Radioisotope Dating Techniques

Radiocarbon dating (¹⁴C)

Radiocarbon (¹⁴C) is the radioactive isotope of the common element carbon. It is formed in the upper levels of the atmosphere

following the interaction of cosmic rays with nitrogen (N₂). Then radiocarbon is oxidized to carbon dioxide (CO2) and is

diffused in the atmosphere. As carbon dioxide is used for photosynthesis, radiocarbon is integrated into all organisms. When a

plant or an animal dies, radiocarbon decays with a half-life of 5730 years. At the radiocarbon dating laboratory the amount of

remaining radiocarbon relative to the stable element is measured. Since radiocarbon’s half-life is known, the age of the sample

can be estimated.

Tritium (³H) dating

Tritium (³H), the heaviest isotope of hydrogen, is radioactive and has a half-life of 12.4 years. Water in contact with the atmosphere

will have some tritium in it, and this tritium will be decaying to a stable, inert isotope called helium-3. Tritium can be

effectively used to investigate hydrologic mixing and transport processes. Due to its short half-life and steady state

concentration, tritium is the ideal tracer for studies requiring a time resolution to the nearest month over the past 150 to 200

years. In using tritium as a hydrological tracer, the analysis should be conducted at the lowest practical detection levels because

of the inherently low tritium levels found in natural waters (5 to 15 Tritium Units, or TUs (0.12 Bq/L)). Through electrolytic

enrichment of tritium in water samples prior to measurement, these low detection levels may be reached, affording greater

accuracy and precision.

Tritium dating can be used in Environmental Sciences, Hydro-geology, Geology, Climatology and other disciplines of science.

Uranium series dating

Uranium series dating is based on the radioactive decay of uranium in calcium carbonate and other minerals that precipitate

from solution. Natural ²³⁸U decays into ²³⁴Th, while the other isotope of U, ²³⁴U, decays into ²³⁰Th. Because uranium is soluble

in water and thorium is not, minerals that precipitate from solution often contain U but very little Th. Through time Th is

formed in the mineral as the U decays. The Th is itself a radioactive element and it decays into daughter products (i.e. ²³⁴Th

decays into ²³⁴U and ²³⁰Th decays into ²²⁶Ra). Ra is also radioactive, and it decays into Rn in a very short time.

If the mineral is of sufficient age, it contains the entire chain of U decay products, from ²³⁸U through to ²⁰⁶Pb. The critical

elements in this series are ²³⁸U, ²³⁴U and ²³⁰Th because of their half-lives. For any mineral that initially contained only U, the

time since its formation can be calculated from the ²³⁸U/ ²³⁴U and ²³⁴U/ ²³⁰ Th ratios.