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LAT Large Area Telescope


The primary instrument required for the GLAST mission is an imaging, wide field-of-view telescope (the Large Area Telescope, or LAT) that covers the energy range from 20 MeV to 300 GeV. The two characteristics that the LAT will measure for each incoming gamma-ray are the energy of the photon and the angle at which the light ray hits the detector. These measurements will enable scientists to determine the location on the sky that produced the gamma-ray and the energy contained in that gamma-ray. The telescope consists of a tracker, followed by an energy-measuring calorimeter. The entire telescope is surrounded by anti-coincidence shielding, in order to eliminate signals which might be generated by background particles, such as cosmic rays. When a gamma-ray comes in contact with the converter material, it interacts to create a electron and positron pair. This interaction is called pair production. Each electron and positron then travels through the subsequent layers of tracking detectors and converters. For very energetic particles, further interactions occur, which produce additional pairs. The tracking detectors record information about the paths taken by the particles that are generated in the shower. The calorimeter records information about the energy of the particles in the shower. On-board analysis of the tracker and calorimeter data provides initial information about the energy and direction of the shower and helps filter out additional background signals. The on-board triggering system will reject over 100,000 background signals for each gamma-ray photon that it identifies. The remaining signals are telemetered to the ground where they are further processed to determine the energy and direction of the gamma-ray photon.
An artistic interpretation of GLAST in orbit above the Earth. The satellite is accompanied by a cut away image of the Large Area Telescope (LAT).
The above figure depicts the GLAST LAT.

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LAT Silicon-strip Detectors (SSDs)

The GLAST Large Area Telescope consists of a four-by-four array of tower modules. Each tower module consists of 17 interleaved pairs of silicon-strip detectors and tungsten converter sheets, and an additional two pairs of silicon-strip detectors without converters. Silicon-strip detectors (SSD's) are able to more precisely track the electron or positron produced from the initial gamma-ray than previous types of detectors. SSDs will have the ability to determine the location of an object in the sky to within 0.5 to 5 arc minutes.

In each pair of silicon strip detectorss, there are two planes of silicon, one plane has the strips oriented in the "x-direction", while the other has the strips oriented in the perpendicular "y-direction". When a particle interacts in the silicon, its position on the plane can therefore be determined in two dimensions. The third dimension of the track is determined by analyzing signals from adjacent planes, as the particle travels down through the telescope towards the calorimeter.

An example of the four-by-four array tower of LAT is shown along with a cross section of an indivdual tower. One tower consists of three layers: converter/tracker layers (SSD), anticoincidence shield, and the cesium-iodide calorimeter..

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LAT Cesium-Iodide Calorimeter

The cesium-iodide calorimeter design for the GLAST LAT produces flashes of light that are used to determine how much energy is in each gamma-ray. A calorimeter ("calorie-meter") is a device that measures the energy (heat in calories) of a particle when it is totally absorbed. Once a gamma ray penetrates through the anticoincidence shield, the silicon-strip tracker and lead converter planes, it then passes into the cesium-iodide calorimeters. This causes a scintillation reaction in the cesium-iodide, and the resultant light flash is photoelectrically converted to a voltage. This voltage is then digitized, recorded and relayed to earth by the spacecraft's onboard computer and telemetry antenna. 96 Cesium-iodide blocks are arranged in eight layers in two perpendicular directions, to provide additional positional information about the shower.

Three cesium-iodide blocks are lined up next to each other, showing the three stages of the preperation before being inserted into a tower module. The three blocks lie perpendicular to a meter stick to illustrate their size.

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LAT On-board Processor

The data acquisition system (DAQ) is the brain behind the GLAST LAT, as it makes the initial distinction between false signals and real gamma ray signals, and decides which of the signals should be telemetered to the ground. The DAQ consists of specialized electronics and 32-bit radiation-hard processors that record and analyze the information generated by the silicon-strip detectors and the calorimeter. The DAQ will be shielded from the incredible rigors of space-flight, such as extreme high and low temperatures as well as high energy cosmic rays, which can cause the electronics to malfunction.

GLAST's Digital Acquisition System (DAQ). A electronic board mount.

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