Radioluminescence

Radioluminescent 1.8-curie (67 GBq) 6-by-0.2-inch (150 mm × 5.1 mm) tritium vials are simply tritium gas-filled, thin glass vials whose inner surfaces are coated with a phosphor. The "gaseous tritium light source" vial shown here is brand new.

Radioluminescence is the phenomenon by which light is produced in a material by bombardment with ionizing radiation such as beta particles. Radioluminescence is used for emergency exit signs or other applications where light must be produced for long periods without external energy sources. Formerly, radioluminescent paint was used for clock hands and instrument dials allowing them to be read in the dark.

Tritium

Main article: Tritium illumination

Tritium is used as a source of beta particles in a large variety of applications where electricity is not available for illumination. For example, gun sights and emergency exit signs.

Radium

Main article: Radium dials

See also: Radium girls

A 1950s radium clock, exposed to ultraviolet light to increase luminescence

Historically a mixture of radium and copper-doped zinc sulfide was used to paint instrument dials giving a greenish glow. Phosphors containing copper doped zinc sulfide (ZnS:Cu) yield blue-green light; copper and manganese doped zinc sulfide (ZnS:Cu,Mn), yielding yellow-orange light, are also used. Radium based luminescent paint is no longer used due to the radiation hazard posed to those manufacturing the dials. These phosphors are not suitable for use in layers thicker than 25 mg/cm², as the self-absorption of the light then becomes a problem. Furthermore, zinc sulfide undergoes degradation of its crystal lattice structure, leading to gradual loss of brightness significantly faster than the depletion of radium.

ZnS:Ag coated spinthariscope screens were used by Ernest Rutherford in his experiments discovering the atomic nucleus.

Mechanism

Radioluminescence occurs when an incoming radiation particle collides with an atom or molecule, exciting an orbital electron to a higher energy level. The electron then returns to its ground energy level by emitting the extra energy as a photon of light.

See also

• Cherenkov radiation
• Gas mantle
• List of light sources
• Undark
• Radium Dial Company
• United States Radium Corporation
• Radioluminescence in phosphor