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The Energetic Gamma Ray Experiment Telescope (EGRET)

Sphoto and drawing of EGRET cross-section

The Energetic Gamma Ray Experiment Telescope (EGRET) provides the highest energy gamma-ray window for the Compton Observatory. Its energy range is from 30 million electron volts (20 MeV) to 30 billion electron volts (30 GeV). EGRET is 10 to 20 times larger and more sensitive than previous detectors operating at these high energies and has made detailed observations of high energy processes associated with diffuse gamma-ray emission, gamma-ray bursts, cosmic rays, pulsars, and active galaxies known as gamma-ray blazars.

The EGRET instrument produces images at these energies using high voltage, gas filled, spark chambers. High energy gamma rays enter the chambers and produce an electron-positron pair of particles which cause sparks. The path of the particles is recorded allowing the determination of the direction of the original gamma ray. The particle energies are recorded by a NaI crystal beneath the spark chambers providing a measure of the original gamma-ray energy.