Detecting Gamma-Rays with Moderate Resolution and Large Field of View: Particle Detector Arrays and Water Cherenkov Technique

Abstract

The Earth is continuously bombarded by cosmic rays and gamma-rays extending over an immense range of energies. Discovered in 1912 by Victor Hess, the cosmic radiation has been studied from balloons, from space, from the ground, and from underground. The resulting fields of cosmic-ray astrophysics (focused on the charged particles), gamma-ray astrophysics, and neutrino astrophysics have diverged somewhat. But for the air showers in the GeV and TeV energy ranges, the ground-based detector techniques have considerable overlaps.Very high-energy (VHE) gamma-ray astronomy is the observational study measuring the directions, flux, energy spectra, and time variability of the sources of these gamma-rays. These measurements constrain the theoretical models of the sources and their interactions between the sources and detection at Earth. With the low flux of gamma-rays, and the background of charged particle cosmic rays, the distinguishing characteristic of gamma-ray air shower detectors is large size and significant photon to charge particle discrimination.Air shower telescopes for gamma-ray astronomy consist of an array of detectors capable of measuring the passage of particles through the array elements. To maximize signal at energies of a TeV or so, the array needs to be built at high altitude as the maximum number of shower particles is high in the atmosphere. These detectors have included sparse arrays of shower counters, dense arrays of scintillators or resistive plate counters (RPC), buried muon detectors in concert with surface detectors, or many-interaction-deep water Cherenkov detectors (WCD).In general, these detectors are sensitive over a large field of view, and the whole of the sky is a typical sensitivity and perhaps two-thirds of the sky selected for clean analysis, but with only moderate resolution in energy, typically due to shower-to-shower fluctuations and the intrinsic sampling of the detector. These telescopes, though, operate continuously, despite weather, moonlight, day or night, and without needing to be pointed to a specific target for essentially a 100% duty cycle. In this chapter, we will examine the performance and characteristics of such detectors. These are contrasted with the Imaging Air Cherenkov Telescopes which also operate in this energy range, and both current and future proposed experiments are described.KeywordsGamma-ray astronomyInstrumentationAir shower physics