Fermi – an eye for gamma rays

The Fermi space telescope belonging to the US space agency NASA investigates cosmic gamma radiation. It is a joint astrophysics and astroparticle physics project, and is operated in collaboration with the US Department of Energy with important contributions by academic institutions and partners in France, Germany, Italy, Japan, Sweden and the USA. Fermi was launched on 11 June 2008; the satellite is named after the Italian nuclear physicist and Nobel laureate Enrico Fermi (1901-1954).

Gamma radiation, the range in the electromagnetic spectrum with the shortest wavelength, is so energetic that it would pass through a lens or a mirror straightaway, and therefore cannot be observed with conventional optical telescopes. Fermi’s main instrument, the Large Area Telescope (LAT), thus operates more like a particle detector. When a gamma ray reaches the telescope, it impacts on a metal foil producing a pair of charged particles which continue to fly through the detector. The LAT has over 880,000 silicon strips in total to detect these charged particles and enable the astronomers to reconstruct the path of the original gamma radiation with unprecedented sensitivity and resolution.

The Large Area Telescope scans the whole sky once every three hours and can detect gamma radiation in an energy range of 20 million to approx. 300 billion electronvolts (20 MeV to 300 GeV). It observes 20 per cent of the entire sky at any one time.



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