Fermi Gamma-ray Space Telescope

Overview of the LAT

The principal objective of the Large Area Telescope (LAT) is to conduct a long term high sensitivity gamma-ray observations of celestial sources in the energy range from ~20 MeV to >300 GeV. The LAT is a wide field-of-view (FoV) imaging gamma-ray telescope with large effective area combined with good energy and angular resolution. The LAT data are being used for a) rapid notification of gamma-ray transient events and monitoring of variable sources b) constructing a catalog of celestial gamma-ray sources along with spectra and light curves for many of these sources c) localization point sources to 0.3 - 2 arcmin d) spatial mapping and spectra of extended sources such as supernova remnants, molecular clouds, and nearby galaxies e) measuring the diffuse gamma-ray background, f) searching for signals from dark matter and g) measuring the flux and spectrum of cosmic ray electrons.

LAT Characteristics

Parameter Value or Range
Energy Range ~20 MeV to >300 GeV
Energy Resolution <15% at energies >100 MeV
Effective Area >8,000 cm2 maximum effective area at normal incidence
Single Photon Angular Resolution <0.15�, on-axis, 68% space angle containment radius for E > 10 GeV; < 3.5�, on-axis, 68% space angle containment radius for E = 100 MeV
Field of View 2.4 sr
Source Location Determination <0.5 arcmin for high-latitude source
Point Source Sensitivity <6x10-9 ph cm-2 s-1 for E > 100 MeV, 5σ detection after 1 year sky survey
Time Accuracy <10 microseconds, relative to spacecraft time
Background Rejection (after analysis) <10% residual contamination of a high latitude diffuse sample for E = 100 MeV - 300 GeV.
Dead Time <100 microseconds per event

Detector Methodology

The LAT is a pair conversion detector. Gamma rays penetrate into the detector (the tracker) and interact with a high Z converter material, in this case tungsten, to produce an electron-positron pair. This pair is tracked through the instrument by silicon strip detectors. Since the gamma-ray energy is much larger than the rest mass of the electron and positron, both members of the pair continue predominantly in the direction of the incident gamma ray. So the reconstructed direction on the incoming gamma-ray is limited by multiple scatterings of the pair components in the tracker material as well as the spatial resolution of the tracker. At the bottom of the LAT is a calorimeter made of CsI(Tl) that is thick enough to provide an adequate energy measurement of the pairs in the LAT energy band.

Schematic of LAT Structure

Schematic Structure of the LAT

Charged cosmic ray particles incident on the LAT, that outnumber the gamma rays by factors of 102 - 105 also produce tracks in the LAT, resulting in a potentially overwhelming background. In order to reject such background events, the LAT is surrounded by an anti-coincidence detector (ACD), consisting of scintillator tiles, which detect these events and issue a veto signal. In some cases secondary charged particles from the electromagnetic shower, created by an incident high energy photon in the calorimeter (potentially a valid event), travel back up through the tracker and cross the ACD. These particles can Compton scatter and thereby create signals from the recoil electrons (backsplash effect) and result in valid gamma rays being vetoed. The EGRET detector on CGRO had a monolithic ACD that vetoed many high energy gamma rays and dramatically reduced its sensitivity to high energy photons. In order to suppress the backsplash effect in the LAT, its ACD is segmented, and only events that trigger an ACD tile on the path of the incoming particle are vetoed. The segmentation also results in a more uniform threshold over the ACD. This segmentation of the LAT ACD dramatically increases the LAT's sensitivity to high-energy gamma rays dramatically compared to EGRET.

The output from the LAT consists of the pulse-height signals produced as charged particles deposit energy in different parts of the tracker and calorimeter. By combining the pulse heights with the x-y coordinates of each silicon strip detector hit one can reconstruct the particle trajectory and energy losses. The analysis both on-board and on the ground reconstructs the tracks of the charged particles from these data, and then characterizes the interaction that produced the charged particles; this analysis can distinguish between events resulting from photons and background, as well as determine the incident photon direction and estimate the energy.

Detector Structure

The LAT consists of an array of 16 tracker (TKR) modules, 16 calorimeter (CAL) modules, and a segmented ACD. The TKR and CAL modules are mounted to the instrument central structure.

Each TKR module consists of 18 XY tracker planes. Each XY plane has an array of silicon-strip tracking detectors (SSDs) for charged particle detection. The first 12 planes have tungsten plates 0.035 radiation lengths thick in front of the SSDs, the next 4 planes have tungsten plates 0.18 radiation lengths thick, and the last 2 planes, immediately in front of the calorimeter, do not have any converters. The SSDs in each plane actually consist of two planes of silicon strips, one running in the x and the other in the y direction, thereby localizing the passage of a charged particle. Gamma rays incident from within the LAT's field of view preferentially convert into an electron-positron pair in one of the TKR's tungsten plates. The initial directions of the electron and positron are determined from their tracks recorded by the SSD planes following the conversion point. Multiple scattering of the pair components in the first conversion plane results in an angular deflection that results in a limit to the low angular resolution, especially at low energy. Cosmic rays also interact within the TKR modules. Reconstruction of the interactions from the tracks identify the type of particle as well as its energy and incident direction.

LAT Structure

Structure of the LAT

Each CAL module consists of 96 CsI(Tl) crystals, arranged in eight alternating orthogonal layers, with 1536 crystals total. The crystals are read out by dual PIN photodiodes at each end. The CAL's segmentation and read-out provide precise three-dimensional localization of the particle shower in the CAL. At normal incidence the CAL's depth is 8.6 radiation lengths. The LAT calorimeter is a total absorption (not sampling) calorimeter with excellent energy resolution.

The ACD is composed of 89 plastic scintillator tiles, each read out through wave-length shifting fibers by two photomultiplier tubes (PMT) for redundancy. In order to provide maximum ACD efficiency for charged particle detection, the tiles are overlapped in one direction. Eight scintillation ribbons (also read out by a PMT at each end) close the gaps between the tiles in the other direction.

The LAT's Data Acquisition System (DAQ) performs onboard filtering in order to reduce the rate of background events that will be telemetered to the ground. The DAQ processes the captured event data into a data stream with an average bit rate of 1.2 Mbps for the LAT. The DAQ also performs the command, control, and instrument monitoring; housekeeping, and power switching. Onboard processing can be modified by uploading new software, if necessary.

The astrophysical photons of primary interest will be a tiny fraction of the particles that will penetrate into the LAT's TKR. The LAT on-board analysis reduces raw LAT trigger rate, which can approach 10 kHz, to ~400 events per second which are sent to the ground for further analysis. Of these ~400 Hz only ~2-5 Hz are astrophysical photons. The data for an event that passes the on-board analysis cuts is stored in a packet with a time stamp and details of the signals from the various LAT components. Because the number of signals for a given event varies, the data packets have variable length. These data packets describing each event are the LAT's primary data product. The LAT transfers these packets to the spacecraft's Solid State Recorder (SSR) for subsequent transmission to the ground.

The LAT Collaboration

The Fermi LAT collaboration includes scientists from Stanford University(PI: Prof. P. Michelson); SLAC ; NASA's Goddard Space Flight Center (Project Scientist: E. Hays); University of California at Santa Cruz; Naval Research Laboratory; University of Washington; Sonoma State University; Ohio State University; University of Denver; Purdue University in the United States; Stockholm University and Royal Institute of Technology in Sweden; Commissariat a l'Energie Atomique, Departement d'Astrophysique, Saclay; Institut National de Physique Nuclearie et de Physique des Particules; and Institut de Recherche en Astrophysique et Planétologie (IRAP) in France; Instituto Nazionale di Fisica Nucleare; Agenzia Spaziale Italiana; Instituto di Fisica Cosmica, CNR; and Universita e Politecnico di Bari in Italy; Hiroshima University; Institute of Space and Astronautical Science and the Tokyo Institute of Technology in Japan; and Institut de Ciencies de l'Espai in Spain. In addition, 29 institutions world-wide host Affiliated Scientists.

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