International School of Cosmic Ray Astrophysics <<Maurice M. Shapiro>>

21^st Course: Astroparticle Physics: Yesterday, Today, and Tomorrow
1-7 August, 2018

It is difficult to make direct measurements of cosmic rays above an energy of ~1015 eV as the flux is very low. To study cosmic rays of higher energies use must be made of extensive air showers, cascades of particles generated in the atmosphere that sweep to ground like a giant dinner plate moving at the speed of light. In my first lecture I will briefly review the history of the discovery of the phenomenon - for which Pierre Auger is usually, but incorrectly, given the credit. I will explain the basic physics that controls air-shower development and then review the pioneering projects of the 1950s and 1960s during which period nearly all of the techniques used in modern work were developed. Specifically I will discuss the work at MIT that led to advances made at Chacaltaya and Volcano Ranch where the first large arrays were constructed, the importance of Cherenkov radiation in several aspects of shower work, and the key invention of shower detection through fluorescence radiation. What can with hindsight be seen as an exploratory phase, ended in the late 1980s with the conclusion that even areas of 100 km2 were too small to establish the parameters needed to pin down the origin of cosmic rays above 1018 eV. This led to the conception and construction of the Pierre Auger Observatory, the largest cosmic ray detector ever built. It covers 3000 km2 in Western Argentina where 1600 water-Cherenkov detectors are overlooked by sets of fluorescence telescopes in a 'hybrid mode'. Since 2004 high-precision measurements of cosmic rays with energies above ~ 3 x 1017 eV have been made. I will describe the important features of the Observatory so that you can appreciate the results from it that will be discussed in my second lecture.