Isotopes of oxygen

Oxygen-14 (half-life 70.62 seconds) is the second most stable radioisotope of oxygen, and decays by positron emission to nitrogen-14. Oxygen-14 ion beams are of interest to researchers of proton-rich nuclei; for example, one early experiment at the Facility for Rare Isotope Beams in East Lansing, Michigan, produced a 14O beam by proton bombardment of 14N, using it to determine the absolute strength of the electron capture transition. ==Oxygen-15==

Oxygen-15 (half-life 122.27 seconds) is the most stable radioisotope of oxygen, decaying by positron emission to nitrogen-15. It is thus the isotope of oxygen used in positron emission tomography (PET). It can be used in, among other things, water for PET myocardial perfusion imaging and for brain imaging. It is produced for this application through deuteron bombardment of nitrogen-14 using a cyclotron. : + → + n Oxygen-15 and nitrogen-13 are produced in air when gamma rays (for example from lightning) knock neutrons out of 16O and 14N: : + γ → + n : + γ → + n decays to , emitting a positron. The positron quickly annihilates with an electron, producing two gamma rays of about 511 keV. After a lightning bolt, this gamma radiation dies down with half-life of 2 minutes, but these low-energy gamma rays go on average only about 90 metres through the air. Together with rays produced from positrons from nitrogen-13 they may only be detected for a minute or so as the "cloud" of and floats by, carried by the wind. ==Oxygen-16==

Oxygen-16 (symbol: 16O or ) is a stable isotope of oxygen, with 8 neutrons and 8 protons in its nucleus, making it a doubly magic nuclide. It is the most abundant isotope of oxygen, accounting for about 99.76% of all oxygen. The relative and absolute abundances of oxygen-16 are high because it is a principal product of stellar evolution. It can be made by stars that were initially made exclusively of hydrogen. Most oxygen-16 is synthesized at the end of the helium fusion process in stars. The triple-alpha process creates carbon-12, which captures an additional helium-4 to make oxygen-16. It is also created by the neon-burning process. Prior to the definition of the dalton based on , one atomic mass unit was defined as one sixteenth of the mass of an oxygen-16 atom. Since physicists referred to only, while chemists meant the natural mix of isotopes, this led to slightly different mass scales. ==Oxygen-17==

Oxygen-17 (17O) is a stable isotope of oxygen with a low isotopic abundance of about 0.038%. is primarily made by burning hydrogen into helium in the CNO cycle, making it a common isotope in the hydrogen burning zones of stars.) at high magnetic fields. Water used as nuclear reactor coolant is subjected to intense neutron flux. Natural water starts out with 0.038% of 17O; heavy water starts out incidentally enriched to about 0.055% in that isotopes. Further, the neutron flux slowly converts 16O in the cooling water to 17O by neutron capture, increasing its concentration. The neutron flux slowly converts 17O (with much greater cross section) in the cooling water to carbon-14, an undesirable product that can escape to the environment: : 17O (n,α) → 14C Some tritium removal facilities make a point of replacing the oxygen of the water with natural oxygen (mostly 16O) to give the added benefit of reducing 14C production. The isotope was first hypothesized and subsequently imaged by Patrick Blackett in Rutherford's lab in 1925: It was a product out of the first man-made transmutation of 14N and 4He2+ conducted by Frederick Soddy and Ernest Rutherford in 1917–1919. Its presence in Earth's atmosphere was later detected in 1929 by Giauque and Johnson in absorption spectra, demonstrating its natural existece. ==Oxygen-18==

Oxygen-18 (, Ω) is one of the stable isotopes of oxygen, with roughly 0.20% abundance, and considered one of the environmental isotopes. Most is produced when Nitrogen| (made abundant from CNO burning) captures a nucleus, becoming Fluorine-18|. This quickly (half-life around 110 minutes) beta decays to making that isotope common in the helium-rich zones of stars. Measurements of the 18O/16O ratio (known as δ) are often used in paleoclimatology. Water molecules with a lighter isotope are slightly more likely to evaporate and less likely to fall as precipitation, so Earth's freshwater and polar ice have slightly less () than air () or seawater (). This disparity allows the study of historical temperature patterns via the analysis of ice cores. Assuming that atmospheric circulation and elevation has not changed significantly over the poles, the temperature of ice formation can be calculated as equilibrium fractionation between phases of water that is known for different temperatures. Water molecules are also subject to Rayleigh fractionation as atmospheric water moves from the equator poleward which results in progressive depletion of , or lower δ values. The δ ratio can also be used in paleothermometry for certain types of fossils. The fossil material used is generally calcite or aragonite, however oxygen isotope paleothermometry has also been done of phosphatic fossils using SHRIMP. For determination of ocean temperatures over geologic time, multiple fossils of the same species in different stratigraphic layers would be measured, and the difference between them would indicate long term changes. 18O has also been used to trace ocean composition and temperature which seafood is from. In the study of plants' photorespiration, the labeling of atmosphere by oxygen-18 allows for the measurement of oxygen uptake by the photorespiration pathway. Labeling by gives the unidirectional flux of uptake, while there is a net photosynthetic evolution. It was demonstrated that, under preindustrial atmosphere, most plants reabsorb, by photorespiration, half of the oxygen produced by photosynthesis. Then, the yield of photosynthesis was halved by the presence of oxygen in atmosphere. ==Oxygen-20==

Oxygen-20 has a half-life of and decays by β− decay to 20F. It is one of the known cluster decay ejected particles, being emitted in the decay of 228Th with a branching ratio of about . == See also ==