False-colour photograph of an alpha particle (blue track) in a hydrogen-filled cloud chamber. The wispy side shoots (red) are due to electrons which the alpha has knocked out of atoms in its path. Only when an alpha comes very close to an atomic electron does the electron acquire sufficient velocity to form a short track of its own. Such energetic electrons, which in this image travel up to 2.9mm, are known as insdelta raysins. Alpha particles, or alpha rays as they were originally known, are in fact the nuclei of helium atoms & consist of a particularly stable configuration of two protons & two neutrons.