Fermi Gamma-ray Space Telescope
What Is LAT?

Deciphering the genetic code of the universe is no easy task. Yet that's just what FGST's Large Area Telescope (LAT) seeks to accomplish. Integrated at the SLAC National Accelerator Laboratory in 2005 and 2006 from hardware fabricated at laboratories all around the world, the LAT uses 880,000 silicon strips to detect high-energy gamma rays with unprecedented resolution and sensitivity, filling in gaps in understanding left by previous missions and pushing new boundaries in particle physics and astrophysics.

As FGST orbits the earth, gamma rays—emanating from jets of plasma streaming from enormous black holes, pulsars and other astronomical sources—strike the LAT. By determining the time of each gamma ray's arrival, the direction from where it came and energy it carries—the fundamental quantities of astronomy—the LAT offers a wealth of new data and a glimpse into the fundamental nature of high-energy processes in the universe.

Gamma rays that encounter the LAT first meet several layers of tungsten metal. Tungsten's massive and highly charged atomic nuclei interact with the high-energy gamma ray in a way that creates a charged pair of particles: one electron and one positron. These particles travel in V-shaped trajectories, with the electron going one way and the positron going another, which are detected by the silicon-strip sensors positioned just below each tungsten layer. Later, these signals are reconstructed by algorithms to obtain the direction and time of the original gamma ray photon.

After traversing through tracking layers, the particles pass into a cesium iodide imaging calorimeter and generate tiny amounts of light—flashes with brightness proportional to the particles' energies.

Through this multi-step process, the LAT detects gamma rays with unprecedented sensitivity, which allow detection of thousands of new sources and possibly even new classes of sources.

In the mid-1990s, the instrument's predecessor, the Energetic Gamma Ray Experiment Telescope (EGRET), made the first sky survey in high-energy gamma rays with a sensitivity up to a few giga-electron-volts (GeV) of energy. Not only does the LAT increase that energy range to 300 GeV, it also provides vastly more sensitivity for faint sources in the universe. An auxiliary instrument, the FGST Burst Monitor, scrutinizes lower energies for the FGST mission.

The LAT observes from space because the gamma rays that it is designed to detect are blocked by the Earth's atmosphere. The problem of detecting relatively rare gamma rays in the midst of a continual cacophony of cosmic rays is reduced by an additional detector surrounding the LAT's 16 tracker towers and calorimeters, and then left to sophisticated data analysis, similar to that found in high-energy physics accelerator experiments. The first round of analysis is performed by flight software on the LAT, which filters out about 80 percent of background signals. This ensures that fainter sources stand out more cleanly against the thousands of signals seen every second and reduces the volume of data sent back to earth.

Further, more detailed, analysis is then be performed in ground processing of the LAT data, which is delivered to SLAC several times per day during the mission.

Try out our online LAT simulator to see what gamma rays will look like as they hit the LAT.

NASA’s FGST mission is an astrophysics and particle physics partnership, developed in collaboration with the U.S. Department of Energy, along with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden, and the United States.

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Cutaway illustration of LAT
Cutaway illustration of LAT — image courtesy of NASA
LAT's 16 towers of particle detectors
LAT's 16 towers of particle detectors — image courtesy of NASA