All of the known stable isotopes occur naturally (see primordial isotope). Twenty radioactive isotopes of plutonium have been characterized. The nuclei of several hafnium isotopes can each absorb multiple neutrons. Diamonds are dated by analyzing inclusions using the decay of radioactive isotopes. All other isotopes of aluminium are radioactive. Polonium has 42 known isotopes, all of which are radioactive. Examples of fissile isotopes are uranium-235 and plutonium-239. A nuclear reactor consumes certain specific fissile isotopes to produce energy. There are no stable isotopes of astatine. There are nine known isotopes of helium, but only helium-3 and helium-4 are stable. See table of nuclides for a complete list of isotopes. Naturally occurring ruthenium is composed of seven stable isotopes. These are the only two stable isotopes of helium. Fluorine-20 is one of the more unstable isotopes of fluorine. U-235 and U-238 isotopes by Gaseous diffusion.