Forward Detector (FD)

The forward detectors [28,29] are situated at either end of the detector to measure very low angle particles. The FDs are in fact made up of four separate subdetectors; the main calorimeter (FK), the forward tube chambers (FB), the $\gamma$-catcher (FE) and the far forward luminosity monitor (FF). Their principle purpose ss to measure the luminosity OPAL received by identifying Bhabha events, for which the cross-section is well known.

The main calorimeter is made up of 16 azimuthal segments of lead-scintillator sandwich. Each segment has a presampler of 4 radiation lengths thickness and a main calorimeter of 20 radiation lengths. The azimuthal angular resolution is 2$^{\circ}$ and the energy resolution of FK in units of GeV is:

$$\frac{\sigma_{E}}{E} \simeq \frac{18\%}{\sqrt{E}}$$ (2.4)

The polar angle resolution ranges from 4$^{\circ}$ at the inner edge, to 10$^{\circ}$ at the outer edge. The tube chambers (FB) consist of three planes of proportional tubes, two of which are mutually perpendicular and the third at 45$^{\circ}$ to these. Information from FB is combined with that from FK to provide a more precise position measurement. Resolutions of 2 mrad can be achieved in the polar angle and a spatial resolution of 3 mm is possible.

Filling the gaps between the electromagnetic endcap and the forward calorimeter are the gamma catchers (FE). They are small annular 7 radiation length lead-scintillator calorimeters. They have an electromagnetic energy resolution of about 20%.

The far forward monitors (FF) are used to measure positions and energies of showers from electrons and positrons in the 5-10 mrad region, close to the horizontal plane. They are also used to measure OPAL trigger rates during data taking. They consist of 20 radiation length lead-scintillator calorimeters and are stationed $\pm$7.85 m from the interaction point.