2. The AIROBICC array
The AIROBICC array is part of the cosmic ray (CR) HEGRA detector complex located at the Roque de los Muchachos on the Canary Island La Palma (28.75° N, 17.89° W, 2240 m a.s.l.). It is an array of 7x7 stations (currently enlarged to 8x8 plus a subarray of 4x4 to increase detector density in the center) with a regular grid spacing of 30 m, thus covering more than 32000 m2. Each station consists of a 40 cm diameter reflecting cone which focuses the incoming light onto a fast 20 cm diameter photomultiplier tube (PMT). The set is placed inside a protective hut with a lid which can be opened through remote control. The PMT is covered with a blue filter (300-480 nm) to improve signal to noise ratio (S/N). The PMT output is amplified in the hut and sent through a 150 m long cable to a constant fraction discriminator (CFD) placed in the electronics container. The CFD is set to a level equivalent to 5 of the night sky background fluctuations. Whenever six or more AIROBICC stations exceed the CFD threshold within 200 ns, a trigger is produced. Calibrations are performed every 20 minutes to measure the relative delay between stations, the response of the TDC and the ADC pedestals. The mean dead time after each recorded event is 8 ms. Detailed descriptions of AIROBICC and the other components of the HEGRA complex can be found in Karle et al. 1995a, Fonseca et al. 1995a, Fonseca et al. 1995b.
The ADC signals (AIROBICC + scintillator array) are used to locate the EAS core position with a typical error 18 m. The Cherenkov light front is fitted by a cone to determine the arrival direction of the incident CR. The angular resolution (angular distance containing 63% of events for a point source) is 0.29° when 12 stations are fired (standard cut) and it continuously improves with increasing number of triggered huts. The absolute pointing accuracy has been estimated through comparison with the first HEGRA Cherenkov telescope to be better than 0.2° (Karle et al. 1995a). The FOV, which is limited by the acceptance of the Winston cone, is 1 sr and the trigger rate is 20 Hz. The energy threshold (50% trigger probability) is estimated to be 16(25) TeV for s and 29(37) TeV for hadrons with zenith angle ) using flux and Monte Carlo studies (Martí nez et al. 1995). The price that has to be paid for the advantages of the Cherenkov technique (lower energy threshold, better angular resolution) is that observations are restricted to moonless clear nights (10% duty cycle).
© European Southern Observatory (ESO) 1998
Online publication: August 6, 1998