Radioactive Substances
Some HID lamps make use of radioactive substances such as krypton-85 and thorium. These isotopes, as they are also known, help start the lamps and improve lamp operating characteristics.
Krypton-85 is a gas and is found mixed in with the argon which is in the in the arc tube of the lamp. The thorium which is a solid is used in the electrodes.
These isotopes produce ionizing radiation. It is because of their particular ionisation properties that they are used in lamps. They produce alpha and beta radiation which causes high ionization inside the lamp but without being able to escape from the lamp. The amount of gamma radiation produced by the isotopes, which can escape from the lamp, is negligible.
Read more about this topic: High-intensity Discharge Lamp