Definition
A radioactive isotope of hydrogen whose nucleus contains one proton and two neutrons. Tritium emits low-energy beta radiation and is used in aviation as the energy source in self-luminous instrument markings, exit signs, and emergency lighting. The beta particles excite a phosphor coating, producing a soft glow that requires no external power and continues for years.
Plain English
A mildly radioactive form of hydrogen that makes certain instrument markings and signs glow on their own, without batteries or electricity.
Context Anchor
Seen in maintenance descriptions for self-lighted cockpit markings, emergency signs, or small instrument light sources.
Derivation
From the Greek 'tritos' meaning 'third.' Hydrogen has three forms: ordinary hydrogen (one proton), deuterium (one proton, one neutron), and tritium (one proton, two neutrons) -- the third and heaviest form. Knowing it is the 'third' hydrogen helps explain why it behaves chemically like hydrogen but is heavier and unstable.
Why Pilots Care
It supplies reliable low-light visibility for critical instruments and safety markings during night operations or power loss.
Intuition Check
Tritium is radioactive, but the beta radiation it emits cannot penetrate the glass tube containing it. The hazard arises only if the tube is broken and the gas is inhaled or ingested.
Example Sentence 1
The cabin exit signs are tritium-illuminated, so they remain visible even after a complete electrical failure.
Example Sentence 2
During preflight the pilot noted the tritium exit signs still provided enough glow for an emergency evacuation in darkness.