International School of Cosmic Ray Astrophysics
14th Course: "Neutrinos and Explosive Events in the Universe"
A NATO Advanced Study Institute
2-13 July 2004
Ettore Majorana Centre
Erice, Sicily, Italy
Topic: The Nature of Cosmic-ray Primaries – a Brief Review
Lecturer: Maurice Shapiro
The mutability of cherished physical theories is exemplified by the early history of the cosmic-ray discipline. Widely held views as to the identity of the penetrating primaries changed from decade to decade. First they were gamma rays; then, successively, electrons (and positrons), protons, heavier nuclei up to iron, culminating in the periodic table of the elements. Paradoxically the progression came full circle with the advent of gamma-ray astronomy, albeit the fluxes are orders of magnitude lower than those of the nuclei. High-energy neutrino astronomy, in gestation since the 1970’s, is now established. The isotopic composition of the primaries differs completely from the universal abundances. Until the advent, in the 1950’s, of GeV machines, nature’s accelerator provided the means for the discovery of new elementary particles.