Editing Heavy water (section)

===Heavy-oxygen water===
Water enriched in the heavier oxygen isotopes [[oxygen-17|{{chem|17|O}}]] and [[oxygen-18|{{chem|18|O}}]] is also commercially available. It is "heavy water" as it is denser than normal water ({{chem|H|2}}{{chem|18|O}} is approximately as dense as {{chem|D|2|O}}, {{chem|H|2}}{{chem|17|O}} is about halfway between {{chem|H|2|O}} and {{chem|D|2|O}})—but is rarely called heavy water, since it does not contain the excess deuterium that gives D{{sub|2}}O its unusual nuclear and biological properties. It is more expensive than D{{sub|2}}O due to the more difficult separation of {{sup|17}}O and {{sup|18}}O.<ref>Mosin, O. V, Ignatov, I. (2011) Separation of Heavy Isotopes Deuterium (D) and Tritium (T) and Oxygen ({{sup|18}}O) in Water Treatment, Clean Water: Problems and Decisions, No. 3–4, pp. 69–78.</ref> H{{sub|2}}{{sup|18}}O is also used for production of [[fluorine-18]] in [[radiopharmaceutical]]s and [[radiotracer]]s, and [[positron emission tomography]]. Small amounts of {{chem|17|O}} and {{chem|18|O}} are naturally present in water, and most processes enriching heavy water also enrich heavier isotopes of oxygen as a side-effect. This is undesirable if the heavy water is to be used as a neutron moderator in nuclear reactors, as {{chem|17|O}} can undergo neutron capture, followed by emission of an [[alpha particle]], producing radioactive {{chem|14|C}}. However, [[doubly labeled water]], containing both a heavy oxygen and hydrogen, is useful as a non-radioactive isotopic tracer.

Compared to the isotopic change of hydrogen atoms, the isotopic change of oxygen has a smaller effect on the physical properties.<ref>Steckel, F., & Szapiro, S. (1963). Physical properties of heavy oxygen water. Part 1.—Density and thermal expansion. Transactions of the Faraday Society, 59, 331-343.</ref>