g in new production BaBar RPCs
Malter eect
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
n
e
= e where is the Townsend coef-
cient (depends on gas and
E)
and
is the shower length
Streamer mode (space charge
dominated
discharge)
occurs
when 20
n
e
= 5
× 10
8
Streamer is limited in part by
the high resistivity of the bake-
lite
- HV
Muon
-
Ar
-
Ar
-
Ar
-
Ar
-
Ar
-
-
Ar
Ar
-
-
-
- - -
-
-
E
pads
Bakelite
Bakelite
pads
= 40,000 V/cm
LC Santa Cruz
2
David Strom UO Typical gas mixture
Argon to provide for ecient gas amplication
Isobutane (or another hydrocarbon) to absorb UV photon
Freon ( e.g. 134a , C
4
H
2
F
4
) 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: ammable mixtures easily produced, especially at low 134a
fractions!
Streamer production relatively tolerant to N
2
, O
2
and H
2
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 mm
2
) increase with
Ar fraction. Charge
distributions
of
streamers is relatively narrow
Fraction of double streamers
small Charge
distributions
of
avalanches
exponential
in
parallel plate geometry
2001 NSS, Onodera, et al.
0
2000
4000
6000
0
100
200
300
400
500
600
700
800
900
All Strips
Triplet charge (pC)
0
50
100
150
200
400
600
800
Strip 10
Triplet charge (pC)
0
200
400
600
200
400
600
800
Strip 15
Triplet charge (pC)
0
200
400
600
800
200
400
600
800
Strip 20
Triplet charge (pC)
0
250
500
750
1000
200
400
600
800
Strip 25
Triplet charge (pC)
LC Santa Cruz
4
David Strom UO Bakelite (or glass) resistivity controls time needed (typically millisec-
onds) to rebuild eld after a streamer occurs
In BaBar bakelite was required to have
= 28 120 × 10
10
cm
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 eect is large:
/ 10%/ C
It is speculated that at high temperature streamers lower values of
can lead to large discharges and signicant aging of the detectors.
LC Santa Cruz
5
David Strom UO Mechanical Tolerances Townsend
coecients
rapidly increase with electric
eld (from Imonte simula-
tion)
If gap width increased,
Townsend
coecient
de-
creases faster than streamer
length
increases
Chamber becomes ine-
cient when < 20
This analysis courtesy of
C. Lu, Princeton
LC Santa Cruz
6
David Strom UO Basic result:
dV
d gap
2300V/mm
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 eld
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 Babar problems
Possibly due to linseed oil
bridges Temperature rose to 36 C in
the experimental hall Currents increased
Many chambers temporarily
disconnected Eciency can be increased by
lowering amount of Freon
See 200 and 420 days But eciency still declines con-
tinuously
Eciency History
Average RPC Efficiency
0.2
0.4
0.6
0.8
1
0
100
200
300
400
500
600
700
800
Barrel
1999
June
Jan.
2000
July
Jan.
2001
July
All RPCs
RPCs with eff 10
%
0.2
0.4
0.6
0.8
1
0
100
200
300
400
500
600
700
800
Forward Endcap
1999
June
Jan.
2000
July
Jan.
2001
July
0.2
0.4
0.6
0.8
1
0
100
200
300
400
500
600
700
800
Backward Endcap
1999
June
Jan.
2000
July
Jan.
2001
July
Henry Band
LC Santa Cruz
9
David Strom UO
Ineciency appears to be mainly
concentrated around edges of
the chambers
There is some evidence that the
eciency also occurs near the
rows of spacers
High voltage plateaus become
very broad
Eciency Maps
LC Santa Cruz
10
David Strom UO Eciency 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 Materials Studies and Models
Eects of linseed oil columns will
depend on the resistivity of the lin-
seed oil:
 ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ 
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 ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ 
 ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ 
 ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ 
 ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ 
¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢
¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢
¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢
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¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢¡¢
£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£
£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£
£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£
£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£
£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£
£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£¡£
¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤
¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤
¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤
¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤
¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤
¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤¡¤
Bakelite
Bakelite
R
R
R
bak
bak
gap
gap
V
HV
A model of high and low resistivity
linseed oil columns:
The resistivity of linseed oil
depends on how it has cured and
if contaminants are present
Sample
resistivity
[10
9
cm]
polymerized US linseed oil
145.9
(skin/oil mix)
US linseed oil
42.3
(cured in air for 30 days)
US linseed oil
27.9
(cured in air for 3 days)
uncured US linseed oil
14.4
uncured linseed
7.7
(production oil)
uncured oil
0.21
(removed from bad RPC )
measurements
from
SLAC
and
Princeton
Unclear why oil removed from bad
RPC has so low resistivity
LC Santa Cruz
13
David Strom UO Experience with prototypes for endcap replacement
24 endcap modules (12 chambers) were
replaced with prototypes 12/00
The prototype chambers have a single
coat of 30% linseed oil, 70% pentane.
Inner surface of opened chambers
smooth
Some damage seen in one of two cham-
bers heated in test stand
Thinner linseed oil surface more sen-
sitive to dust, contamination
Modules in the shallow layers of the
detector have stable, good eciency
Eciency (no beam) for layer 18
prototypes
0
0.2
0.4
0.6
0.8
1
100
200
Days of operation
Efficiency
0
0.2
0.4
0.6
0.8
1
100
200
Days of operation
Efficiency
Modules in the deepest layer of the calorimeter show signicant
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.
Since detailed monitoring began, the
charge through the gas has grown linearly
with time. The
decline
in
eciency
started
at
about
120
days
corresponding
to
500 C/m
2
(
10
8
streamers/cm
2
) in
the gas.
A model which takes the temperature
of the leakage current into account and
which assumes that
I
leakage
Q
gas
describes the data well.
Can this model explain the
decline in eciency?
0
50
100
150
200
250
300
350
140
160
180
200
220
240
260
280
Day of operation
Current (microamps)
Measured current
Predicted current
Predicted and measured current
at injection.
LC Santa Cruz
15
David Strom UO Water vapor (70% relative
humidity at 20 ) was added
to
the
gas
of
test
stand
chamber 6 on day 528. Rate
was nominally 1 cm
3
/min,
but
was
much
lower
for
chamber
6
because
it
is
somewhat leaky. On day 529 a high rate
of gas was own through
chamber 6
(ow rate o-scale on ow
meters,
15 cm
3
/min) Current
immediately
de-
creased in 6 Eciency
immediately
improved in 6
Voltage 7250 V
0
20
40
60
80
100
524
526
528
530
532
534
536
days
current in chamber 6 (microamps)
high rate of water
0
0.2
0.4
0.6
0.8
1
524
526
528
530
532
534
536
Ch 5
Ch 6
Ch 4
days (Oct 1 = 1)
Efficiency
water added to gas
high rate
LC Santa Cruz
16
David Strom UO Discussion
The observed behavior of chamber 6 is consistent with the Malter eect:
Bakelite
- HV
Graphite
Oil
Insulating
skin
e-
e-
Ar ions
E
Ar
-
Ar
+
Ar
+
Ar
+
-
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 eld across the linseed
oil surface
Electrons can then be accelerated into the gas volume where avalanches
are produced (Malter Eect)
The large current from Malter electrons keeps the gap voltage below