Fermi LAT

This animation shows a gamma ray (purple) entering a corner tower of the Fermi Gamma-ray Space Telescope’s Large Area Telescope. When the gamma ray interacts with a tungsten plate, it produces an electron (red) and a positron (blue). Detectors track these particles as they cascade down the tower and reconstruct the original path (purple) of the gamma ray that created them. Credit: NASA's Goddard Space Flight Center Conceptual Image Lab