39Ar-40Ar Method

Nuclear dating plays an important role in the fields of Earth Sciences and Archaeology as one of the few viable techniques for determining the age of geological samples. Argon-39/Argon-40 dating is the most widely used variant of nuclear dating due to its accuracy, the small sample size required and the wide sample age range for which it is valid. The initial step in the dating process is the irradiation of the geological sample in a neutron flux to convert a portion of the the naturally occurring stable isotope K-39 to Ar-39, a radioisotope with a moderate half-life of 269 years. Another isotope of argon, Ar-40, will also be present in the sample as the decay product of the naturally occurring long-lived radioisotope K-40 with a half life of 1,280,000,000 years. The age of the sample can be determined indirectly by measuring the ratio of Ar-39:Ar-40 via mass spectrometry.

McMaster Nuclear Reactor is well-suited for use as a neutron source for the 39-Argon/40-Argon dating technique because of its near-optimal fast neutron flux, the convenience and flexibility of the irradiation facility, and the relatively low sample heating resulting from its water-cooled design. Since the optimum irradiation period depends on the age of the samples, it is more convenient and cost-effective to use a number of small capsules irradiated for differing lengths of time than to use the large capsules required in many other facilities.

EPR Method

EPR dating is an alternative to nuclear dating that combines a type of molecular spectroscopy with MNR’s Cobalt-60 source. Electron Paramagnetic Resonance (EPR) spectroscopy is a method of monitoring the electronic configuration or spin state of a paramagnetic molecule or lattice. A sample is placed in an external magnetic field that aligns the electron spins, and a microwave radiation source is applied over a range of frequencies. Absorption of microwaves will occur when the energy of the radiation matches the energy required to induce a spin transition (“spin flip”) in the sample. This resonance absorption is directly proportional to the population of unpaired electrons in that particular energy state.

FossilExposure of materials to naturally occurring ionizing radiation (e.g. uranium, thorium, plutonium) over geological time causes the formation of paramagnetic defects in the crystal lattice that can be observed by EPR spectroscopy. Using an “additive dose” method of dating, a sample is deliberately exposed to gamma radiation using, for example, the Co-60 source housed at MNR, to examine the dose-response relationship between the applied radiation field and the extent of the paramagnetic defects. From this information, it is possible to extrapolate the dose a particular sample has received over time due to naturally occurring radiation, and therefore the age of the sample.

Materials suitable for EPR dating analysis include those containing apatite, aragonite, calcite, hydroxyapatite or silicate-based minerals. This method of dating has been applied extensively for samples of bones and teeth, fossilized organisms, volcanic and sedimentary rocks and geological deposits of ages ranging from 5,000 to 1,000,000 years.