Abstract
The UA2 experiment at CERN is currently upgrading its apparatus to take advantage of the higher accelerator luminosity to be provided from November 1987. The first large particle tracking detector using scintillating optical fibres will form a major part of this upgrade. The detector is built by the Cambridge, CERN and Saclay groups. The Scintillating Fibre Detector is a cylinder of radius 38 to 44 cm, composed of 60000 plastic optical fibres of length over 2 m. The 1 mm diameter fibres are optically clad and aluminised, and are arranged in 24 layers. The detector will be used to track charged particles produced in pp interactions, and also contains a lead converter to cause neutral and charged electromagnetic particles to shower. The signals from the fibres are amplified, multiplexed and digitised by 32 readout chains. These consist of: • —A large aperture, de-magnifying image intensifier, providing a gain of ∼ 40000. Extremely fast phosphors are employed for the three stages, to match the short time (4 ps) between particle interactions; • —Charge-coupled devices (CCDs) converting the optical image into a single train of electrical pulses for 2000 fibres. A novel use of the anti-blooming electrode of the CCD enables unwanted images to be cleared in <1 ps; • —A Fastbus digitiser, which processes the video signal, providing a large degree of data reduction. A full-scale prototype of 960 fibres with readout has been tested in a particle beam. It has demonstrated a good spatial resolution of the tracks (< 0.20 mm), and an excellent discrimination between electrons and hadrons based on the light detected after the converter.
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