Cherenkov Telescopes
Cherenkov radiation is produced when charged particles move in a medium with a higher velocity than the phase velocity of electromagnetic waves in this medium. If the medium is non-conductive, the charged particle polarizes the medium's atoms along its trajectory. Due to the polarization of the atoms electromagnetic waves are emitted which normally interfere destructively (i. e. cancel each other) and so, do not produce a net emission. If, however, the particle moves through the medium faster than light the waves cannot cancel each other out and Cherenkov light is produced. In particle physics, nuclear physics and astro particle physics Cherenkov radiation is used for detecting highly energetic charged particles and for measuring their velocity or energy.
Cherenkov telescopes indirectly observe cosmic gamma radiation between approximately 30 GeV and 30 TeV and so there is an overlap with satellites (which have a detection range up to 100 GeV). Cherenkov telescopes detect the Cherenkov light of particle showers which are caused by highly energetic photons in the Earth's atmosphere. For example, a photon with an energy of 1 TeV produces enough Cherenkov light to 'illuminate' a circle on the ground with a diameter of 250 m and a photon density of 100 photons per square meter. The telescope is composed of a mirror (see picture) which images the Cherenkov light onto a camera (held out in front of the mirror) and thus, produces an image of the particle shower. Based on the detected intensity and the position of the image in the camera the direction of the primary photon as well as its energy can be determined.
Due to the low concentration of Cherenkov photons, imaging Cherenkov telescopes can only be operated on cloudless nights and in regions without interfering background light (as from human settlements).
A further problem is that charged particles in this cosmic radiation (for example protons) also produce showers and Cherenkov light in the atmosphere. The measurement of a gamma-ray source can only take place after subtracting the background of these charged particle events. A more exact reconstruction of photons and an improved rejection of protons can be achieved by observing the particle showers in the atmosphere by several Cherenkov telescopes at the same time.
Humboldt University Berlin is a member of the H.E.S.S. collaboration. The H.E.S.S.-Experiment (High Energy Stereoscopic System) consists of four Cherenkov telescopes in the Khomas Highland of Namibia which have taken data since the end of 2003. Currently a fifth, larger telescope is being built (H.E.S.S. II) which opens the possibility of observing the sky of the southern hemisphere below 100 GeV. In addition to this, the working group Experimental Elementary Particle Physics I at Humboldt University Berlin is involved in the preparation of CTA (Cherenkov Telescope Array), a next-generation gamma-ray observatory consisting of 50 to 100 Cherenkov-telescopes.
Detectors | Cherenkov Telescopes | Collider Experiments | Neutrino Telescopes