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.
Helium-3 was thought to be a radioactive isotope until helions were also found in samples of natural helium, which is mostly helium-4, taken both from the terrestrial atmosphere and from natural gas wells. Other than 1H, helium-3 is the only stable isotope of any element with more protons than neutrons. Helium-3 occurs as a primordial nuclide, escaping from the Earth's crust into the atmosphere and into outer space over millions of years.