Design and performance of the silicon strip tracker of the Fermi Large Area Telescope

Abstract

The Large Area Telescope (LAT) is the primary instrument on-board the Fermi Gamma-ray Space Telescope (Fermi), an observatory on a low Earth orbit that was launched on 11 June 2008 to monitor the high energy γ-ray sky. The LAT tracker is a solid-state instrument: tungsten foils convert the gamma rays into electron-positron pairs which are then tracked in silicon planes in order to reconstruct the incoming photon direction. The tracker comprises 36 planes of single-sided silicon strip detectors, for a total of 73 square meters of silicon, read out by nearly 900,000 amplifier-discriminator channels. The system operates on only 160 W of conditioned power while achieving > 99% single-plane efficiency within its active area and better than 1 channel per million noise occupancy. We describe the tracker design and performance, and discuss in particular the excellent stability of the hardware response during the first three years of operation on orbit.