Radioluminescence

Radioluminescence occurs when an incoming particle of ionizing radiation collides with an atom or molecule, exciting an orbital electron to a higher energy level.

The continuous radioactive decay of the isotope's atoms releases radiation particles which strike the molecules of the phosphor, causing them to emit light.

The constant bombardment by radioactive particles causes the chemical breakdown of many types of phosphor, so radioluminescent paints lose some of their luminosity during their working life.

Zinc sulfide undergoes degradation of its crystal lattice structure, leading to gradual loss of brightness significantly faster than the depletion of radium.

[2] In addition to alpha and beta particles, radium emits penetrating gamma rays, which can pass through the metal and glass of a watch dial, and skin.

A typical older radium wristwatch dial has a radioactivity of 3–10 kBq and could expose its wearer to an annual dose of 24 millisieverts if worn continuously.

[2] Another health hazard is its decay product, the radioactive gas radon, which constitutes a significant risk even at extremely low concentrations when inhaled.

[3] The latest generation of radioluminescent materials is based on tritium, a radioactive isotope of hydrogen with half-life of 12.32 years that emits very low-energy beta radiation.

Tritium is used because it is believed to pose a negligible threat to human health, in contrast to the previous radioluminescent source, radium, which proved to be a significant radiological hazard.

Radioluminescent 1.8- curie (67 GBq ) 6-by-0.2-inch (152.4 mm × 5.1 mm) tritium vial used as a light source. It consists of a sealed glass tube containing radioactive tritium gas whose inner surfaces are coated with a phosphor .
A 1950s radium clock, exposed to ultraviolet light to increase luminescence
Self-luminous white radium paint on the face and hand of an old clock.
Watch face illuminated by tritium tubes