohmic current. • HF is an aggressive acid: it can harm the inner surface finding the eventual weak points of the oil protective coating. • The effect is self sustaining HF damage  rate increase  more HF • Systematic measurements of this process can help to understand many ageing processes and can be also a new probe to investigate the discharge phenomena

3. TISAB + H2O Measurement setup • The F- can be measured by trapping the ions e.g. by bubbling the gas in water where the fluorine is detectable as Fluoride. • The TISAB (Total Ionic Strength Adjousting Buffer) neutralizes the effect of the electrode interfering substances such as OH- or metal traces. It keeps the PH around 5.5 Serial line SCALER: Singles doubles DAQ F˙ was measured by an Ion selective electrode probe(0.02 ppm F˙ sensitivity) , read by a PHmeter (used as impedance converter and prompt monitor) and recorded by DAQ pH meter fluoride probe Gas out (teflon) To exit bubbler Teflon container Magnetic stirrer RPC current Gas in Gas T,RH probe Gas system

4. Now doubled both ROMA2 SETUP BABAR SETUP

5. To avoid external electrode interferences Study of the F˙ pollutant dynamics Understand and control systematics continuous cumulative measurements in TISAB + H2O buffer Measurement strategies Short current pulse stimulation: Tpulse << gas change rate Step response (long sample accumulation) Study the F attachment (RPC as a pulse attenuator) Ageing (long term) studies Determine the absolute total F produced per given charge and conditions Production rate dependence from the working conditions (measurement in a stationary state) Study of the F “weak” attachment F accumulated in long term runs Deep extraction cycles with Ar plasma

6. Typical run in continuous mode • Linear fit of the slope • Sudden change of the working point • Calibration constants • Instantaneous F˙ readout • Instantaneous F˙ rate readout

7. F vs. Isobutane (Binary mix in streamer mode) • Measurement at fixed current (15.5 mA) and counting rate on 2 different chambers • Lower rate (40 Hz/cm2) • The effect of Iso-butane: • The 15%/3% ratio is 4.99 on an aged chamber: a factor 5 of iso-butane reduces by5 the F˙ • The 20%/5% ratio is 2.9 for the second chamber: a factor 4 in iso-butane reduces the F˙ by a factor of 3 • The iso-butane F˙ suppression mechanism seems to be more effective in streamer than in avalanche. (IEEE2004)

8. F- vs I production law and exception… Measurement Performed on the same chamber but over about 1 month with very different env. conditions Deviated from the “standard low”: Streamer contamination And 0.5% SF6 Saturation at higher currents (av. or stream.) for SF6 free mix Why saturation? Recombination Mechanism of F? Avalanche mode: F- depends linearly on current only

9. Evaluation of the F- entrapment In first approx. It depends from the flow To estimate it one chamber is alternatively used as a filter following… The difference is measured The “real” production rate can be guessed by applying the absorbed fraction as a correction F- entrapment in the RPC & Flow Rate (A) HV on (B) HV off

10. Fixed rate & current (10 uA) Variable flow Binary mixture 10% I-butane OLD and very used chamber By applying the measured entrapped fraction to the first chamber we guess the total effective F- produced that is more or less constant and flow independent To be verified: the attachment of a passive chamber may be different when turned on… (Argon suggestion…) What if the chamber was new? F- entrapment in the RPC & Flow Rate

11. Pulsed measurements of F- (mmoles) 2’:30’’ flow=60cc/min+ 2’:30’’ flow=0 300’’ flow=0 300’’ flow=60 cc/min uninterrupted

12. F- entrapment: pulse test • New chamber 10x10 cm^2 • 10%Ibutane binary mix • 300 s @10mA pulse: 3mC= 1.9E16 electrons • Pulse given with gas closed • 60 cc/min flow for the tail with the chamber off (4h) • 60 cc/min Ar+10mA for forced extraction (5 days) • Final value about 6 mmoles= 3.6E18 molecules • Prompt signal 0.25 mmoles • Released energy: V*I*T=30J= 18.7E19 eV • For each electron we have ~ 50 F radicals. Primary Ionization potential of TFE is 13.6 and 17.4 • To free each F radical we have ~ 100 • eV available. C-F dissociation energy in TFE is 4.5 eV The numbers are compatible

13. Time evolution: a simple model for the HV=0 extraction-deposition • F’gas = -1/V F’dep–F/VFgas variation offraction fraction flowed F in the gas evaporatedaway • F’dep = JFgas– KFsup) variation of proportional to the F deposited respective fractions The solution for Fgas is like: C1el1t + C2el2t The measurement is the integral of Fgas and can be fitted as in the plot The process constants can be studied for different conditions

14. Time evolution: Argon forced extraction The experimental curve is simply Ftot(t)=A(1-e-t/t) The plot fits 1-Ftot(t)/A (A=6.4 mmoles) The extraction rate is linear with The current in Argon at a given time The tau of the process is of the Order of 2 days. The measurement was repeated showing the same tau within 10% This number should depend on the current/surface ratio. This is obtained cleaning an old 20x20 aged RPC tau~ 8.4 days

15. Fluoride measurements conclusions • A relevant fluorine production rate has been measured in the RPC gas • The concentration of i-C4H10 in C2H2F4 based gas mixtures seems to be essential for keeping the Fluorine production rate at low level • A significant fraction of the fluorine tends to accumulate in the chamber walls and can not be removed only by flushing many fresh gas volumes. The lost fraction depends on the gas flow rate. • The F- effective production rate seems independent from the flow • The F- production low is current-only dependent for SF6 free mixtures, no matter of the w.p. and operative mode. • A large saturation effect is visible at high current. Still under study. • The SF6 seems to take a relevant part in the SF6 production but it does not follow the production law in streamer mode (does not saturate). Hints for streamer models? • The pulsed F test has been introduced to study in detail the production balance and the attachment. • Two type of attachment are identified: ionic bound and molecular bound, being the last dominant for a chamber which is new. The molecular bound can be broken by means of Argon plasma operation • Two simple models for the F deposition/extraction are introduced and seems to explain the process dynamic. • ….Some answers but many new questions….