The Large Area Telescope ( Fermi /LAT , hereafter LAT ) , the primary instrument on the Fermi Gamma-ray Space Telescope ( Fermi ) mission , is an imaging , wide field-of-view , high-energy \gamma -ray telescope , covering the energy range from below 20 MeV to more than 300 GeV .
The LAT was 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 .
This paper describes the LAT , its pre-flight expected performance , and summarizes the key science objectives that will be addressed .
On-orbit performance will be presented in detail in a subsequent paper .
The LAT is a pair-conversion telescope with a precision tracker and calorimeter , each consisting of a 4 \times 4 array of 16 modules , a segmented anticoincidence detector that covers the tracker array , and a programmable trigger and data acquisition system .
Each tracker module has a vertical stack of 18 x,y tracking planes , including two layers ( x and y ) of single-sided silicon strip detectors and high- Z converter material ( tungsten ) per tray .
Every calorimeter module has 96 CsI ( Tl ) crystals , arranged in an 8 layer hodoscopic configuration with a total depth of 8.6 radiation lengths , giving both longitudinal and transverse information about the energy deposition pattern .
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 ( 2.4 sr ) and ensuring that most pair-conversion showers initiated in the tracker will pass into the calorimeter for energy measurement .
Data obtained with the LAT are intended to ( i ) permit rapid notification of high-energy \gamma -ray bursts ( GRBs ) and transients and facilitate monitoring of variable sources , ( ii ) yield an extensive catalog of several thousand high-energy sources obtained from an all-sky survey , ( iii ) measure spectra from 20 MeV to more than 50 GeV for several hundred sources , ( iv ) localize point sources to 0.3 – 2 arc minutes , ( v ) map and obtain spectra of extended sources such as SNRs , molecular clouds , and nearby galaxies , ( vi ) measure the diffuse isotropic \gamma -ray background up to TeV energies , and ( vii ) explore the discovery space for dark matter .