120
H. J. Hilke and W. Riegler
velocities and has proven to be capable under certain conditions to avoid or even
to etch away deposits, in particular in the presence of minute Si impurities. But
its aggressive radicals may also etch away chamber components, especially glass
[15]. In any case the water content has to be carefully controlled to stay below
0.1%, if CF 4 is used, to avoid etching even of gold-plated wires. Also DME,
offering low diffusion, has in some cases provided long lifetime. It has, however,
shown to attack Kapton and to be very sensitive to traces of halogen pollutants at
the ppb level.
• During production, high cleanliness has to be observed, e.g. to avoid resistive
spots on the cathodes. The sense wire has to be continuously checked during
wiring to assure the required quality of its geometrical tolerances and of the gold
plating.
• The gas amplification should be kept as low as possible.
• In any case, a final detector module with the final gas system components should
be extensively tested under irradiation. As an accelerated test is usually required
for practical reasons, to obtain the full integrated charge for some 10 years of
operation in a 1 year test, an uncertainty un- fortunately will remain, because a
rate dependence of the ageing cannot be excluded.
4.3 Detector Designs and Performance
4.3.1 Single Wire Proportional Tubes
Despite the revolution started with the multiwire proportional chambers (MWPC),
single wire tubes are still widely used, mostly as drift tubes. They have circular or
quadratic cross-section and offer independence of the cells, important, e.g., in case
wire rupture. We present three examples.
ATLAS has chosen for the muon system large diameter (3 cm) aluminum tubes
operated with Ar/CO 2 (93/7%) at 3 atm, with the addition of about 300 ppm of
water to improve HV stability. A pair of 3 or 4 staggered tube layers, separated
by a support frame, form a module. The disadvantages of the gas mixture, a nonlinear space-drift time relation and relatively long maximum drift time, had to be
accepted in order to obtain a high radiation tolerance. The spatial resolution for a
single tube under strong γ - irradiation producing space charge is shown in Fig. 4.16.
An average resolution per tube of 80 μm is expected with a maximum background
rate of 150 hits/cm 2 s. With a relative positioning of the wires during construction
to 20 μm, an adjustment of the tube curvature to the gravitational wire sag and a
relative alignment and continuous monitoring of the pair of layers inside a chamber,
a combined resolution for the 6–8 1ayers of ~35 μm is aimed at. These chambers
provide only one coordinate, the other one being measured in other subdetectors of
the experiment.
H. J. Hilke and W. Riegler
velocities and has proven to be capable under certain conditions to avoid or even
to etch away deposits, in particular in the presence of minute Si impurities. But
its aggressive radicals may also etch away chamber components, especially glass
[15]. In any case the water content has to be carefully controlled to stay below
0.1%, if CF 4 is used, to avoid etching even of gold-plated wires. Also DME,
offering low diffusion, has in some cases provided long lifetime. It has, however,
shown to attack Kapton and to be very sensitive to traces of halogen pollutants at
the ppb level.
• During production, high cleanliness has to be observed, e.g. to avoid resistive
spots on the cathodes. The sense wire has to be continuously checked during
wiring to assure the required quality of its geometrical tolerances and of the gold
plating.
• The gas amplification should be kept as low as possible.
• In any case, a final detector module with the final gas system components should
be extensively tested under irradiation. As an accelerated test is usually required
for practical reasons, to obtain the full integrated charge for some 10 years of
operation in a 1 year test, an uncertainty un- fortunately will remain, because a
rate dependence of the ageing cannot be excluded.
4.3 Detector Designs and Performance
4.3.1 Single Wire Proportional Tubes
Despite the revolution started with the multiwire proportional chambers (MWPC),
single wire tubes are still widely used, mostly as drift tubes. They have circular or
quadratic cross-section and offer independence of the cells, important, e.g., in case
wire rupture. We present three examples.
ATLAS has chosen for the muon system large diameter (3 cm) aluminum tubes
operated with Ar/CO 2 (93/7%) at 3 atm, with the addition of about 300 ppm of
water to improve HV stability. A pair of 3 or 4 staggered tube layers, separated
by a support frame, form a module. The disadvantages of the gas mixture, a nonlinear space-drift time relation and relatively long maximum drift time, had to be
accepted in order to obtain a high radiation tolerance. The spatial resolution for a
single tube under strong γ - irradiation producing space charge is shown in Fig. 4.16.
An average resolution per tube of 80 μm is expected with a maximum background
rate of 150 hits/cm 2 s. With a relative positioning of the wires during construction
to 20 μm, an adjustment of the tube curvature to the gravitational wire sag and a
relative alignment and continuous monitoring of the pair of layers inside a chamber,
a combined resolution for the 6–8 1ayers of ~35 μm is aimed at. These chambers
provide only one coordinate, the other one being measured in other subdetectors of
the experiment.