Separation of gamma and hadron initiated air showers with energies between 20 and 500 TeV

The discrimination between air showers initiated by gamma rays and by hadrons is one of the fundamental problems in experimental cosmic-ray physics. The physics of this 'gamma/hadron separation' is discussed in this paper. We restrict ourselves to the energy range from about 20 to 500 TeV, and take only the information contained in the lateral Cerenkov light distribution and the number of electrons at the detector level into consideration, An understanding of the differences between air showers generated by gamma rays and those due to hadrons leads us to formulate suitable observables for the separation process. Angle integrating Cerenkov arrays (AICA) offer a promising new approach to ground-based gamma-ray astronomy in the energy region from about 20 to 500 TeV. In order to establish this technique, an efficient suppression of the overwhelming hadronic background radiation is required. As an example for our general discussion, we present one method for gamma/hadron separation in AICAs called 'LES'. It is based on the simultaneous determination of the shower size and some characteristic parameters of the lateral distribution of the Cerenkov light. The potential inherent within this technique is demonstrated in quantitative detail for the existing 'AIROBICC' AICA. We also propose an objective measure of the intrinsic sensitivity of a detection scheme in ground-based gamma-ray astronomy, the 'reduced quality factor'. It is shown that AICAs may reach a sensitivity to gamma-ray point sources in the high VHE range similar to that of the Cerenkov-telescope imaging technique in the low VHE region.