5He and 5Li, and the triple alpha process skips over all species between 4He and 12C, these elements are not produced in the main reactions of stellar nucleosynthesis. In addition, nuclei of these elements (such as 7Li) are relatively weakly bound, resulting in their rapid destruction in stars and no significant accumulation, although new theory suggests that 7Li is generated primarily in novae erruptions. It was thus postulated that another nucleosynthesis process occurring outside stars was necessary to explain their existence in the universe. This process is now known to occur in cosmic rays, where lower temperature and particle density favor reactions leading to the synthesis of lithium, beryllium, and boron. In addition to the above light elements, tritium and isotopes of aluminium, carbon (carbon-14), phosphorus (phosphorus-32), chlorine, iodine and neon are formed within solar system materials through cosmic ray spallation, and are termed cosmogenic nuclides. Since they remain trapped in the atmosphere or rock in which they formed, some can be very useful in the dating of materials by cosmogenic radionuclide dating, particularly in the geological field. In formation of a cosmogenic nuclide, a cosmic ray interacts with the nucleus of an in situ solar system atom, causing cosmic ray spallation. These isotopes are produced within earth materials such as rocks or soil, in Earth's atmosphere, and in extraterrestrial items such as meteorites. By measuring cosmogenic isotopes, scientists are able to gain insight into a range of geological and astronomical processes. There are both radioactive and stable cosmogenic isotopes. Some of the well-known naturally-occurring radioisotopes are tritium, carbon-14, and phosphorus-32. The timing of their formation determines whether nuclides formed by cosmic ray spallation are termed primordial or are termed cosmogenic (a nuclide cannot belong to both classes). The stable nuclides of lithium, beryllium, and boron found on Earth are thought to have been formed by the same process as the cosmogenic nuclides but at an earlier time in cosmic ray spallation predominantly before the solar system's formation, and thus they are by definition primordial nuclides and not cosmogenic. In contrast, the radioactive nuclide beryllium-7 falls into the same light element range but has a half-life too short for it to have been formed before the formation of the solar system, so that it cannot be a primordial nuclide. Since the cosmic ray spallation route is the most likely source of beryllium-7 in the environment, it is therefore cosmogenic. See