Influence Of The Radioactivity Of Thorium On The Starting Of Metal Halide Lamps

Thorium is sometimes added to the chemical fill of metal halide lamps to improve the color rendering properties of the emitted radiation. Alternately, a few monolayers of elemental thorium on the surface of the tungsten electrodes in the arc tube can aid in the starting of the lamp by reducing the operating temperature of the thermionically emitting electrode by lowering the work function of the tungsten from Ф = 4.6 eV to ≃ 2.7 eV. Thorium is also weakly radioactive and the ionizing radiation produced in the radioactive decay of thorium is a source of free electrons in the arc tube and can also aid in reducing the statistical lag time of the gas breakdown process during the starting of the lamp. The expected effect that the presence of thorium has on the statistical lag time can be found by calculating the electron production rate in the arc tube that is attributable to thorium, and comparing this rate to that estimated from background radiation sources.