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There are three known stable of oxygen (8O): , , and . Radioisotopes are known from 11O to 28O (particle-bound from mass number 13 to 24), and the most stable are with half-life 122.27 seconds and with half-life 70.62 seconds. All remaining radioisotopes are even shorter in lifetime. The four heaviest known isotopes (up to ) decay by neutron emission to , whose half-life is 77 milliseconds; 24O, along with 28Ne, have been used in the model of reactions in the crust of neutron stars. The most common decay mode for isotopes lighter than the stable isotopes is β+ decay to nitrogen, and the most common mode after is beta decay to fluorine.
has high relative and absolute abundance because it is a principal product of stellar evolution and because it is a primary isotope, meaning it can be made by [[star]]s that were initially [[hydrogen]] only.Most is nucleosynthesis at the end of the helium fusion process in ; the triple-alpha process creates , which captures an additional nucleus to produce . The neon burning process creates additional .
Both and are secondary isotopes, meaning their synthesis requires seed nuclei. is primarily made by burning hydrogen into helium in the CNO cycle, making it a common isotope in the hydrogen burning zones of stars. Most is produced when Nitrogen (made abundant from CNO burning) captures a nucleus, becoming . This quickly (half-life around 110 minutes) to making that isotope common in the helium-rich zones of stars. Temperatures on the order of 109 are needed to nuclear fusion oxygen into sulfur.
An atomic mass of 16 was assigned to oxygen prior to the definition of the dalton based on . Since physicists referred to only, while chemists meant the natural mix of isotopes, this led to slightly different mass scales.
Solid samples (organic and inorganic) for oxygen isotopic ratios are usually stored in silver cups and measured with pyrolysis and mass spectrometry. Researchers need to avoid improper or prolonged storage of the samples for accurate measurements.
Due to natural oxygen being mostly , samples enriched with the other stable isotopes can be used for isotope labeling. For example, it was proven that the oxygen released in photosynthesis originates in , rather than in the also consumed , by isotope tracing experiments. The oxygen contained in in turn is used to make up the sugars formed by photosynthesis.
In heavy-water nuclear reactors the neutron moderator should preferably be low in and due to their higher neutron absorption cross section compared to . While this effect can also be observed in light-water reactors, ordinary hydrogen (protium) has a higher absorption cross section than any stable isotope of oxygen and its number density is twice as high in water as that of oxygen, so that the effect is negligible. As some methods of isotope separation enrich not only heavier isotopes of hydrogen but also heavier isotopes of oxygen when producing heavy water, the concentration of and can be measurably higher. Furthermore, the (n,α) reaction is a further undesirable result of an elevated concentration of heavier isotopes of oxygen. Therefore, facilities which remove tritium from heavy water used in nuclear reactors often also remove or at least reduce the amount of heavier isotopes of oxygen.
Oxygen isotopes are also used to trace ocean composition and temperature which seafood is from.
Oxygen-14 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.
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.
Oxygen-15 and nitrogen-13 are produced in air when (for example from lightning) knock neutronsIf protons were knocked out, the stable isotopes 13C and 15N would be formed. out of 16O and 14N:
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.
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