Hydrogen isotope biogeochemistry assumed that helium-3 was the radioactive component. The work of Luis Walter Alvarez and Robert Cornog first isolated tritium and reversed Rutherford's incorrect notion. Alvarez reasoned that tritium was radioactive, but did not measure the half-life, although calculations at the time suggested over ten years. At the end of World War II, the physical chemist Willard Libby detected the residual radioactivity of a tritium sample with a Geiger counter, providing a more accurate understanding of the half-life, now accepted at 12.3 years. The discovery of hydrogen isotopes also impacted the field of physics in the 1940s, as Nuclear Magnetic Resonance
Tritium radioluminescence for night use and small lights (which need to be more reliable than battery powered lights yet not interfere with night vision or be bright enough to easily give away one's location) used mostly by military personnel fall under the latter application. Tritium lighting is made using glass tubes with a phosphor layer in them and tritium gas inside the tube. Such a tube is known as a "gaseous tritium light source" (GTLS), or "beta light" (since the tritium undergoes beta decay). The tritium in a gaseous tritium light source undergoes beta decay, releasing electrons that cause the phosphor layer