Transcript GRPC_OPERA - Indico

Glass Resistive Plate Chambers
in the OPERA experiment
A.Candelaa, M.D’Inceccoa, A.Di Giovannia, N.Di Marcoa, C.Gustavinoa, M.Lindozzia, S.Micanovicb, D.Orlandia, M. Stipcevicb, E.Tatanannia, I. Zambonib
a) Laboratori Nazionali del Gran Sasso-INFN,Assergi (L’Aquila),67010 Italy
b) Rudjer Boskovic Institute (IRB), Zagreb, Croatia
Oscillation Project with Emulsion tRacking
Apparatus: an hybrid detector
The VETO system of the OPERA
experiment
The CNGS (CERN Neutrinos to Gran Sasso) long baseline
project is focused on the appearance of ντ in a νμ beam.
Its objective is to prove explicitly the νμ−ντ nature of the
atmospheric oscillation. The OPERA experiment is placed
in the Gran Sasso underground laboratory, 730 km from
CERN. The CNGS beam is a wide band neutrino beam
optimized for ντ appearance with a mean neutrino energy
of 17 GeV. The OPERA experiment is based on the direct
observation of the ντ decay topology. The basic target unit
is a brick of 10.2 x 12.7 x 7.5 cm made of 56 lead plates (1
mm thick) and 57 emulsion films (about 200 000 bricks in
total). The emulsion films are made of a plastic base with
two emulsion layers of 45 μm. After emulsion film scanning
performed by automated microscopes, charged tracks will be reconstructed allowing for vertex
and decay kink finding. Interspersed between the target bricks stacked in vertical walls, planes
of scintillator strips [1] allow to localize on line neutrino interactions and select the corresponding
bricks. There are two target blocks of 31 brick planes interleaved with 31 scintillator planes. To
identify muons and measure their momentum and charge to fight charm background, each target
block is followed by a spectrometer, composed of a dipolar magnet. It is equipped with 22 planes
of Resistive Plate Chambers (RPC) with bakelite electrodes. The measurement of muon momentum
is complemented by 6 sections of drift tubes for precision tracking through the magnetic field.
During data taking, the bricks tagged by electronic detectors will be daily extracted by two
automated manipulators. After film development, scanning stations will operate quasi on-line to
fully complete data acquisition with the vertex location and the search of τ decay topology.
Resistive Plate Chamber are gas detector
composed of two parallel plates generating a
uniform electric field. When a particle ionizes
the gas between the electrodes, an avalanche
process occurs. When the electric field is
intense enough, the avalanche reaches the
critical size and generates the streamer, a
plasma filament between the electrodes,
producing an intense current pulse.
Spacer
Glass electrode
Resistive coating
HV insulator
Read-out copper strip
The Glass RPC construction
The glue is applied by digital The glue is placed on the
dispenser
glass
Resistive Plate Chambers or RPC
Polycarbonate frame
The VETO system is installed to reduce the
extraction and analysis of bricks, in which fake
events are generated by the interactions of
neutrinos with the rock and the concrete around
the OPERA detector. The VETO is based on the
use of glass RPCs (GRPC). It consists of two
1004X923 cm2 GRPC layers, each one made of 32
chambers organized into 8 raws. Each raw has 4
chambers, three of them are 2.60 m long and 1.14 m
high, while the fourth is of 2.40 × 1.14 m2. The
VETO planes are equipped with horizontal and
vertical copper strips, 3 cm wide, to pick-up the
signals produced by crossing particles. The Front End Boards (FEB) collect and
transmit the signals to the data acquisition system.
Spacer positioning on the
glass
The glue is applied over
the spacers
Superimposition of the
glass layers
“Vacuum gluing” (24 hours)
Gas inlet and polycarbonate The detector is ready!
frame for detector sealing
The gap of the detector
before the sealing
In the VETO RPC the electrodes are layers of float glass 3 mm thick,
with a bulk resistivity of about 5 x 10^12 Ωcm @ 20˚C. The HV is
supplied to the electrodes by means of a new resistive coating (surface
resistivity ~ 400 kΩ/□).
The distance between the spacers has been chosen
taking into account the requirement of a minimal dead
area and the effect of the deformation produced by the
electric field and the gas pressure. In the VETO GRPC
the spacers are 20 cm apart (0,25% dead area, maximal
glass sagitta ~10 microns) [2].
Water Vapour Monitoring System
Total charge is a function of the voltage. A large increase of the
charge is observed when C2H2F4<48%, Iso-C4H10<4% because of the
larger number of the afterpulses. Best performances are reached
for Argon/iso-C4H10/C2H2F4/SF6 = 47/4/48/1% [3].
NB: to easily compare the behaviour of different gas mixtures, the
voltages are rescaled in such a way the efficiency curves can be
superimposed with Vknee=±3.9 kV.
The VETO gas system is in stainless steel and tygon r2075 pipeline to minimize the
water vapour content in the gas flowing through the detectors. Each wall has a
distribution system with 8-fold parallel channels, each one equalized by a flow resistor
(needle). Its impedance is about 300 times larger than that of four GRPC string. The
relief bubbler protects the detectors from overpressure. At the beginning and at the
end of each line, hygrometers (HiH-4602-C) are read out by a field point system.
hygrometer
Flow resistor
Test on Glass RPC: radiography of the chambers
(1)
(2)
(3)
(4)
The quality controls on the GRPCs of the OPERA experiment
are made in a dedicated external hall at the LNGS [4].
Mechanical properties are first investigated in a fully
automatized station, measuring the gas leakage and checking
the gluing of the spacers. Then the response uniformity is
tested, measuring the efficiency on the entire detector with
cosmic rays. The test program is divided into three steps:
- Average efficiency (1) and counting rate (2) as a function of
gap voltage;
- local RPC efficiency at the nominal working voltage (3): the
position of the spacers is apparent.
Dg(mm) @ 1/tg(α) De(%)
De(rms)= 20%
Dg(rms)@40mm
(Dg/g@2%)
As the electric field is inversely proportional to the
gap (E=V0/g), it is possible to estimate the gap
uniformity by measuring the local efficiency well
before the working point (4). As shown in the figure,
the uniformity of the gap is limited by the float glass
technology, at the level of 2%.
GRPC
Exhaust bubbler
Relief bubbler
Water vapour contamination in the gas mixtures
plays the main role in the GRPC ageing [5], and
must be kept at the level of hundred ppm or
below. To control the contamination, a humidity
monitoring system has been developed. It
consists of 32 hygrometers (organized as shown
in the figure above). The sensors have a
sensitivity of about 10 ppm. Their calibration is
performed with the setup shown in the figure.
The plot shows the response of all the 32
sensors after the insertion of the plastic tube:
it is apparent how the humidity “cloud”
propagates from the first to the last sensor.
References
[1] Y. Gornushkin et al., The target tracker detector for OPERA experiment,
Nuclear Science Symp. 2004 IEEE, 126 - 129 Vol. 1
[2] A. Di Giovanni et al., The VETO system of the OPERA experiment,
Nucl.Phys.Proc.Suppl.158:40-43,2006
[3] C. Gustavino et al., Performance of glass RPC operated in streamer mode with
four-fold gas mixtures containing SF6, NIM A 517 (2004) 101-108
[4] C. Gustavino et al., The OPERA cosmic ray test facility at the Gran Sasso,
NIM A 533 (2004) 221–224
[5] A. Candela et al., Ageing and recovering of glass RPC, NIM A 533 (2004) 116-120
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