Researchers examine an overlooked mode of weathering that has implications for fundamental physics and the dating of some of the oldest rocks in the Earth and Solar System.
Potassium-40 has a half-life of 1,25 billion years, so it doesn't decay as often, but when it does, its effects are significant. Potassium-40 is one of the main sources of radioactivity we come into contact with on a daily basis. It is a very common (2,4% of all potassium) isotope of a very common metal (0,012% by mass of the earth's crust). The abundance of heat emanating from these decays caused the initial estimates of the age of the Earth by Lord Kelvin to be wrong. These decays are the main source of argon-1, which makes up about 40% of the atmosphere.
The small amount of radioactivity found in foods like bananas is mostly due to potassium-40, which is a major source of noise in some highly sensitive particle physics detectors. This isotope and the byproducts of its breakdown can be used to date rocks and geological processes that date back to the beginning of Earth's history. However, there has always been considerable doubt about these extensively studied decays.
The rare decay mode of potassium-40 to argon-40 was directly seen for the first time by the KDK Collaboration. Based on the measured decay rate, the probability of this decay mode is lower than previously thought. The findings will have a small but significant impact on the field of geochronology as well as other disciplines that use or attempt to avoid the widespread decay of this element.
What is KDK?
Potassium-40 (40K), which is in the background of many rare event studies, may be important in interpreting the findings of the DAMA dark matter investigation. It is unclear how much background is added to the ground state of 40 Ar by electron capture decay of 40K. The Modular Total Absorption Spectrometer at Oak Ridge National Laboratory, a 40K source, and an X-ray detector are used by the KDK (potassium decay) team to measure this branching rate.
The degradation process of potassium-40 is somewhat complex. It does not have a long lineage like uranium. However, it has some interesting properties such as most of the remaining 10% decays to argon-40 mentioned above via electron capture, and about 40% of potassium-90 decays to calcium-40 via decay. When a rock begins to solidify, it contains some potassium-40 but very little argon-40.
As potassium-40 decomposes over time, argon-40 is formed and trapped in the rock. By analyzing the concentration of these various components, geologists can determine the age of the rock. One method of doing this is known as potassium-argon dating; It involves measuring total potassium (mainly potassium-39) and figuring out how much potassium-40 is based on the relative abundances already known. Then, this value is combined with the argon-40 measurement to determine the age.
A different dating technique, more commonly used today, is to convert a small amount of potassium-39 from a rock into argon-39. This argon-39 replaces the potassium content and therefore the potassium-40 content. Therefore, geologists can determine the age of the rock using the ratio of argon-39 to argon-40. The advantage of this argon-argon dating method is that mass spectrometry measurements focus on isotopes of the same element rather than comparing isotopes of various elements, and this can be done more quickly and accurately. Age determination undergoes a series of additional reactions and modifications as a result of the diffuse neutron activation process that causes potassium-argon transmutation in a reactor.
To convert the argon and potassium abundances from both approaches to an age, it is necessary to measure the relative decay rates (branching rates) to each subspecies, as well as the overall decay rate of potassium-40. This can be a surprise challenge, as it requires exact detection of the parent isotope and a significant number of extremely rare decays.
The KDK Collaboration's research focuses on a small 40% of potassium-40 that decays to argon-10 by electron capture. Since the subsequent (almost instantaneous) decay to the ground state of argon-40 emits a distinct gamma ray, about 10% of this 99% transitions to the excited state of argon-40, which is a useful property. For example, in dark matter observatories where radioactive decays create significant interference, researchers can monitor this gamma ray to help measure the rate of this process and correct for its presence there.
However, a very small percentage of potassium-40 electron capture decays go directly to the ground state of argon-40, so there are only weak, hard-to-isolate x-rays and no gamma rays. In terms of geochronology, the result of each electron capture is the same, but it is much more difficult to measure the proportion of the subset that goes directly to the ground state. Although it has been predicted for a long time, some widely used decay models ignore this completely and account for up to 40% of decays to argon-2.
The KDK experiment shows, through careful measurement of the x-ray and gamma-ray spectra produced by an enriched potassium source, that it is actually close to half that amount.
This finding indicates that other relevant decay rates need to be reassessed and is the first direct measurement of the decay rate of potassium-40 to the argon-40 ground state. Therefore, some potassium-argon dates may need to be adjusted by about 1%, changing the age of some ancient meteorites and rocks by tens of millions of years.
The immediate effects for argon-argon dating would be minimal, according to the researchers. Due to the relative nature of argon-argon dating, rock samples as well as aged standards are placed in the nuclear reactor to ensure that the same amount of potassium-39 is converted to argon-39 in both.
This method has the advantage that some physical constant uncertainties, such as decay rates, are largely eliminated as they affect variables that determine the age of both standards and co-irradiated samples. Their absolute ages will not change either, as most common standards are based on procedures such as calibrating other chronometers using other decay schemes or calibrating large numbers of dated layers in a sedimentary array using astronomical cycles. However, the argon-argon approach has the disadvantage that it attributes all dates to the systematic biases present in these other methods.
The medium-term goal of the field is to improve the direct calibration of the argon-argon method using potassium-argon dating to the point where this calibration can be used for independent comparison with approaches such as uranium-lead. This would require a detailed and precise calculation of all physical constants for potassium-40's degradation to argon-40 and its absorption by minerals, including unusual decay modes that affect potassium-40's overall decay constant and branching rate. Corrections like the ones we've covered here will become more important as high-precision geochronology progresses.
Source: physics.aps.org/articles/v16/131
📩 01/08/2023 13:13