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LAT instrument

image of the assembled LAT towers
Image of the assembled LAT towers
simulated image of a particle event in a LAT tracker tower
Simulated image of a particle event in a LAT tower
image of the ACD with workers
Image of the completed anti-coincidence detector

It's pretty difficult to determine the energy of any given gamma ray, and even harder to figure out from what direction it came. Focusing them is nearly impossible: most gamma rays have so much energy that they pass right through lenses and mirrors. However, the Fermi Large Area Telescope, or LAT, exploits a property of gamma rays which makes this task easier. Einstein's famous equation, E=mc2 , states that matter can be converted to energy, and vice-versa. A gamma ray is basically a bundle of energy - enough energy, in fact, that it can be converted to matter, which is not possible with ordinary, visible light. If a gamma ray photon with enough energy interacts with matter, it can create a pair of oppositely charged particles of matter and anti-matter, such as an electron and positron. These particles are a lot easier to examine than the original gamma ray, making it possible to determine their energy and direction, and thus find those same properties of the original gamma ray (plus its time of arrival at the detector). Because the LAT uses the particles created from a gamma ray, scientists call the LAT a "pair conversion telescope." Even though it's an astronomical instrument, the LAT measures the properties of the pair of particles using the same sort of detectors used in particle physics-- the study of the elementary components of the Universe.

The Tracker

The LAT consists of 16 towers in a 4x4 grid. The upper part of each tower is a tracker, a stack of particle detectors called "silicon strips," which accurately measure the paths of the particles. Interleaved between these tracking detectors are thin sheets of tungsten, which provide the material in which the pair conversion takes place. When a gamma ray slams into them, it is converted into an electron/positron pair. It takes a certain amount of energy to create this pair; if the gamma ray doesn't have that much energy it won't convert, and if it has more than this amount, the extra energy is given to the particle pair in the form of velocity. The more energy the original gamma ray had, the faster the particles will travel and the straighter their tracks in the detectors will be. Each particle moves in a slightly different direction, creating an inverted "V" that points back toward the direction in the sky from which the gamma ray came. The tracks thus carry information about the arrival direction of the original gamma ray. These pictures from the tracker are used by astronomers to map the gamma ray sky, one photon at a time.

The Calorimeter

The particles pass into the bottom part of the tower, into a device called a "calorimeter", which measures the amount of energy carried by the particles. If the gamma ray had a very large amount of energy, than the particles created can in turn create their own particles, causing a "cascade shower" of particles. Each particle creates a little flash of light when it hits the cesium-iodide detectors in the calorimeter. This flash of light is then converted into a voltage, which is then measured. The more energy the original gamma ray had, the more intense will be the light seen in the calorimeter and the bigger the electrical signal. The calorimeter is made of "logs" of cesium-iodide stacked in a criss-cross pattern. By looking at where the light is seen, the calorimeter can confirm that the energy came from the original pair conversion event. The LAT electronics system then adds the direction information from the tracker with the energy information from the calorimeter to send to the ground for further analysis.

The Anti-Coincidence Detector

Of course, there's a fly in the ointment: high-velocity particles, called cosmic rays, are common in space, and in fact vastly outnumber the gamma rays the LAT detects. These cosmic rays can hit the tracker and calorimeter and be confused for gamma rays, making it impossible to see any of the astronomical targets -- it would be like looking for a firefly sitting on a searchlight! To prevent that, surrounding the LAT is a device called the "anti-coincidence detector", or ACD. The ACD is made of tiles which are sensitive to cosmic rays, but not gamma rays. Any incoming gamma rays will pass right through it, but when a cosmic ray hits the ACD it makes a little flash of light. This light is detected by sensitive instruments which then send a signal to the LAT telling it, essentially, "ignore anything you detect from the direction of this tile at the same time this flash of light occurred". That way, anything seen in the tracker and calorimeter at that time can be ignored as a cosmic ray, letting the desired gamma rays be detected. The on-board electronics for the LAT combines the information from the ACD with information from the tracker and calorimeter to reject 99.999% of all the incoming cosmic rays, which is what is required to be able to clearly see the gamma rays from an astronomical source.

Conclusion

The Fermi LAT is able to determine the position of a gamma ray source to about 1 arcminute, roughly the same resolution of the human eye; a giant leap in gamma ray resolution. It has detected exotic beasts such as black holes, pulsars, and continues to search for the ghostly fingerprints of mysterious dark matter. Fermi's many discoveries have revolutionized our understanding of the Universe at high energies.

Fermi Gamma-ray Space Telescope - Education and Public Outreach - Active Galaxies Education Unit