TABLE OF CONTENTS The phenomenon of the natural radioactivity of some chemical elements was first appreciated by Becquerel in 1896. He observed that photographic emulsions wrapped in black paper and placed near a uranium compound, potassium uranyl sulfate, were blackened. This effect was subsequently attributed to the emission of a radiation by the uranium with properties not dissimilar to those of the already known X-rays, in that it was capable of ionising air, and the activity of a uranium compound could be measured by the rate at which a known quantity could bring about the discharge of an electroscope. The emission of these rays was a fundamental property of the uranium atom, the activity being independent of the nature of the compound, of its valence state, of the temperature or of the previous history of the material. The spontaneous emission of radiation of this type is now known as radioactivity.
This ionising radiation differs from non-ionising radiation, such as light or radio waves, in its possession of sufficient energy to remove electrons from the atoms of matter through which it passes, and therein lies its particular hazard. This is in contrast to non-ionising radiation, which does not normally possess this property.
Following Becquerel's discovery, the work of Rutherford and Soddy, and of P. and M. Curie, established that the nuclei of some natural elements were not completely stable. These unstable elements were found to emit radiation of three main types, two having the properties of charged particles...
