Ultra-high energy cosmic rays: Setting the stage

Abstract

The history of ultra-high energy cosmic ray physics is reviewed from the post-war era of arrays such as Volcano Ranch, Haverah Park and Akeno to the development of air-fluorescence and current hybrid arrays. The aim of this paper is to present the background information needed for a better understanding of the current issues in this field that are discussed in much greater depth in the rest of this conference. a. Pre-history: the discovery of cosmic radiation and the study of its penetrating power. This led to great debates between Millikan and Compton about the kinds of particles that composed it. The discovery of the latitude effect followed by the East-West effect conclusively proved that the majority of the cosmic ray particles were positively charged (protons and nuclei). b. Ancient history: the discovery that high energy cosmic radiation produced extensive air showers and that much of the cosmic radiation on the surface was the result of secondary processes in the atmosphere. This was also the era when the presence of very high energy particles in the primary radiation was clearly demonstrated (by B. Rossi, P. Auger and others). c. Middle Ages: the post-war development of large area surface detectors covering areas up to 10km 2 coming from the pioneering work of B. Rossi., J. Linsley and others. d. Renaissance: the development of fully calorimetric techniques based on air-fluorescence (by the Fly's Eye group, based on earlier work of J. Keuffel, K. Greisen and others) and the extension of ground arrays to 100km 2 (AGASA). e. Modern times: the refinement of air-fluorescence techniques (HiRes) and the construction of hybrid air-fluorescence/surface array detectors of unprecedented scale (Pierre Auger Observatory (PAO), Telescope Array (TA)). 2. POST-WAR DEVELOPMENTS