The backward silicon tracker of the H1 experiment at HERA

The Backward Silicon Tracker (BST) of the H1 Experiment at HERA measures the polar angle of electron tracks with a precision of about 0.2 mrad. The BST recorded its first high-luminosity data in 1997 and underwent significant upgradation in 1998. In this paper, the layout, operation and performance of the BST are described and some physics results are discussed. The BST has been operating reliably. Thus, it is foreseen for the high-luminosity phase of HERA to base an additional silicon tracker in the forward region of H1 on the technology and the experience gained with the operation of the BST.