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Astron. Astrophys. 359, 682-694 (2000)
2. The experimental setup
The air-shower detector complex HEGRA covers an area of
180![[FORMULA]](img11.gif)
180 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
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