In the 1940s, as Los Alamos scientists raced to unlock and harness the physics of fission, a parallel scientific revolution was taking shape, one that would take nuclear science from splitting atoms to saving lives: biomedical isotopes. The science of isotopes was fundamental to the mission at Los Alamos from day one. Some isotopes fission readily, like the uranium used in nuclear reactors (U-235), while others don’t, like the uranium found in nature (U-238). The difference boils down to neutrons.