Cosmic Rays

Abstract

The history of the discovery of cosmic rays is fascinating in itself and corresponds to a pioneering phase of Astrophysics in the early 20th century, when Relativity and Quantum Mechanics changed our perspective of the physical world. In this context the Austrian physicist Victor Hess was studying the radioactivity of elements (then only recently discovered) and related problems, and decided to attempt direct measurement of the degree of ionization in the high atmosphere. This problem was intriguing because there was evidence in favor of an increase in ionization with height. Hess refined an electroscope to measure ionization and took it in person over 3 km up with the help of balloons (Fig. 12.1) between 1911 and 1912. His data showed that this result was correct, and that it depended little on whether the measurement was made during the day or at night. The data obtained during a solar eclipse were particularly important for attempts to characterize the phenomenon. The origin and propagation of cosmic rays are addressed here, in particular, the so called Ultra-High energy range with all the associated puzzles and questions.