These isotopes are stable, which is why they are with us today, but unstable isotopes are also present in minute amounts.About one carbon atom in a trillion (10) contains a radioactive nucleus with 6 protons and 8 neutrons — carbon 14. They break down and change into a completely different type of atom. For example, carbon-14 decays to nitrogen-14 when it emits beta radiation.It is not possible to predict when an individual atom might decay.But it is possible to measure how long it takes for half the nuclei of a piece of radioactive material to decay.This is called the half-life of the radioactive isotope.
A useful application of half-lives is radioactive dating. Ninety-five percent of the activity of Oxalic Acid from the year 1950 is equal to the measured activity of the absolute radiocarbon standard which is 1890 wood. This is the International Radiocarbon Dating Standard.Another standard, Oxalic Acid II was prepared when stocks of HOx 1 began to dwindle. The ratio of the activity of Oxalic acid II to 1 is 1.29330.001 (the weighted mean) (Mann, 1983). There are other secondary radiocarbon standards, the most common is ANU (Australian National University) sucrose.The ratio of the activity of sucrose with 0.95 Ox was first measured by Polach at 1.50070.0052 (Polach, 1976b:122).