Study of gas mixture and ageing of the MRPC Chambers

1
Study of
gas mixture and ageing of the MRPC Chambers
used for the
Alice-ToF
Eugenio
Scapparone (INFN-Bologna)

for the
Alice-Tof Group RPC2003, Clermont-Ferrand,
October 21, 2003
Intense RD performed on MRPC over many years
excellent results in terms of time resolution
and efficiency.

• Gas mixtures is an important element for the
  successful performance
• of the MRPC. We need a fast mixture, with high
  Townsend coeff.,
• for small gaps ( d250 mm) a 170
  mm-1.
• Good stability for 5-10 electric field
  change gap width
• mechanical tolerance not critical.
• Several mixtures carefully studied in terms of
  efficiency and time
• resolution.
• - Can such performance be spoiled by ageing ?

E. Scapparone

RPC2003
2
By using the gas mixture C2F4H2(90) Sf6 (5)
Isobutane(5),
tests performed over 200 pads give
lt e gt (99.6 0.5) lt st gt (62 6) ps
at 12.5 kV
Example(not the best pad, but just randomly
selected to perform gas mixture test)
Efficiency()
st(ps)
Comfortable streamer free plateau
HV(kV)
Can MRPC performance be further improved by
changing gas mixture ?
3
1) SF6 variation

Efficiency()
By increasing the SF6 fraction plateau
moves to higher HV
E. Scapparone

RPC2003
4
SF6 variation
s(ps)
- A fraction of SF6 below 5 spoils the
resolution - A large fraction does not help
E. Scapparone

RPC2003
5
Why ? Just a streamer production effect ? .look
at TOT ( ? Q)
SF6 0
SF62
9
HV 13 kV
T(ns)
T(ns)
SF65
Resolution spoiled for SF60 and for SF62 . At
SF60 we see some streamers but for SF62 there
is no evidence for them.. more sophisticated
mechanism at work, under investigation.
T(ns)
Conclusion we need Sf6 and 5 looks a good
choice
E. Scapparone

RPC2003
6
2) ISOBUTANE variation
Efficiency()
( iso 5)
s(ps)
HV(kV)
Isobutane not required.just add a little bit
more SF6
E. Scapparone

RPC2003
7
Try other quenching gas
3) Sf6 and Isobutane replaced by CO2
Efficiency()
s(ps)
Not a good candidate.time resolution spoiled
E. Scapparone

RPC2003
8
4) C5 F4 H2 C5 F5 H
s(ps)
No appreciable difference in efficiency plateau,
but significant degradation of the time resolution
E. Scapparone

RPC2003
9
Conclusion the standard mixture
C2F4H2 (90) SF6 (5) Isobutane(5), giv
es very good results.

• Similar results without Isobutane ( just
  increasing SF6 at 7 )
• We need SF6 ( 5 )
• C2F4H2 works better than C2F5H
• CO2 ?.no, thanks!

E. Scapparone

RPC2003
10
Ageing

• but
• MRPC work in avalanche mode ( Total charge Q 2
  pC )
• Charge increases slowly with HV ( factor 5 for 2
  kV change)
• small gap distance ( 250 mm)streamers ( if any
  ) are small
• large electrodes area (safer for erosion)

E. Scapparone

RPC2003
11
GIF results
two MRPC strips irradiated at GIF


1 week
Time(min)
7 109 events /cm2 _at_ 50 Hz/cm2 T1.4 108 sec
1620 days ( 54 years, running Alice at 30
days/year at full luminosity)
Total charge 0.014 C/cm2 Total charge 2
pC
12
GIF results
No current increase
No current increase
I (mA)
Source on
Source on
Source off
Source off
E. Scapparone

RPC2003
13
Chemical analysis (Chromatography) of the
outgoing
gas
from
both
MRPCs
(CH1, CH2) by CERN EST/SM
-
CP
measured concentration of Fluorine under the
limit of


- Active detector volume is 2 of the total
volume of the gas box - Diffusion for the gas
exchange between strip and the surrounding gas.

• No sign of degradation

- No increase of dark current
- No degradation in efficiency
- No degradation in time resolution
E. Scapparone

RPC2003
14
Other 2 strips now exposed at GIF already 0.01
C/cm2 collected. We exposed these strips to
PS-T10 beam last week.
E. Scapparone

RPC2003
15
Conclusions The gas mixture we use C2F4H2(90)
Sf6 (5) Isobutane(5) , nicely matches our
requirements in terms of efficiency and time
resolution. CO2 and C2F5H spoil the time
resolution. Isobutane could be eliminated.
Ageing is not a problem for our MRPC. After 1620
days equivalent no evidence for current increase,
no HF formation, no spoiling of efficiency
and/or time resolution).
E. Scapparone

RPC2003