Introduction

Radioactivity, from the 1890s to the 1990s

Radioactivity is a characteristic of the nuclei of atoms. The nuclei, and with them the atoms as a whole, undergo spontaneous changes known as radio-active or nuclear transformations and also as decays or disintegrations. The energy released per nuclear transformation and carried away as nuclear radiation is, as a rule, some 103 to 106 times greater than the energy released per atom involved in chemical reactions.

Radioactivity was discovered in 1896 by the Frenchman H. Becquerel. The discovery occurred while he was experimenting with phosphorescence in compounds of uranium, an investigation aiming only at knowledge for its own sake. However, practical applications of radioactivity appeared not long after its discovery and have multiplied ever since.

Radioactivity could not have been discovered much before 1896 because at naturally occurring intensities it is undetectable by the unaided human senses. The photographic technique which contributed to its discovery was not adequately developed until well into the nineteenth century. But by the end of that century it played a major part in two discoveries which changed the path of science and of history: Röntgen's discovery of X rays in Germany in late 1895, followed by Becquerel's discovery of radioactivity in France in early 1896. These completely unexpected events opened the doors to totally new physical realities, to the emerging world of the nuclei of atoms and the high-energy nuclear radiations emitted by these nuclei.

Following Becquerel's discovery, it took about 35 years of intense scientific work before radioactive atoms could be produced from stable atoms by man-made procedures.