Which isotope was designated as the standard reference for measuring atomic masses?

Carbon-12 was adopted as the standard reference for atomic mass measurements by the International Union of Pure and Applied Chemistry (IUPAin 1961. The atomic mass unit (amu) is defined as exactly one-twelfth of the mass of a carbon-12 atom. This choice was practical because carbon-12 is both stable and abundant in nature. Carbon-11 is radioactive with a short half-life of about 20 minutes, Carbon-14 is radioactive with a half-life of approximately 5,730 years (used in radiocarbon dating), and Carbon-13 is a stable isotope comprising about 1.1% of natural carbon.