A "rare isotope" refers to a type of atom that occurs very infrequently in nature. It is an atom with a different number of neutrons compared to the more common version of that element. Full definition
Isotopic analysis of an animal's tissues can yield clues to its diet because of natural variability in the abundance of rare isotopes of elements such as carbon and nitrogen. (sciencedaily.com)
That's consistent with the funding needed for construction of the new Facility for Rare Isotope Beams at Michigan State University. (sciencemag.org)
Today, Merminga manages her dream project: leading the design and construction of a new accelerator facility to produce rare isotopes for nuclear physics and medicine. (sciencemag.org)