Wiktionary
argon-40
n. (context isotope English) The major stable isotope of argon, (nuclide 40 18 Ar), having eighteen protons and twenty-two neutrons; it amounts to about 99.6% of the element in nature
Usage examples of "argon-40".
Argon-39 and argon-40 dating had shown the chunk of troctolite breccia to be 3.
If you know how long it takes for potassium-40 to become argon-40, and you measure the amounts of each in a sample, you can work out how old a material is.