1

Fluoride contamination of the RPC working gas and ageing phenomena

in collaboration with

Dip. Chimica University of Roma “Tor Vergata”

Bio-Electro analytical group (BEAT)

RPC 2005

Seoul – 12 october 2005Giulio Aielli

2

Introduction

The RPCs make large use of electronegative gases to control and limit the discharge process. F compound gases such as C2H2F4 and 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

Measurement setup

TISAB+ H2O

fluoride probe

DAQpH meter

Gas system

Gas T,RH probeRPC current

SCALER:Singlesdoubles

Magnetic stirrer

Gas in

Gas out (teflon)To exit bubbler

Teflon container

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

Serialline

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

4

ROMA2 SETUP BABAR SETUPNow doubled both

5

Measurement strategies

continuous cumulative measurementsin TISAB + H2O buffer

Study of the F˙ pollutant dynamics Understand and control systematics

To avoid external electrode interferences

Short current pulse stimulation: Tpulse << gas change rate

Determine the absolute total F produced per given charge and conditions

Ageing (long term) studiesStudy the F attachment (RPC as a pulse attenuator)

Step response (long sample accumulation)

Production rate dependence from the working conditions (measurement in a stationary state)

F accumulated in long term runs

Deep extraction cycles with Ar plasma

Study of the F “weak” attachment

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)F- vs. Iso-butane in streamer (aged chamber)

y = 43.051x-0.9777

0

2

4

6

8

10

12

14

16

18

0 2 4 6 8 10 12 14 16

Iso-butane %

[F-]

(n

Mo

les/

s )

F- vs. Iso-butane in streamer (new chamber)

y = 8.9315x-0.6228

0

1

2

3

4

5

6

7

8

0 5 10 15 20 25 30Iso-butane %

[F-]

(n

Mo

les/

s )

Measurement at fixed current (15.5 A) 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…

0.E+00

5.E-04

1.E-03

2.E-03

2.E-03

3.E-03

3.E-03

4.E-03

4.E-03

5.E-03

5.E-03

0 2 4 6 8 10 12 14 16

gap current (uA)

F- r

ate

(um

ol/1

0s)

ternary fixed WPbinary fixed Sbinary fixed WPternary fixed S

Ibu 10%

Avalanche mode: F- depends linearly on current only

Saturation at higher currents (av. or stream.) for SF6 free mix

Deviated from the

“standard low”:Streamer

contaminationAnd 0.5% SF6

MeasurementPerformed on the same chamber but over about 1 month with very different env. conditions

Why saturation?Recombination Mechanism of F?

9

F- entrapment in the RPC & Flow Rate

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

(A)HV on

(B)HV off

10

F- entrapment in the RPC & Flow Rate

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?0.0E+00

5.0E-04

1.0E-03

1.5E-03

2.0E-03

2.5E-03

3.0E-03

3.5E-03

4.0E-03

4.5E-03

5.0E-03

0 10 20 30 40 50 60

flow (cc/min)

corr

ecte

d p

rod

uct

ion

rat

e (u

mo

l/10

s)

Ibu 10%

2.0E-01

2.5E-01

3.0E-01

3.5E-01

4.0E-01

4.5E-01

5.0E-01

5.5E-01

6.0E-01

0 10 20 30 40 50 60

flow (cc/min)

abso

rpti

on

fac

tor

Ibu 10%

11

Pulsed measurements of F- (moles)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 1000 2000 3000 4000

0.269

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

7000 8000 9000 10000 11000

0.0413

0.0753

0.0716

0.0823

0.0724

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

15000 16000 17000 18000 19000

0.3238

0.470

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 @10A 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+10A for

forced extraction (5 days) Final value about 6

moles= 3.6E18 molecules Prompt signal 0.25 moles Released energy:

V*I*T=30J= 18.7E19 eV

0

1

2

3

4

5

6

0 5000 10000 15000 20000 25000 30000 35000 40000 45000

Time (X10s)

F- (

mole

s)•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 –/V Fgas

variation of fraction fraction flowed F in the gas evaporated away F’dep = JFgas – KFsup)

variation of proportional to the F deposited respective fractions

The solution for Fgas is like: C1e1t + C2e2t

The measurement is the integral of Fgas and can be fitted as in the plot

0 100 200 300

0.10

0.12

0.14

0.16

0.18

0.20

0.22

0.24

Data: Data2_ConcModel: ExpAssocEquation: y = y0 + A1*(1 - exp(-x/t1)) + A2*(1 - exp(-x/t2))Weighting: y No weighting Chi 2/DoF = 1.4929E-6R 2 = 0.99457 y0 0.09505 ±0.00118A1 0.03759 ±0.00061t1 79.05063 ±2.73383A2 0.08866 ±0.00116t2 6.74983 ±0.17324

Con

cent

ration

Time (x10S)

The process constants can be studied for different conditions

14

Time evolution: Argon forced extraction

y = 1.0841e-6E-05x

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 5000 10000 15000 20000 25000 30000 35000 40000 45000

time (x10s)

The experimental curve is simply Ftot(t)=A(1-e-t)The plot fits 1-Ftot(t)/A(A=6.4 moles)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 sametau within 10%

This number should depend on the current/surface ratio.This is obtained cleaning an old 20x20 aged RPC tau~ 8.4 days

y = 1.0026E+00e-1.3871E-05x

R2 = 9.9959E-01

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 20000 40000 60000 80000 100000 120000

time (x10s)

15

Fluoride measurements conclusions

I. A relevant fluorine production rate has been measured in the RPC gasII. The concentration of i-C4H10 in C2H2F4 based gas mixtures

seems to be essential for keeping the Fluorine production rate at low level

III. 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.

IV. The F- effective production rate seems independent from the flowV. The F- production low is current-only dependent for SF6 free

mixtures, no matter of the w.p. and operative mode.VI. A large saturation effect is visible at high current. Still under study.VII. 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?

VIII. The pulsed F test has been introduced to study in detail the production balance and the attachment.

IX. 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

X. Two simple models for the F deposition/extraction are introduced and seems to explain the process dynamic.

XI. ….Some answers but many new questions….