Introduction

The study of radioactivity has taken place continuously since the beginning of the century, and the great wealth of artificially radioactive nuclides which has become available since the invention of the cyclotron and the nuclear reactor has given great impetus to this work during the latter half of this period. The phenomenon of α-decay was first recorded by Rutherford in 1899 in uranium minerals, and is a process confined, with very few exceptions, to heavy elements, that is, to elements with mass numbers greater than 200. The existence of natural decay series denoted by the mass numbers 4n, 4n + 2, 4n 4 + 3, was recognised during the early part of this period, but the fourth series (4n+1) was not discovered until much later, because its longest lived member (237Np) has a half-life of only 2.2 × 106 years, which prevents the occurrence of members of this series in nature.

The existence of a more penetrating radiation was also recognised at an early stage, and had in fact been responsible for the original discovery of radioactivity by Becquerel in 1896. This more penetrating radiation consisted of the β-particles, soon to be identified with the negative electrons produced in discharge-tube experiments.