Silicon Microvertex Subdetector (SI)

The SI detector [15] is designed to give accurate measurement and detailed information on the primary vertices of interaction between the electron and positron particles in the beam. It is designed so that it can also measure the positions of any secondary vertices resulting from the decays of particles that could have been produced in the primary interaction, such as $\tau$-leptons and heavy flavour hadrons. The SI consists of two cylinders of silicon ladders. The inner cylinder has a radius of 61 mm, consisting of 11 ladders and the outer a radius of 75 mm, consisting of 14 ladders. Each ladder has two back-to-back single sided sets of silicon wafers, one giving co-ordinates in the r-$\phi$ plane and the other along the $z$-axis.

To measure the path of charged particles the strips have a voltage bias across them. When a charged particle passes through them charge from ionisation in the silicon is collected on the read-out strips. By combining information from all the strips on both layers, the path of the particle travelling through the silicon can be reconstructed.