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Geochronological “clock” readjusted

Accurate determination of the half-life of potassium-40

PTBnews 3.2023
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In geochronological sciences, for example when determining the age of rocks, potassium-40 is known to be one of the most important tools. To this end, its half-life and further decay data must be known as accurately as possible. These have been redetermined in sophisticated experiments.

Liquid scintillation samples on the Karlsruhe nuclide map with an (outdated) potassium-40 half-life value

When radioactive isotopes are used to determine age, their continuous decay comes into play. Potassium-40 decays into both calcium-40 and argon-40 thus providing several possibilities to determine the age of a rock. If the rock was once liquid, the ratio of potassium-40 and argon-40 can be used to determine the time of solidification. Scientists often use the so-called argon-argon method, where the rock is first irradiated with fast neutrons which convert potassium-39 nuclei to argon-39. This makes very accurate dating possible. However, the accuracy of any dating based on potassium-40 depends on knowing the partial half-lives based on the total half-life and the probabilities of decay to argon-40 and/or calcium-40.

In 2004, PTB had already determined the half-life of potassium-40 using liquid scintillation counting to determine the activity. For this purpose, PTB used potassium salts with a natural isotopic composition, which contained only little potassium-40. Therefore, only low count rates were obtained. PTB’s experiment was considered by many scientists to be the most accurate half-life determination of potassium-40 by activity measurement. However, in geochronology, researchers to some extent came to slightly different results with the help of age comparisons. To this end, such rocks were used such rocks whose age had been determined using the uranium-lead method.

In cooperation with partner institutions, PTB has now performed a very broad series of experiments. In these experiments, two solutions were used where potassium-40 is enriched to a proportion that is higher by a factor of approx. 265 than in naturally occurring materials. In one of the solutions, the number of potassium-40 nuclei was determined very accurately by mass spectrometry at the Australian National University (ANU). The specific activity was again measured with liquid scintillation counters, this time using two different measurement methods. They provided consistent results. The combination of specific activity and the number of K-40 nuclei determined by mass spectrometry leads to a half-life of 1.2536(27) billion years. The probabilities for decay to argon-40 and calcium-40 were also newly determined. With these decay data, the measurement results of the previous PTB publication can be harmonized with the new measurement as well as with the results obtained by the geochronologists.

With the new determination of the potassium-40 decay data, the dating of terrestrial and extraterrestrial rocks will now be on a more solid basis.