Radioactive elements can emit alpha particles (helium nuclei), electrons, and gamma rays (high-energy electromagnetic radiation). The term radioluminescence, therefore, means that an appropriate material is excited to luminescence by a radioactive substance. When alpha particles bombard a crystal phosphor, tiny scintillations are visible to microscopic observation. This is the principle of the device used by an English physicist, Ernest Rutherford, to prove that an atom has a central nucleus. Self-luminous paints, such as are used for dial markings for watches and other instruments, owe their behaviour to radioluminescence. These paints consist of a phosphor and a radioactive substance, e.g., tritium or radium. An impressive natural radioluminescence is the aurora borealis: by the radioactive processes of the sun, enormous masses of electrons and ions are emitted into space in the solar wind. When they approach the Earth, they are concentrated by its geomagnetic field near the poles. Discharge processes of the particles in the upper atmosphere yield the famous luminance of the auroras.
Energy-levels-of-a-luminescent-centreFigure 1: Energy levels of a luminescent centre (see text).[Credits : Encyclopædia Britannica, Inc.]
Transition-of-an-electron-from-the-valence-band-to-theFigure 2: Transition of an electron from the valence band to the conduction band by light …[Credits : Encyclopædia Britannica, Inc.]
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