Transcript Document

Production and test of the LHCb Muon Chambers
D. Pinci1 and A. Sarti2
on behalf of LHCb
1
http://lhcb.web.cern.ch/lhcb/
Università di Roma “La Sapienza” and INFN sez. Roma1, Italy
2 INFN-Laboratori Nazionali di Frascati, Italy
Abstract
The main goals for the Muon System of LHCb (presentely under construction) is to provide a high-Pt muon trigger at the first experiment trigger level (Level-0) and a very good muon identification in the off-line analysis. The
performance needed for the level-0 trigger results in very stringent requirements on the quality of the Muon System detectors. We report the status of the production of the Multi-Wire Proportional Chambers for the LHCb Muon
System and we describe the test systems for the quality control adopted in the production sites. After wire winding and gluing the wire pitch is measured by means of digital cameras with a precision of about 20 μm. For the
evaluation of the wire mechanical tension two systems were developed which find the wire mechanical resonance frequency. A resolution of about 1 g is obtained in both cases. Once all elements of the chamber are assembled the
gas tightness of the detector is verified by monitoring the decrease of an overpressure applied. After a suitable high voltage conditioning the dark current is recorded and the gain uniformity of each gap is measured by acquiring the
137
currents drawn by four gaps, while scanning the detector surface with a Cs source. Finally, the detectors are equipped with the front end electronics and their detection efficiency and time resolution are tested using cosmic rays.
The Muon System of LHCb
The Multi-Wire Proportional Chambers for LHCb
The chambers in the stations M2-M5 are made by 4 layers (gaps) while the chamber in M1 will comprise only two gaps. Each gap is 5 mm high with an anode plane in
the middle made by 30 μm diameter wire with a pitch of 2 mm. The gaps are filled with an Ar/CO2 /CF4 (40/55/5) gas mixture and will work at a gain of about 5x104.
In M2-M5 the four gaps are
ganged in pairs. In a pair
(double-gap), the signals from
corresponding pad (wire or
cathode) are added together
via a simple wire-OR and sent
to FE electronics. A further
logical OR between the two
double-gaps is performed by
the electronics.
“M3R3” chamber
Muon stations
The LHCb experiment is dedicated to the study of the decays of the
beauty hadron. In many of these decays muons are present in the final
state so that muon triggering and identification is important. The LHCb
Muon System will be composed of 5 stations (for a total active surface of
435 m2) equipped with 1380 Multi-Wire Proportional Chambers of 20
different sizes. The LHCb level-0 trigger, based on a 5-fold coincidence
within 25 ns, requires a fast and precise (< 20%) measurement of the
muon transverse momentum and a high efficiency in the bunch-crossing
identification. This translates in the requirements of a high efficiency
(>99%) in 20 ns and good spatial resolution for the detectors. The LHCb
MWPCs were designed in order to fulfill all those requirements and are
now under construction in different sites: CERN (CH), Ferrara (ITA),
Firenze (ITA), Frascati (ITA) and San Petersburg (RU).
Wire tension meter
The 4
gaps
chamber
time spectrum
The use of a high yielding and fast gas
mixture allows to achieve very good time
performance in proportional mode operation.
Wire pitch measurement
These requirements define
the limits of the HV working
region.
During tests at CERN beams,
a working region width of
about 170 V was found.
Gas leakage test
The wire position is
precisely determined by
the pitch of the wiring
machine combs, however
it is important to check
that no wire is out of the
acceptance.
In order to minimize the gas refill rate, the maximum leakage allowed for
each chamber is 2 mbar/h.
To verify the gas tightness of
a chamber, we inflate it with
nitrogen up to an
overpressure DP of 5 mbar,
with respect to the
atmospheric pressure. Then,
we record the DP decrease
as a function of time, during
about one hour.
The requirement on the wire pitch (WP) is:
WP = 2 mm ± 50 µm (95% of wires) ± 100 µm (5% of wires)
l : mass per unit length of the wire
- efficiency in 20 ns higher
than 95%;
- average pad-cluster size
lower than 1.1;
A detailed numerical calculation of the dependence of the electric field on
the MWPC mechanical parameters (e.g. wire position and mechanical
tension) was performed in order to provide the requirements for the
chamber production.
Typical time-spectrum of a four-gap chamber
shows an rms lower than 4 ns.
The wire mechanical tension must be in the range 50÷90 g in oder to
provide a good electrostatic stability.
The wire tension t can be found by measuring its mechanical
resonance frequency n0 with the formula:
The requirements that each
double-gap must satisfy are:
The measurement is
heavily affected by
variations of the
temperature. In order to
correct this effect, a
second chamber is used
as a reference.
The WP measurement is performed with an
automatic device, based on two TV cameras
scanning the panel and a software for image
acquisition and analysis.
An accuracy of about 20 µm is obtained.
l: length of the wire
To measure n0, mechanical oscillations of the wire to be tested are
induced by applying a periodic high voltage (about 900 V) with a
frequency of 300 ÷ 400 Hz, between this wire (Cw) and a nonoscillating sense (Sw) wire placed parallel and close to it at a
distance d of about 1 mm.
The capacitance C between the two wires is given by:
Measurements of the wire pitch
The measure of the gas leakage, is obtained by correcting the DP(t) behavior
by using the data of the reference chamber.
Sample image from scanning device
The range 2 mm ± 50 µm
corresponds to the red lines
drawn at 211 ± 5 pixels.
where a and b are radii of wires and l is the SW length.
The oscillations
result in a variation
of C.
The maximum
variation of C
occurs at the
mechanical
resonance of the
chamber wire.
Test with cosmic rays
Gap gain uniformity
Uniformity of the gas gain is tested with a
radioactive source. The current drawn by
each gap is monitored while the lead case
containing the source is moved by means of
a mechanical arm.
An alternative method was
developed by the University of
Ferrara, Firenze and Roma “Tor
Vergata”.
Wire oscillations are induced via
a mechanical excitation. A laser
beam, reflected by the wire
under study is recorded by a
photodiode. From a simple
Fourier analysis the resonance
frequency is obtained. This
method allows to measure 1
wire/s.
137Cs
These measurements allow to check the
gain uniformity within each gap and to
compare different chambers among them.
wire
Y position in the gap
Machanical
excitation
Profile view of a MWPC on the
source test table.
Current (nA)
Photodiode
source case (LNF)
400
300
200
100
02/05/05
21/02/05
0
13/12/04
The quality control systems have shown
to operate well and to be able to
provide a fast and reliable feeback on
the quality of the detectors assembled.
500
04/10/04
The chambers are equipped with
the CARIOCA read-out electronics.
Signals are acquired by VME TDCs
allowing efficiency, cluster-size and
time resolution studies.
Scheduled
CERN
Fi
Fe
LNF
PNPI
26/07/04
A trigger system
based on three large
plastic scintillators
gives the trigger
signal and the
reference time with a
jitter of about 2.8 ns.
600
17/05/04
allowed range
rms = 4 ns
About 460 MWPCs have been already
built. In the last six months the
production has been keeping the
scheduled rate.
It should be possible to install all M2-M5
and a sizable part of M1 for 2007 data
taking. The production phase is
foreseen to finish in April 2007.
08/03/04
A resolution of about 1 g is obtained and a panel (660 wires)
can be checked in less than 1 hour.
Production and test summary
Number of assembled chambers
Example of the cosmic ray
illumination of a chamber
The use of a cosmic ray
stand allows to measure
the performances of the
chambers produced.
The system allows to
house and test up to 6
chambers fully equipped
at the same time (600
FEE channels).
Chamber assembly date
The panel winding provides very good quality panels. In average only about 1
wire over 4000 has to be replaced because pitch or mechanical tension were
out from the allowed range.
The measurement of gas leakage showed the required sensitivity. Less than
5% of the chambers had a gas leakage which has been found and fixed in all
cases.
An automated conditioning procedure allows to bring all chambers to a
voltage of 2.9 kV, well above the working point, withuot demage. In all cases
a residual dark current of less than 5 nA per gap was found.
The results of the measurement of the gas gain uniformity on the double-gaps
surface are shown in the plot.
The gain spread defined as the
maximum between Gmax/G0 and G0/Gmin
should be less than 1.4.
No one chamber has shown both the
gaps with a gain spread higher than
that limit.
X position in the gap
Panel
Example of a result of the scan with radioactive source
allowed limit
Conclusions
The systems for the test of the Multi-Wire Proportional Chambers at production sites result to be very useful for the mass production. The performance achieved in the measurement of the wire pitch and wire mechanical tension allows to check, with
the needed accuracy, the quality of the wire winding before chamber assembling. The gas leakage can be measured with the sensitivity required. Once the chambers are closed and conditioned, the study of the gain uniformity inside each gap gives a
fast feedback on the quality of the chamber, allowing to tune the assembly procedure. Test with cosmic rays gives the possibility to study in detail the efficiency and time perfomance of the chambers.
460 chambers have been already assembled and tested and all of them satisfy the requirements on gain uniformity.