ABSTRACT
It is natural that astronomy should develop using those parts of the electromagnetic spectrum to which the atmosphere is transparent and for which detectors were available. This led to the early development of optical astronomy (at the dawn of mankind) and radio astronomy as radio and radar techniques were developed in the period around the Second World War. The earth’s atmosphere effectively blocks all electromagnetic radiation of energies greater than 10 eV. The total vertical thickness of the atmosphere above sea level is 1030 g cm−2 and since the radiation length is 37.1 g cm−2, this amounts to more than 28 radiation lengths. This is equivalent in blocking power to a 1 m thickness of lead. This is true up to the energy of the highest known cosmic rays (some of which may be gamma rays). Much of the electromagnetic spectrum was not available until space techniques, first rockets and balloons, and later satellites, became accessible. Hence, until 1960, almost all astronomical observations came via the radio and optical windows. It may seem nonsensical then to speak of a ‘gamma-ray window’ where ground-based telescopes can make observations since no significant flux of primary gamma rays can penetrate even to the elevation of the highest mountain. However, there is a ‘gamma-ray window’ from about 100 GeV to 50 TeV where it has been possible to successfully pursue gammaray observations of cosmic sources using ground-based instruments. It is a fortunate coincidence in nature that while the gamma ray itself may not survive, the secondary products of its interaction with the atmosphere do survive and can be detected with the simple detectors described here. The techniques that are used in this window are described in this chapter. It is also a coincidence that the minimum energy that the gamma ray must have to be detectable from the ground is just above the maximum energy that has been detected by the space telescopes described in the following chapter.
