Fermi's Large Area Telescope (LAT) detects particles produced in a physical process known as pair production that epitomizes Einstein's famous equation, E=mc2. When a gamma ray, which is pure energy (E), slams into a layer of tungsten in one of the tracking towers that compose the LAT, it creates mass (m) in the form of a pair of subatomic particles, an electron and its antimatter counterpart, a positron. Several layers of high-precision silicon detectors track the particles as they move through the instrument. The direction of the incoming gamma ray is determined by projecting the particle paths backward. The particles travel through the trackers until they reach a separate detector called a calorimeter, which absorbs and measures their energies. The LAT produces gamma-ray images of astronomical objects, while also determining the energy of each detected gamma ray.
This animation shows a gamma ray (purple) entering a corner tower of the Tracker. After the electron (red) and positron (blue) cascade down the tower, their incoming paths (red/blue) combine to show the original path (purple) of the incoming gamma ray that created them.