4 Gaseous Detectors
123
Fig. 4.18 Design of the first multiwire proportional chamber [8]
Fig. 4.19 Principle of ‘centre of gravity’ cathode measurement of cathode strip signals
per chamber was achieved. The stringent quality demands can be inferred from
the definition of the efficiency plateau: the ‘beginning of plateau’ was defined as
efficiency ε 99.98% and the end by 10 times the ‘normal noise’, corresponding
to cosmics rate With this tight definition, the measured plateau length for a
chamber was 50 − 100 V with the magic gas mixture of Ar/isobutane/freon/methylal
(67.6/25/0.4/7%).
In the following decades, drift chambers imposed themselves more and more, but
MWPCs remained valid options, especially when speed was more important than
high spatial resolution. Thus three of the four LHC experiments employ MWPCs,
for triggering and momentum measurements. All of them make use of the signals
induced on the cathode strips. In ATLAS and CMS, who call their chambers cathode
strip chambers (CSC), a high precision measurement is obtained in the bending
coordinate by determining the ‘centre of gravity’ of the strip signals, see Fig. 4.19.
In ATLAS, each third strip is read out (pitch ~5.5 mm), leaving two strips floating,
123
Fig. 4.18 Design of the first multiwire proportional chamber [8]
Fig. 4.19 Principle of ‘centre of gravity’ cathode measurement of cathode strip signals
per chamber was achieved. The stringent quality demands can be inferred from
the definition of the efficiency plateau: the ‘beginning of plateau’ was defined as
efficiency ε 99.98% and the end by 10 times the ‘normal noise’, corresponding
to cosmics rate With this tight definition, the measured plateau length for a
chamber was 50 − 100 V with the magic gas mixture of Ar/isobutane/freon/methylal
(67.6/25/0.4/7%).
In the following decades, drift chambers imposed themselves more and more, but
MWPCs remained valid options, especially when speed was more important than
high spatial resolution. Thus three of the four LHC experiments employ MWPCs,
for triggering and momentum measurements. All of them make use of the signals
induced on the cathode strips. In ATLAS and CMS, who call their chambers cathode
strip chambers (CSC), a high precision measurement is obtained in the bending
coordinate by determining the ‘centre of gravity’ of the strip signals, see Fig. 4.19.
In ATLAS, each third strip is read out (pitch ~5.5 mm), leaving two strips floating,