As Marie Curie proposed almost a hundred years ago, radioactivity comes from the nucleus. This is why physicists refer to those electromagnetic rays (or photons) emitted by nuclei as ‘gamma rays’ and all radiation emanating from the deeper shells of the atom as ‘X-rays’. In fact, all electromagnetic radiation (radio waves, infra red, visible light, ultraviolet, X or gamma) have similar properties: the only differences lie in their energies and corresponding wavelengths. Physicists nowadays tend to classify all photons with energies in excess of 100 keV as ‘gamma rays’.