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.
A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is an atom that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferred to one of its electrons to release it as a conversion electron; or used to create and emit a new particle (alpha particle or beta particle) from the nucleus.