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
On-Line Isotope Mass Separator more than 60 physics experiments are active there. Radioactive nuclei are produced at ISOLDE by impinging a high energy beam of protons on a stationary target. There are a number of target materials depending on the desired final species. Collisions between the proton beam and the target produce a variety of fragments, which are extracted and filtered to yield the desired isotope. The time required for extraction places a lower limit on the half life of isotopes which can be produced by this method. Once extracted, the isotopes are directed either to one of several detectors, an isotope harvesting area,