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 the fission reaction in the fissionable core (pit) of the bomb, after it is compressed to a critical mass by explosives. Actuated by an ultrafast switch like a krytron, a small particle accelerator drives ions of tritium and deuterium to energies above the 15 keV or so needed for deuterium-tritium fusion and directs them into a metal target where the tritium and deuterium are adsorbed as hydrides. High-energy fusion neutrons from the resulting fusion radiate in all directions. Some of these strike plutonium or uranium nuclei in the primary's pit, initiating nuclear chain reaction. The quantity of neutrons produced is