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.
| The above figure depicts
the GLAST LAT. |
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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.
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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.
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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.
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If you have a general question about GLAST, GLAST science or classroom materials, please contact GLAST Answers
If you have a question about analyzing GLAST science data, please contact us via the Feedback form.
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