[Picture of HEGRA array] The HEGRA experiment [Air shower detection]

The High Energy Gamma Ray Astronomy (HEGRA) experiment was located on La Palma (Canary Islands) at a height of 2200 m a.s.l. (at the site of the Observatory operated by the IAC). It consisted of several detector types for observing secondary particles or light from particle cascades in the atmosphere (Extensive Air Showers, EAS). The EAS detected by HEGRA are produced by cosmic ray particles in the energy range 1012 to 1016 eV. With its various types of detectors HEGRA was taking data between 1987 and 2002.

The detectors with the lowest energy threshold were the Atmospheric Cherenkov Telescopes with 'cameras' of photomultiplier tubes. They were sensitive to showers above 1012 eV (1 TeV) but had to look towards possible sources and could be operated only during clear, moonless nights. They detected Cherenkov light from relativistic secondary particles in the air showers. The field of view was about 4.6 degrees. There were a total of six of these telescopes in operation. They were dismantled in September 2002.

Another detector type for the Cherenkov light was AIROBICC (AIrshower Observation By Angle Integrating Cherenkov Counters) with one large photomultiplier looking at the sky above it. 49 of these detectors were spread in a 7-by-7 grid to observe the amplitude and the time of arrival of the front of Cherenkov light. Another 48 were added later on. These counters had a wide field of view but could be operated also only during clear, moonless nights. Their energy threshold was a few 1013 eV. The AIROBICC array has been dismantled.

The first detector type of HEGRA was the array of 1 m2 scintillation counters which were used to measure the numbers and arrival times of secondary particles in air showers arriving at ground level. More than 250 of these counters were in operation, spread over a 180-by-180 m2 area. These detectors were operated day and night at any weather. The energy threshold of the scintillator array was between 40 and 100 TeV, depending on the kind of primary cosmic ray particle. The scintillator array has been dismantled as well.

The scintillator array was sensistive to all types of charged secondary particles. To be able to select secondary muons in air showers there were the Muon 'Towers' with 16 m2 area each. Seventeen of these detectors were installed on La Palma.

There were two more types of detectors at the HEGRA site: the CRT (Cosmic Ray Tracking) detectors and the CLUE (Cherenkov Light Ultraviolet Experiment). A successor to the HEGRA Cherenkov telescopes is being built up at the HEGRA site: MAGIC. A direct successor to the stereoscopic system of Cherenkov telescopes is the H.E.S.S. experiment.

The main HEGRA server used to be in Hamburg but is now off-line. A collection of HEGRA publications located there could be partially recovered from the Internet Archive.
For pointers to various cosmic ray and neutrino experiments worldwide see here.
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