THE LARGE AREA TELESCOPE ON THE<i>FERMI GAMMA-RAY SPACE TELESCOPE</i>MISSION

Abstract

(Abridged) The Large Area Telescope (Fermi/LAT, hereafter LAT), the primary\ninstrument on the Fermi Gamma-ray Space Telescope (Fermi) mission, is an\nimaging, wide field-of-view, high-energy gamma-ray telescope, covering the\nenergy range from below 20 MeV to more than 300 GeV. This paper describes the\nLAT, its pre-flight expected performance, and summarizes the key science\nobjectives that will be addressed. On-orbit performance will be presented in\ndetail in a subsequent paper. The LAT is a pair-conversion telescope with a\nprecision tracker and calorimeter, each consisting of a 4x4 array of 16\nmodules, a segmented anticoincidence detector that covers the tracker array,\nand a programmable trigger and data acquisition system. Each tracker module has\na vertical stack of 18 x,y tracking planes, including two layers (x and y) of\nsingle-sided silicon strip detectors and high-Z converter material (tungsten)\nper tray. Every calorimeter module has 96 CsI(Tl) crystals, arranged in an 8\nlayer hodoscopic configuration with a total depth of 8.6 radiation lengths. The\naspect ratio of the tracker (height/width) is 0.4 allowing a large\nfield-of-view (2.4 sr). Data obtained with the LAT are intended to (i) permit\nrapid notification of high-energy gamma-ray bursts (GRBs) and transients and\nfacilitate monitoring of variable sources, (ii) yield an extensive catalog of\nseveral thousand high-energy sources obtained from an all-sky survey, (iii)\nmeasure spectra from 20 MeV to more than 50 GeV for several hundred sources,\n(iv) localize point sources to 0.3 - 2 arc minutes, (v) map and obtain spectra\nof extended sources such as SNRs, molecular clouds, and nearby galaxies, (vi)\nmeasure the diffuse isotropic gamma-ray background up to TeV energies, and\n(vii) explore the discovery space for dark matter.\n