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Failure mo des of resistive plate chamb ers Outline з Resistive Plate Chamb er (RPC) op eration з Mechanical tolerances з Failures due to resistivity changes э eg Oil bridges з Aging in new pro duction BaBar RPCs з Malter effect з Water in glass RPCs з Conclusions

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RPC op eration з Numb er of electrons at the head of shower is given by ne = e where is the Townsend co efficient (dep ends on gas and E ) and is the shower length
Ar

Muon

pads
- HV

Bakelite
Ar Ar Ar Ar -Ar -

E

= 40,000 V/cm

з Streamer mo de (space charge dominated discharge) o ccurs when 20 ne = 5 ╜ 108

Ar

Bakelite pads

з Streamer is limited in part by the high resistivity of the bakelite
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Typical gas mixture з Argon to provide for efficient gas amplification з Isobutane (or another hydro carb on) to absorb UV photon з Freon ( e.g. 134a , C4H2F4 ) "quench gas", controls charge and physical size of streamers з The detectors will op erate over a very wide range of these gases. з The Isobutane fraction can b e as low as 4% Caution: flammable mixtures easily pro duced, esp ecially at low 134a fractions! з Streamer pro duction relatively tolerant to N2, O2 and H2 O contamination
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з The ratio of Ar/134a can vary from 10 to 0.25 з Streamer charge and size (area is in mm2) increase with Ar fraction. з Charge distributions of streamers is relatively narrow з Fraction of double streamers small з Charge distributions avalanches exp onential parallel plate geometry of in
2001 NSS, Ono dera, et al.
All Strips

6000 4000 2000 0 0 100 200 300 400 500 600 700

800 900 Triplet charge (pC)

150 100 50 0 200

Strip 10

600 400 200

Strip 15

400 600 800 Triplet charge (pC)

0

200

400 600 800 Triplet charge (pC)

800 600 400 200 0 200

Strip 20

1000 750 500 250

Strip 25

400 600 800 Triplet charge (pC)

0

200

400 600 800 Triplet charge (pC)

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Bakelite (or glass) resistivity controls time needed (typically milliseconds) to rebuild field after a streamer o ccurs In BaBar bakelite was required to have

= 28 - 120 ╜ 1010cm at 20 C. Resistivity of bakelite varies substantially with b oth humidity and temp erature. Higher resistivities can b e used for cosmic ray detectors. The temp erature effect is large: / -10%/ C It is sp eculated that at high temp erature streamers lower values of can lead to large discharges and significant aging of the detectors.
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Mechanical Tolerances з Townsend co efficients rapidly increase with electric field (from Imonte simulation) з If gap width increased, Townsend co efficient decreases faster than streamer length increases з Chamb er b ecomes inefficient when < 20 з This analysis courtesy of C. Lu, Princeton
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Basic result:

dV d gap

2300V/mm

In Babar a few "p opp ed buttons" (unglued spacers) can easily lead to a 3mm gap width rather than the nominal 2 mm width. з To avoid excess aging chamb ers should b e kept no more than 500 V ab ove streamer threshold

mechanical tolerance of only 200 ▓m

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Problems asso ciated with linseed oil coating з Linseed oil coatings of inner surface lower the current drawn through the gas and singles of rates of the detectors by a factor of 5 to 10. з The linseed oil is thought to provide two functions: з It makes a smo oth inner surfaces leading to a more uniform electric field з It can absorb UV photons pro duced in the avalanche з Main advantage of glass RPCs is that they avoid this coating

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Efficiency History Babar problems Possibly due to linseed oil bridges з Temp erature rose to 36 C in the exp erimental hall з Currents increased Many chamb ers temp orarily disconnected з Efficiency can b e increased by lowering amount of Freon See 200 and 420 days з But efficiency still declines continuously
1 0.8 0.6 0.4 0.2 0
June 1999

Barrel

Average RPC Efficiency

RPCs with eff 10% All RPCs 100 200
Jan. 2000

300

400
July

500

600
Jan. 2001

700

800
July

1 0.8 0.6 0.4 0.2 0

Forward Endcap

100

200
Jan. 2000

300

400
July

500

600
Jan. 2001

700

800
July

June 1999

1 0.8 0.6 0.4 0.2 0

Backward Endcap

100

200
Jan. 2000

300

400
July

500

600
Jan. 2001

700

800
July Henry Band

June 1999

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Efficiency Maps з Inefficiency app ears to b e mainly concentrated around edges of the chamb ers з There is some evidence that the efficiency also o ccurs near the rows of spacers з High voltage plateau's b ecome very broad

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Efficiency Plateaus

During original testing

After op eration in BaBar

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Test Stand Studies з Can we repro duce the problems in the lab? з SLAC test stand shows that trigger chamb ers made prior to the BaBar pro duction are sensitive to heat. з Other tests (e.g. at Oregon) show that damage can b e done to chamb ers at temp eratures of only 28 C Problems could o ccur even at mo derate temp eratures!

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з Effects of linseed oil columns will dep end on the resistivity of the linseed oil: HV

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з A mo del of high and low resistivity linseed oil columns:

Bakelite

Bakelite

Materials Studies and Mo dels

R bak

R bak

R gap

Vgap

13
uncured linseed (pro duction oil) uncured US linseed oil US linseed oil (cured in air for 3 days) Sample

RPC has so low resistivity Unclear why oil removed from bad
uncured oil (removed from bad RPC ) measurements from SLAC

The resistivity of linseed oil dep ends on how it has cured and if contaminants are present

US linseed oil (cured in air for 30 days)

p olymerized US linseed oil (skin/oil mix)

David Strom э UO and resistivity [109 cm] 145.9 0.21 14.4 27.9 42.3 7.7 Princeton

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Exp erience with prototyp es for endcap replacement
Efficiency (no b eam) for layer 18

з 24 endcap mo dules (12 chamb ers) were replaced with prototyp es 12/00
Efficiency

prototyp es
Efficiency 100 200 Days of operation

з The prototyp e chamb ers have a single coat of 30% linseed oil, 70% p entane. з Inner surface of op ened chamb ers smo oth з Some damage seen in one of two chamb ers heated in test stand

1

1

0.8

0.8

0.6

0.6

0.4

0.4

Thinner linseed oil surface more sensitive to dust, contamination з Mo dules in the shallow layers of the detector have stable, go o d efficiency
0 0 100 200 Days of operation 0.2 0.2

з Mo dules in the deep est layer of the calorimeter show significant damage after 120 days of op eration.
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з The layer 18 prototyp es were exp osed to high levels of background from b eam pro cesses. з Since detailed monitoring b egan, the charge through the gas has grown linearly with time. з The decline
2
Current (microamps) 350

Measured current 300 Predicted current

250

in
8

efficiency

started
2

200

at ab out 120 days corresp onding to
150

500 C/m the gas.

( 10

streamers/cm ) in
100

з A mo del which takes the temp erature of the leakage current into account and which assumes that Ilea
kage

50

0 140

160

180

200

220

240

Qgas

260 280 Day of operation

describ es the data well.

Predicted and measured current at injection.

Can this mo del explain the decline in efficiency?
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Voltage 7250 V з Water vap or (70% relative humidity at 20) was added to the gas of test stand chamb er 6 on day 528. Rate was nominally 1 cm3/min, but was much lower for chamb er 6 b ecause it is somewhat leaky. з On day 529 a high rate of gas was flown through chamb er 6 (flow rate off-scale on flow meters, 15 cm3/min) з Current immediately decreased in 6 з Efficiency immediately improved in 6
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current in chamber 6 (microamps) 100

80

60

40

20
high rate of water

0

524

526

528

530

532

534

536 days

Efficiency

1
water added to gas

Ch 4 0.8 Ch 5 Ch 6 0.6

high rate

0.4

0.2

0 524 526 528 530 532 534 536 days (Oct 1 = 1)

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Discussion The observed b ehavior of chamb er 6 is consistent with the Malter effect:
- HV Graphite Bakelite Oil Insulating skin Ar ions
ee-

Ar

+

Ar

+

Ar

+

Ar -

E

з Chamb er current lo cally depletes charge carriers in linseed oil skin
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з Ions collect on the insulating linseed oil surface

з Accumulated ions will pro duce a large electric field across the linseed oil surface

з Electrons can then b e accelerated into the gas volume where avalanches are pro duced (Malter Effect)

з The large current from Malter electrons keeps the gap voltage b elow streamer threshold. A large current and inefficiency is observed

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з Adding water vap or to the gas decreases the surface resistivity of the linseed oil and prevents the accumulation of ions

з The Malter Effect also explains a common phenomena observed with many chamb ers: when the chamb ers are first switched on their efficiency decreases and the current increases

з The increased current o ccurs as the ions collect on islands of insulator on the linseed oil surface causing the Malter Effect

з As the chamb ers b ecome drier, these islands b ecome larger due to the depletion of ion conductivity (see Jerry's Noteb o ok).

з On 2 of 3 chamb ers tested, the water had no effect
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Glass RPCs з "Float glass" has resistivity of roughly 1012 cm, comparable to the higher resistivity bakelite
10
3

Volume resistivity (10 W cm)

12

10

2

10

1

10

-1

-10

0

10

20

30

40 50 60 o Temperature ( C)

Hoshi, et al LC Santa Cruz

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з In the Belle exp eriment, it was found necessary to control any mosture in the glass very tightly. Rep ortedly - Water can combine with Fluorine which forms in the streamers to pro duce HF -HF can etche the glass allowing for the adsorption of water onto the glass etch -The water forms a conducting layer which "shorts" the surfaces to nearby spacers, reducing the gap voltage b elow streamer threshold.

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David Strom э UO


Gas without freon 134a can b e used (Hoshi et al.,) eg: 4% isobutane, 10% O2, 10% Ar and 76% CO2 has 90% efficiency instead of 95% for freon based mixtures.

percent butane

10 9 8 7 6 5

CO2

Caution: a simple analysis based on adiabatic flame temp eratures and complete combustion indicates that this mixture may still b e flammable.

0.76 CO2+ 0.10 Ar+ 0.10 O2

N2
4 3 2 1 0

0

10

20

30

40

50

60

70 80 fraction inert+02

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Conclusion

з Results are mixed for large scale deployment of RPC

з Detectors are relatively inexp ensive, but are not "easy to build" э careful QA/QC needed during pro duction

з Double gap chamb ers are more robust against failure

з Must b e able to replace faulty RPC chamb ers during the lifetime of the exp eriment.

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David Strom э UO