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Resistive plate chamber performance in the BaBar IFR system
Resistive plate chamber performance in the BaBar IFR system
F. Anulli
2
, S. Bagnasco
3
, R. Baldini
2
, H.R. Band
11
, R. Bionta
7
, J.E. Brau
10
, V. Brigljevic
7
, A. Buzzo
3
,
A. Calcaterra
2
, M. Carpinelli
5
, T. Cartaro
4
, N. Cavallo
4
, G. Crosetti
3
, R. de Sangro
2
, G. De Nardo
4
,
A. Eichenbaum
11
, F. Fabozzi
4
, D. Falciai
2
, F. Ferrarotto
6
, F. Ferroni
6
, G. Finocchiaro
2
, F. Forti
5
, R. Frey
10
,
C. Gatto
4
, E. Grauges
10
, M. Iwasaki
10
, J.R. Johnson
11
, D.J. Lange
7
, L. Lista
4
, M. Lo Vetere
3
, C. Lu
8
,
M. Macri
3
, R. Messner
9
, T.B. Moore
12
, S. Morganti
6
, N. Neri
5
, H. Neal
12
, A. Palano
1
, E. Paoloni
5
,
P. Paolucci
4
, S. Passaggio
3
, F. Pastore
3
, P. Patteri
2
, I. Peruzzi
2
, M. Piccolo
2
, D. Piccolo
4
, G. Piredda
6
,
E. Robutti
3
, A. Roodman
9
, A. Santroni
3
, C. Sciacca
4
, N.B. Sinev
10
, D. Strom
10
, A. Soha
9
, S. Tosi
3
,
J. Vavra
9
, W.J. Wisniewski
9
, D.M. Wright,
7
, Y. Xie
2
, A. Zallo
2
1
Universit`a di Bari, Dipartimento di Fisica and INFN, I-70126 Bari, Italy ,
2
Laboratori Nazionali di Frascati dellINFN, I-00044 Frascati, Italy ,
3
Universit`a di Genova, Dipartimento di Fisica and INFN, I-16146 Genova, Italy ,
4
Universit`a di Napoli Federico II, Dipartimento di Scienze Fisiche and INFN, I-80126 Napoli, Italy ,
5
Universit`a di Pisa, Scuola Normale Superiore, and INFN, I-56010 Pisa, Italy ,
6
Universit`a di Roma La Sapienza, Dipartimento di Fisica and INFN, I-00185 Roma, Italy,
7
Lawrence Livermore National Laboratory, Livermore, CA 94550, USA,
8
Princeton University, Princeton, NJ 08544, USA ,
9
Stanford Linear Accelerator Center, Stanford, CA 94309, USA ,
10
University of Oregon, Eugene, OR 97403, USA ,
11
University of Wisconsin, Madison, WI 53706, USA ,
12
Yale University, New Haven, CT 06511, USA
Abstract
The BaBar Collaboration has operated a system covering
over 2000 m
2
with resistive plate chambers for nearly three
years.
The chambers are constructed of bakelite sheets
separated by 2 mm.
The inner surfaces are coated with
linseed oil. This system provides muon and neutral hadron
detection for BaBar.
Installation and commissioning were
completed in 1998, and operation began mid-year 1999.
While initial performance of the system reached design,
over time, a signicant fraction of the RPCs demonstrated
signicant degradation, marked by increased currents and
reduced efciency.
A coordinated effort of investigations
have identied many of the elements responsible for the
degradation.
I. I
NTRODUCTION
The BaBar instrumented ux return (IFR) contains over
2000 m
2
of resistive plate chambers (RPCs) to provide muon
and neutral hadron detection. The system, composed of 806
modules, is described in detail elsewhere [1] and is depicted in
Figure 1. Both planar and cylindrical chambers are deployed.
The planar RPC structure is illustrated in Figure 2.
These
chambers are constructed of bakelite sheets separated by
rows of 2 mm thick polycarbonate spacers with cylindrical
symmetry and berglass frame.
The inner surfaces of the
bakelite were treated with linseed oil for surface smoothing
and UV absorption. In the original BaBar RPC production at
General Tecnica[2], the inside of each chamber was coated
three times with a 70% oil, 30% pentane mixture. The bakelite
is selected to have bulk resistivity of 10
11
to 10
12
cm. The
external surfaces of the bakelite are coated with graphite with
surface resistivity of 100k/ .
The RPCs operated in
limited streamer mode, at voltages below 8kV. The gas in
the chambers is 4.5% isobutane, with the balance a mixture
between Ar and Freon R134a (C
2
H
2
F
4
). Signals are read out
capacitively, on both sides of the gap, by external electrodes
made of aluminum strips on a mylar substrate.
The iron is segmented into 18 plates, giving a total thickness
of 65 cm in the barrel and 60 cm in the end caps. A novel
feature of the BaBar detector is the graded segmentation of
the iron, which varies from 2 to 10 cm, increasing with the
radial distance from the interaction region. This segmentation
is the result of detailed Monte Carlo studies which have shown
that muon identication at low momentum and K
0
L
detection
improve, for a given amount of absorber, as the thickness of the
iron plates decreases. Another important feature of the BaBar
detector is the use of single gap RPC. This optimized the K
0
L
detection and efciency for low momentum muons, at the cost
of increased sensitivity to the efciency of single RPC gaps.
In the following the problems seen in the original BaBar
system (section II. ) are described.
Dedicated studies using
test stands (section III.) and studies of the detector materials
(section IV.) are then discussed.
Finally our remediations
efforts (section V.) and plans for endcap chamber replacement
and upgrade (section VI.) are reviewed. Barrel
342 RPC
Modules
432 RPC
Modules
End Doors
19 Layers
18 Layers
BW
FW
3200
3200
920
1250
1940
4-2001
8583A3
Fig. 1 Overview of the IFR system.
Aluminum
X Strips
Insulator
2 mm
Graphite
Insulator
Spacers
Y Strips
Aluminum
H.V.
Foam
Bakelite
Bakelite
Gas
Foam
Graphite
2 mm
2 mm
8-2000
8564A4
Fig. 2 Cross section of a planar RPC.
II. I
NITIAL
P
ERFORMANCE
Initial performance of the system was good. All planar RPCs
were tested shortly after construction and before shipment from
Italy to SLAC, and again prior to installation at SLAC. Plateau
curves were good, efciencies were high, and dark currents
were low. All chambers had dark currents below 9 µA/m
2
at the
voltage producing 90% efciency. At operating voltage, almost
all chamber efciencies were above 96%.
The history of the chamber efciency versus time is shown
in Fig. 3. During initial operation in the summer of 1999, the
system reached unanticipated temperatures of 30 C external,
and even higher within the iron gaps containing the RPCs.
Currents quickly rose and exceeded the capacity of the high
voltage system, requiring many chambers to be disconnected
between July and October of 1999.
A cooling system was
installed to restore temperature control. The cooling system
reduced the dark currents so that almost all of the chambers
could be operated again, however, the dark currents at 20 C
were now much higher.
In addition, the average chamber
efciency had fallen to 80%.
Starting in 2000, the amount of Freon 134a was reduced
from 45% to 40% and nally to 35% in order to increase
the efciency. This allowed a higher efciency to be obtained
at lower operating voltages as can be seen from the increases in
efciencies in January and August of 2000. Despite the changes
to the gas, the overall efciencies continued to decline linearly
as can be seen from the gure.
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
Fig. 3 Efciency versus time. The increases in January and August
of 2000 are due to changes in the gas. The variation in the chambers
with efciency below 10% in backward endcap in 2001 are due to
interventions designed to test remediation strategies.
Detailed efciency maps of the individual BaBar chambers
showed that much of the efciency loss was primarily at the
edges of the chamber and possibly near the rows of spacers as
shown in Fig. 4. Investigations in dedicated laboratory tests
described below showed that these were areas where excess
linseed oil tended to collect. In some other circumstances, the
excess linseed oil forms droplets that grow into columns which
provide a conducting path between the two bakelite plates of
the chambers. This locally reduces the voltage across the gap
leading to chamber inefciency.
III. D
EDICATED LABORATORY INVESTIGATIONS
In early 2000 several dedicated studies were launched to
understand the cause of the efciency decline in the BaBar
chambers.
In the rst studies at SLAC, 6 chambers which
had been originally used to measure background at the BaBar
interaction region, were heated to 36 C as shown in Fig. 5.
The current drawn by the chambers was expected to increase
as the resistivity of the bakelite sheets increase by 5%/ C.
However, in addition to this effect, the current was shown
to steadily increase even though the temperature was held
constant at 36 C. After the chambers were returned to room
temperature, the currents were permanently higher and the
efciency reduced as shown in Fig. 5.
Sub