Isotope persisted down to the present because their rate of decay is so slow (e.g. uranium-238 and potassium-40). Post-primordial isotopes were created by cosmic ray bombardment as cosmogenic nuclides (e.g., tritium, carbon-14), or by the decay of a radioactive primordial isotope to a radioactive radiogenic nuclide daughter (e.g. uranium to radium). A few isotopes are naturally synthesized as nucleogenic nuclides, by some other natural nuclear reaction, such as when neutrons from natural nuclear fission are absorbed by another atom. As discussed above, only 80 elements have any stable isotopes, and 26 of these have only one stable isotope. Thus, about two-thirds
Environmental isotopes of these decays are alpha or beta decays, meaning that they all follow first order rate equations of the form formula_2, where λ is the half-life of the isotope in question. This makes it simple to determine the age of a sample based on the various ratios of radioactive isotopes that exist. One way uranium isotopes are used is to date rocks from millions to billions of years ago. This is through uranium-lead dating. This technique uses zircon samples and measures the lead content in them. Zircon incorporates uranium and thorium atoms into its crystal structure, but strongly rejects lead.