[an error occurred while processing this directive]
![]() |
| The above figure depicts the GLAST LAT. |
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.
.
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.

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.
