Radioactivity - What is it?
Radioactivity is the spontaneous dissociation of atomic nuclei which results in a transmutation of the element involved. This transformation causes the emission of energy, frequently in the form of energetic rays which are capable of ionizing the surrounding medium.

The science of Nuclear Physics can be said to have started with the discovery of radioactivity. In order to understand the developments in the 20th century we need to make an excursion into the last years of the 19th century.

But! First we will briefly step back more than two thousand years in order to understand why radioactivity was such a startling phenomenon at the turn of the twentieth century.

 Lucretius - On the Nature of Things

There were many people involved with the elucidation of radioactivity. In this lecture we will not be able to cover the contributions of all these people.

1896 - Wilhelm Roentgen - X-Rays , highly ionizing rays from cathode rays

1896  Henri Becquerel - discovers spontaneously emitted radiation from Uranium

1897 - J.J. Thompson proposes the existence of electrons. This proposal undermines the role of the chemical elements as the "true atoms" of nature. He proposes a theory of how air can become a conductor by a process of ionization. His student, E. Rutherford shows through experimental work that this theory is correct and also discovers the phenomenon of a saturation current in ionized air.

1897 - 1903 Marie and Pierre Curie  - pioneering research on radioactivity leading to the discovery of polonium and radium

1898- 1904 Ernest Rutherford et al. - discovery of transmutation of the elements

1906-1911 Rutherford, Geiger, Marsden - discovery of the atomic nucleus

1919 Rutherford produces a change in an atomic nucleus by bombarding nitrogen with alpha particles from a radium source
⁴He + ¹⁴N -> ¹⁷O + ¹H .

1913-1932 - The nucleus as a test lab for quantum mechanics

1899-1933 Beta decay and neutrinos - history from Queens University

1933 - 1934 Joliot-Curie, Chadwick - The discovery of the neutron and of artificially induced radioactivity.

1930 -> present  Accelerators - pioneers ,Cockroft, Walton, Lawrence

1896 - 1939  Summary  - What had the nucleus taught us so far?