A recommendation letter or letter of recommendation, also known as a letter of reference, reference letter or simply reference, is a document in which the writer assesses the qualities, characteristics, and capabilities of the person being recommended in terms of that individual's ability to perform a particular task or function.
This article is about stable isotope ratios. For the general category of stable nuclides, see Stable nuclide. The term stable isotope has a similar meaning to stable nuclide, but is preferably used when speaking of nuclides of a specific element.his article is about stable isotope ratios. For the general category of stable nuclides, see Stable nuclide. The term stable isotope has a similar meaning to stable nuclide, but is preferably used when speaking of nuclides of a specific element.
Carbon-12 is by far the most abundant carbon isotope, and carbon-12 and -13 are both stable. But carbon-14 is slightly radioactive: it will spontaneously decay into nitrogen-14 by emitting an anti-neutrino and an electron, with a half-life of 5730 years.he theory behind radiocarbon dating is as follows: 1  Terrestrial carbon contains virtually no carbon-14, since any that may have been present would have long since decayed into the stable nitrogen-14. 2  Carbon dioxide in the atmosphere, however, contains a mixture of carbon-12 and carbon-14 in known proportions.