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Failure modes of resistive plate chambers
Outline
• Resistive Plate Chamber (RPC) operation
• Mechanical tolerances
• Failures due to resistivity changes – eg Oil
bridges
• Aging in new production BaBar RPCs
• Malter effect
• Water in glass RPCs
• Conclusions
LC Santa Cruz 1 David Strom – UO
RPC operation
• Number of electrons at the
head of shower is given by
Muon pads
ne = eα - HV
Bakelite
where α is the Townsend coef-
ficient (depends on gas and E) -
Ar
and is the shower length - -
E= 40,000 V/cm
Ar Ar
- -
• Streamer mode (space charge Ar Ar
- -
Ar Ar
dominated discharge) occurs
- - - - - - - -
when
α 20 ⇒ ne = 5 × 108 Bakelite
• Streamer is limited in part by
pads
the high resistivity of the bake-
lite
LC Santa Cruz 2 David Strom – UO
Typical gas mixture
• Argon to provide for efficient gas amplification
• Isobutane (or another hydrocarbon) to absorb UV photon
• Freon ( e.g. 134a , C4H2F4 ) ”quench gas”, controls charge and
physical size of streamers
• The detectors will operate over a very wide range of these gases.
• The Isobutane fraction can be as low as 4%
Caution: flammable mixtures easily produced, especially at low 134a
fractions!
• Streamer production relatively tolerant to N2, O2 and H2 O contam-
ination
LC Santa Cruz 3 David Strom – UO
• The ratio of Ar/134a can vary
from 10 to 0.25
• Streamer charge and size
(area is in mm2) increase with
Ar fraction.
2001 NSS, Onodera, et al.
• Charge distributions of
streamers is relatively narrow 6000
All Strips
4000
2000
0
0 100 200 300 400 500 600 700 800 900
• Fraction of double streamers Triplet charge (pC)
600
small 150 Strip 10 Strip 15
100 400
50 200
0 0
200 400 600 800 200 400 600 800
Triplet charge (pC) Triplet charge (pC)
• Charge distributions of Strip 20
1000
Strip 25
800
750
avalanches exponential in 600
400 500
200 250
parallel plate geometry 0
200 400 600 800
Triplet charge (pC)
0
200 400 600 800
Triplet charge (pC)
LC Santa Cruz 4 David Strom – UO
Bakelite (or glass) resistivity controls time needed (typically millisec-
onds) to rebuild field after a streamer occurs
In BaBar bakelite was required to have
ρ = 28 − 120 × 1010Ωcm
at 20◦ C. Resistivity of bakelite varies substantially with both humid-
ity and temperature. Higher resistivities can be used for cosmic ray
detectors.
The temperature effect is large:
∆ρ/ρ ∼ −10%/ ◦C
It is speculated that at high temperature streamers lower values of ρ
can lead to large discharges and significant aging of the detectors.
LC Santa Cruz 5 David Strom – UO
Mechanical Tolerances
• Townsend coefficients
rapidly increase with electric
field (from Imonte simula-
tion)
• If gap width increased,
Townsend coefficient de-
creases faster than streamer
length increases
• Chamber becomes ineffi-
cient when α < 20
• This analysis courtesy of
C. Lu, Princeton
LC Santa Cruz 6 David Strom – UO
Basic result:
dV
2300V/mm
d gap
In Babar a few ”popped buttons” (unglued spacers) can easily lead to
a 3mm gap width rather than the nominal 2 mm width.
• To avoid excess aging chambers should be kept no more than 500 V
above streamer threshold
⇒ mechanical tolerance of only 200 µm
LC Santa Cruz 7 David Strom – UO
Problems associated with linseed oil coating
• 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.
• The linseed oil is thought to provide two functions:
• It makes a smooth inner surfaces leading
to a more uniform electric field
• It can absorb UV photons produced in the
avalanche
• Main advantage of glass RPCs is that they avoid this coating
LC Santa Cruz 8 David Strom – UO
Efficiency History
Babar problems Average RPC Efficiency
1 Barrel
Possibly due to linseed oil 0.8
bridges 0.6
0.4 RPCs with eff ≥ 10%
All RPCs
• Temperature rose to 36◦ C in
0.2
0 100 200 300 400 500 600 700 800
June Jan. July Jan. July
the experimental hall 1
1999 2000
Forward Endcap
2001
• Currents increased 0.8
0.6
⇒ Many chambers temporarily 0.4
0.2
disconnected 0 100 200 300 400 500 600 700 800
June Jan. July Jan. July
1999 2000 2001
• Efficiency can be increased by 1 Backward Endcap
0.8
lowering amount of Freon 0.6
0.4
⇒ See 200 and 420 days 0.2
0 100 200 300 400 500 600 700 800
• But efficiency still declines con- June
1999
Jan.
2000
July Jan.
2001
July
Henry Band
tinuously
LC Santa Cruz 9 David Strom – UO
Efficiency Maps
• Inefficiency appears to be mainly
concentrated around edges of
the chambers
• There is some evidence that the
efficiency also occurs near the
rows of spacers
• High voltage plateau’s become
very broad
LC Santa Cruz 10 David Strom – UO
Efficiency Plateaus
During original testing After operation in BaBar
LC Santa Cruz 11 David Strom – UO
Test Stand Studies
• Can we reproduce the problems
in the lab?
• SLAC test stand shows that
trigger chambers made prior to
the BaBar production are sensi-
tive to heat.
• Other tests (e.g. at Oregon)
show that damage can be done
to chambers at temperatures of
only 28◦ C
⇒ Problems could occur even
at moderate temperatures!
LC Santa Cruz 12 David Strom – UO
David Strom – UO 13 LC Santa Cruz
RPC has so low resistivity
Unclear why oil removed from bad
Princeton and measurements from SLAC
(removed from bad RPC )
0.21 uncured oil
(production oil)
7.7 uncured linseed
14.4 uncured US linseed oil
(cured in air for 3 days) linseed oil columns:
27.9 US linseed oil
• A model of high and low resistivity
(cured in air for 30 days)
42.3 US linseed oil
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(skin/oil mix) R bak ¡¡¡¡¡¡ ¡¡ ¡¡ ¡ ¡ ¡ ¡ ¡¡ ¡ ¡¡ ¡ ¡ ¡¡¡
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Vgap R gap
[109 Ωcm]
resistivity Sample
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R bak
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Bakelite
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if contaminants are present HV
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seed oil:
depends on how it has cured and depend on the resistivity of the lin-
⇒ The resistivity of linseed oil • Effects of linseed oil columns will
Materials Studies and Models
Experience with prototypes for endcap replacement
Efficiency (no beam) for layer 18
• 24 endcap modules (12 chambers) were
prototypes
replaced with prototypes 12/00
Efficiency
Efficiency
• The prototype chambers have a single
1 1
coat of 30% linseed oil, 70% pentane.
• Inner surface of opened chambers 0.8 0.8
smooth
0.6 0.6
• Some damage seen in one of two cham-
bers heated in test stand
0.4 0.4
⇒ Thinner linseed oil surface more sen-
sitive to dust, contamination 0.2 0.2
• Modules in the shallow layers of the
0 0
detector have stable, good efficiency 100 200
Days of operation
100 200
Days of operation
• Modules in the deepest layer of the calorimeter show significant
damage after ∼ 120 days of operation.
LC Santa Cruz 14 David Strom – UO
• The layer 18 prototypes were exposed
to high levels of background from beam
processes.
350
Current (microamps)
• Since detailed monitoring began, the
Measured current
300
charge through the gas has grown linearly Predicted current
with time. 250
• The decline in efficiency started 200
at about 120 days corresponding to
150
∼500 C/m2 (∼ 108 streamers/cm2 ) in
100
the gas.
• A model which takes the temperature 50
of the leakage current into account and
which assumes that 0
140 160 180 200 220 240 260 280
Day of operation
Ileakage ∝ Qgas
describes the data well.
Predicted and measured current
at injection.
⇒ Can this model explain the
decline in efficiency?
LC Santa Cruz 15 David Strom – UO
Voltage 7250 V
• Water vapor (70% relative
current in chamber 6 (microamps)
humidity at 20◦) was added 100
to the gas of test stand 80
chamber 6 on day 528. Rate 60
was nominally 1 cm3/min,
40
but was much lower for
chamber 6 because it is 20
high rate of water
somewhat leaky. 0
524 526 528 530 532 534 536
days
• On day 529 a high rate
of gas was flown through 1
Efficiency
water added to gas
chamber 6 0.8
Ch 4
high rate
(flow rate off-scale on flow Ch 5
Ch 6
meters, ∼ 15 cm3/min) 0.6
• Current immediately de- 0.4
creased in 6 0.2
• Efficiency immediately
0
improved in 6 524 526 528 530 532 534 536
days (Oct 1 = 1)
LC Santa Cruz 16 David Strom – UO
Discussion
The observed behavior of chamber 6 is consistent with the Malter effect:
- HV
Graphite
Bakelite
e- e-
Oil
+ + +
Insulating Ar Ar Ar
skin
Ar ions Ar E
- -
• Chamber current locally depletes charge carriers in linseed oil skin
LC Santa Cruz 17 David Strom – UO
• Ions collect on the insulating linseed oil surface
• Accumulated ions will produce a large electric field across the linseed
oil surface
• Electrons can then be accelerated into the gas volume where avalanches
are produced (Malter Effect)
• The large current from Malter electrons keeps the gap voltage below
streamer threshold. A large current and inefficiency is observed
LC Santa Cruz 18 David Strom – UO
• Adding water vapor to the gas decreases the surface resistivity of
the linseed oil and prevents the accumulation of ions
• The Malter Effect also explains a common phenomena observed
with many chambers: when the chambers are first switched on their
efficiency decreases and the current increases
• The increased current occurs as the ions collect on islands of insu-
lator on the linseed oil surface causing the Malter Effect
• As the chambers become drier, these islands become larger due to
the depletion of ion conductivity (see Jerry’s Notebook).
• On 2 of 3 chambers tested, the water had no effect
LC Santa Cruz 19 David Strom – UO
Glass RPCs
• ”Float glass” has resistivity of roughly 1012 Ω cm, comparable to the
higher resistivity bakelite
3
10
Volume resistivity (10 W cm)
12
2
10
10
1
-1
10
-10 0 10 20 30 40 50 60
o
Temperature ( C)
Hoshi, et al
LC Santa Cruz 20 David Strom – UO
• In the Belle experiment, it was found necessary to control any mosture
in the glass very tightly. Reportedly
- Water can combine with Fluorine which forms in the
streamers to produce 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 be-
low streamer threshold.
LC Santa Cruz 21 David Strom – UO
Gas without freon 134a can be used (Hoshi et al.,) eg:
4% isobutane, 10% O2, 10% Ar and 76% CO2 has 90% efficiency
instead of 95% for freon based mixtures.
10
percent butane
9 CO2
8
Caution: a simple analysis based 7
on adiabatic flame temperatures 6 0.76 CO2+ 0.10 Ar+ 0.10 O2
and complete combustion indi- 5
N2
cates that this mixture may still 4
be flammable. 3
2
1
0
0 10 20 30 40 50 60 70 80
fraction inert+02
LC Santa Cruz 22 David Strom – UO
Conclusion
• Results are mixed for large scale deployment of RPC
• Detectors are relatively inexpensive, but are not ”easy to build” –
careful QA/QC needed during production
• Double gap chambers are more robust against failure
• Must be able to replace faulty RPC chambers during the lifetime of
the experiment.
LC Santa Cruz 23 David Strom – UO
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