Nuclear and Particle Physics. An Introduction

from 1896, the year that Henri Becquerel observed that photographic plates were

being fogged by an unknown radiation emanating from uranium ores. He had

accidentally discovered radioactivity : the fact that some nuclei are unstable and

spontaneously decay. In the years that followed, the phenomenon was extensively

investigated, notably by the husband and wife team of Pierre and Marie Curie and

by Ernest Rutherford and his collaborators, 2 and it was established that there were

three distinct types of radiation involved: these were named (by Rutherford) a-, ft-

and 7-rays. We know now that a-rays are bound states of two protons and two

neutrons (we will see later that they are the nuclei of helium atoms), /J-rays are

electrons and 7-rays are photons, the quanta of electromagnetic radiation, but the

historical names are still commonly used.

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