A ground based gamma ray telescope of high sensitivity and low energy threshold
The subject of this thesis is ground based gamma ray astronomy using the atmospheric Cerenkov technique. Chapter 1 defines the gamma ray region of the electromagnetic spectrum, introduces celestial gamma rays as a component of the flux of energetic particles known as cosmic rays and considers the physical mechanisms by which celestial gamma rays may be produced and absorbed. The phenomenon of Cerenkov radiation, and Its production within the extensive air showers which result from the Interaction of energetic cosmic rays with the atmosphere. Is the subject of Chapter 2. Chapter 3 introduces the early Durham telescopes, and considers the possible improvement to instrument sensitivity afforded by invokatlon of more sophisticated background rejection strategies. The Mk.5 telescope, which is the subject of Chapter 4, Is the vehicle by which the Durham group has sought to assess the relative merits of various signal enhancement strategies, and prove the viability of a design for a high resolution Imaging detector with the additional capacity to make observations at very low energies; the Mk.6 telescope. Chapter 5 introduces a novel concept of background rejection based upon the stereoscopic Imaging of extensive air showers, and describes modifications to the Mk.3 necessary to provide a complement to the Mk.5 in this capacity. Chapter 6 presents the analysis of a burst of periodic gamma ray emission from AE Aquarii, which demonstrates the efficacy of the medium resolution 'mono' Imaging and stereoscopic imaging signal enhancement techniques employed by the Mk.5 and modified Mk.3 telescopes. The culmination of the evolution of the Durham telescopes is the Mk.6, described in Chapter 7. Its sensitivity is enhanced by high resolution imaging and the capacity to make observations at energies deficient in background events. Chapter 8 summarises the contemporary status of atmospheric Cerenkov astronomy.