Pair Production Detectors for Gamma-Ray Astrophysics

Electron-positron pair production is the essential process for high-energy γ-ray astrophysical observations. Following the pioneering OSO-3 counter telescope, the field evolved into use of particle tracking instruments, largely derived from high-energy physics detectors. Although many of the techniques were developed on balloon-borne γ-ray telescopes, the need to escape the high background in the atmosphere meant that the breakthrough discoveries came from the SAS-2 and COS-B satellites. The next major pair production success was EGRET on the Compton Gamma Ray Observatory, which provided the first all-sky map at energies above 100 MeV and found a variety of γ-ray sources, many of which were variable. The current generation of pair production telescopes, AGILE and Fermi LAT, has broadened high-energy γ-ray astrophysics with particular emphasis on multiwavelength and multimessenger studies. A variety of options remain open for future missions based on pair production with improved instrumental performance.