Very-high energy (VHE) gamma quanta contribute only a minuscule fraction - below one per million - to the flux of cosmic rays. Nevertheless, being neutral particles they are currently the best "messengers" of processes from the relativistic/ultra-relativistic Universe because they can be extrapolated back to their origin. The window of VHE gamma rays was opened only in 1989 by the Whipple collaboration, reporting the observation of TeV gamma rays from the Crab nebula. After a slow start, this new field of research is now rapidly expanding with the discovery of more than 150 VHE gamma-ray emitting sources. Progress is intimately related with the steady improvement of detectors and rapidly increasing computing power. We give an overview of the early attempts before and around 1989 and the progress after the pioneering work of the Whipple collaboration. The main focus of this article is on the development of experimental techniques for Earth-bound gamma-ray detectors; consequently, more emphasis is given to those experiments that made an initial breakthrough rather than to the successors which often had and have a similar (sometimes even higher) scientific output as the pioneering experiments. The considered energy threshold is about 30 GeV. At lower energies, observations can presently only be performed with balloon or satellite-borne detectors. Irrespective of the stormy experimental progress, the success story could not have been called a success story without a broad scientific output. Therefore we conclude this article with a summary of the scientific rationales and main results achieved over the last two decades.

Very-high energy gamma-ray astronomy: A 23-year success story in high-energy astroparticle physics

The techniques, the status and prospects of very high energy (VHE) gamma ray astronomy with ground based telescopes will be reviewed

Very high energy gamma ray astronomy

Methods of determining the nature of the primary cosmic ray particles are reviewed, using information on high-energy cosmic rays in the atmosphere. Results obtained in various observations are critically summarized and discussed. Only results which seem relatively well established are quoted as further observations are definitely needed before any firm conclusions can be drawn.

https://iopscience.iop.org/article/10.1088/0954-3899/20/7/001/pdf

Mass composition of primary cosmic rays in the range 1014-1017 eV

Very high energy (VHE) gamma quanta contribute only to a minuscule fraction – below one per million – to the flux of cosmic rays. Nevertheless, being neutral particles they are currently the best “messengers” of processes from the relativistic/ultra-relativistic Universe because they can be extrapolated back to their origin. The window of VHE gamma rays was opened only in 1989 by the Whipple collaboration, reporting the observation of TeV gamma rays from the Crab nebula. After a slow start, this new field of research is now rapidly expanding with the discovery of more than 150 VHE gamma-ray emitting sources. Progress is intimately related with the steady improvement of detectors and rapidly increasing computing power. We give an overview of the early attempts before and around 1989 and the progress after the pioneering work of the Whipple collaboration. Before describing the important experiments that paved the way towards the current observations, a short review of gamma rays as cosmic messengers and the general concept of detecting and filtering them from the vast majority of charged cosmic rays will be given in the introduction. The main part of the chapter on VHE gamma-ray astronomy highlights the various experiments that have pushed the field to the current success. At the end, a short overview of the challenging physics questions will be given summarizing the goals of the experiments in order to give a coherent picture of the underlying reasons behind the experiments. In coherence with the energy range of cosmic rays observable from Earth, we will set the lower energy limit to about 30 GeV above which ground-based observations with the most powerful instruments are now possible. Below this energy range observations can only be performed with balloon or satellite-borne detectors. Their detection range is below the classical cosmic-ray energy range.

Very-High Energy Gamma-Ray Astronomy: A 23-Year Success Story in Astroparticle Physics

Results from a three-dimensional hybrid Monte Carlo calculation for the secondary gamma composition at sea level for gamma- and proton-initiated extensive air showers (EAS) with primary energies 10, 100, 1000, and 10 000 TeV are presented. For each primary energy, the distributions of the number of secondary gammas and their lateral spread with corresponding fits are reported for secondary threshold energies of 3, 10, 30, and 100 MeV. The distributions for secondary gammas are compared to those for secondary electrons. Results show a large enhancement of low-energy secondary gammas compared to the number of secondary electrons for both primaries. The secondary gamma composition of hadronic and electromagnetic EAS are presented. An experimental measurement of the secondary gamma component using the project GRAND detectors has been obtained; there is good agreement between the experimental result and the Monte Carlo predictions thus yielding an experimental check of this hybrid Monte Carlo method.

The secondary gamma composition from hadronic and electromagnetic extensive air showers at 10, 100, 1000, and 10000 TeV

Search for Steady and Sporadic Emission of Neutral Radiation Above 50 TeV with the HEGRA Array

An air shower array named HEGRA has been built up at the Canarian Island La Palma. The array, consisting of 37 scintillation detectors at its first stage, is in operation since July 1988. Until the end of May 1989, 10 million events have been recorded. An extension of the installation is in progress under the HEGRA Collaboration.

The HEGRA project for UHE gamma ray astronomy

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Search for Steady and Sporadic Emission of Neutral Radiation Above 50 TeV with the HEGRA Array

The HEGRA project for UHE gamma ray astronomy