How to detect gamma-rays from ground: an introduction to the detection concepts

Indirect detection of gamma rays with ground-based observatories is currently the most sensitive experimental approach to characterize the gamma-ray sky at energies $>0.1$\,TeV. Ground-based detection of gamma-rays relies on the electromagnetic showers that gamma rays initiate in the Earth's atmosphere. In this chapter we will review the properties of electromagnetic air showers as well as the differences with respect to cosmic-ray showers that enable the rejection of the cosmic ray background. The experimental techniques that have been developed for ground-based detection of gamma rays will be introduced. These fall onto three main categories: air shower particle detectors, sampling Cherenkov arrays, and imaging atmospheric Cherenkov telescopes. Hybrid concepts as well as other experimental approaches are also discussed.