2. The experimental setup
The air-shower detector complex HEGRA covers an area of 180180 m2 at a height of 2200 m a.s.l. (790 g/cm2) (Lindner 1997; Karle et al. 1995; Krawczynski et al. 1996; Rhode et al. 1996). In the present analysis only data of the scintillator array and part of the AIROBICC array were used. The former consists of 243 huts with plastic scintillators of an area of 0.96 m2, covered with 5 mm of lead on a grid with 15 m spacing (with a denser part with 10 m spacing in the centre of the array). The part of the latter used in this analysis is formed by 49 open photo-multipliers fitted with Winston cones, restricting the viewed solid angle to 0.835 sr and measuring the air Cherenkov light of EAS on a grid with 30 m spacing. The gain nonlinearity of all components in the Cherenkov-light measurement was carefully checked, both with a LED light source with variable light intensity and a direct charge source. While the used Cherenkov light photomultiplier tubes were found to be linear, the used amplifier showed an antilinearity (gain rises with input amplitude) which was corrected in the data analysis. Above 10 PeV the amplifier begin to show signs of saturation and therefore no data above 10 PeV are included in the present analysis. The trigger conditions used for the data analysed here demand a signal from at least 14 scintillator or 6 AIROBICC stations within 150 ns. This corresponds to an energy threshold for primary protons and iron nuclei of 25 and 80 TeV respectively.
© European Southern Observatory (ESO) 2000
Online publication: July 7, 2000