The OPAL Tracking Detectors

OPAL's central tracking system consists of (in order of increasing radius) a silicon microvertex detector, a vertex detector, a jet chamber, and z-chambers.

All the tracking detectors work by observing the ionization of atoms by charged particles passing by: when the atoms are ionized, electrons are knocked out of their atomic orbitals, and are then able to move freely in the detector. These ionization electrons are detected in the different parts of the tracking system.

The largest tracking chamber is the central jet chamber, shown in red on the above picture. The ionization caused by a charged particle can be measured at many points along its path, and so its trajectory can be deduced. The curvature of the track in the magnetic field of the detector can also be measured, and from this we deduce the particle's momentum. The amount of ionization that a given particle causes per unit length as it flies through the jet chamber depends on the type and momentum of the particle. This so-called dE/dx ionization energy loss therefore helps in identifying the type of the particle - electron, pion, proton, etc.

Inside the jet chamber lie the central vertex chamber (shown in magenta above) and, innermost, the silicon microvertex detector (shown in cyan). Together these are used to locate decay vertices of short-lived particles, and to improve the momentum resolution.

The vertex and jet chambers are most precise in measuring the co-ordinates of tracks in the plane perpendicular to the beam axis (the bending plane of the magnetic field, known as the "r-phi" plane); they are complemented by special "z-chambers" at the outside edge of the jet chamber, which provide precise measurements of the perpendicular (z) coordinates of the tracks. The z-chambers are shown in red on the above picture: they are the thin rectangles at the outer edge of the jet chamber