The Large Area Telescope

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Abstract
The Large Area Telescope (LAT), one of two instruments on the Gamma‐ray Large Area Space Telescope (GLAST) mission, is an imaging, wide field‐of‐view, high‐energy pair‐conversion telescope, covering the energy range from ∼20 MeV to more than 300 GeV. The LAT is being built by an international collaboration with contributions from space agencies, high‐energy particle physics institutes, and universities in France, Italy, Japan, Sweden, and the United States. The scientific objectives the LAT will address include resolving the high‐energy gamma‐ray sky and determining the nature of the unidentified gamma‐ray sources and the origin of the apparently isotropic diffuse emission observed by EGRET; understanding the mechanisms of particle acceleration in celestial sources, including active galactic nuclei, pulsars, and supemovae remnants; studying the high‐energy behavior of gamma‐ray bursts and transients; using high‐energy gamma‐rays to probe the early universe to z ⩾ 6; and probing the nature of dark matter. The components of the LAT include a precision silicon‐strip detector tracker and a CsI(Tl) calorimeter, a segmented anticoincidence shield that covers the tracker array, and a programmable trigger and data acquisition system. The calorimeter’s depth and segmentation enable the high‐energy reach of the LAT and contribute significantly to background rejection. The aspect ratio of the tracker (height/width) is 0.4, allowing a large field‐of‐view and ensuring that nearly all pair‐conversion showers initiated in the tracker will pass into the calorimeter for energy measurement. This paper includes a description of each of these LAT subsystems as well as a summary of the overall performance of the telescope.

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