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• Physics 16, 131
Researchers revisit a uncared for decay mode with implications for basic physics and for relationship a few of the oldest rocks on Earth and within the Photo voltaic System.
With a half-life of 1.25 billion years, potassium-40 doesn’t decay usually, however its decays have a big effect. As a comparatively frequent isotope (0.012% of all potassium) of a quite common metallic (2.4% by mass of Earth’s crust), potassium-40 is likely one of the major sources of radioactivity we encounter in every day life. Its decays are the first supply of argon-40, which makes up nearly 1% of the ambiance, and the copious quantity of warmth launched from these decays threw off early estimates of the age of Earth made by Lord Kelvin. Potassium-40 is essentially chargeable for the meager radioactivity in our meals (resembling bananas), and it’s a important supply of noise in some extremely delicate particle physics detectors. This isotope and its decay merchandise are additionally helpful instruments in relationship rocks and geological processes that return to the earliest elements of Earth historical past. And but some long-standing uncertainty surrounds these well-studied decays. The KDK Collaboration has offered the primary direct statement of a uncommon decay mode of potassium-40 to argon-40 [1, 2]. The measured decay price implies a smaller chance of this decay mode than beforehand assumed. The outcomes could have restricted however vital implications for the sphere of geochronology, in addition to for different fields that both use or search to keep away from the results of the decay of this ubiquitous ingredient.
Potassium-40 has a considerably sophisticated decay scheme. It’s no uranium, with its chains of long-lived descendants. But it surely does have some attention-grabbing options, with about 90% of potassium-40 decays going to calcium-40 by decay and a lot of the remaining 10% going to the aforementioned argon-40 by electron seize. When a rock solidifies, it begins off with a certain quantity of potassium-40 however nearly no argon-40 (Fig. 1). Over time, potassium-40 decays, producing argon-40 that is still trapped within the rock. Geologists can estimate the rock’s age by measuring the focus of those totally different parts. A method to do that—so-called potassium–argon relationship—is to measure the whole potassium (principally potassium-39) and calculate the quantity of potassium-40 from the identified relative abundances. This worth is then mixed with an argon-40 measurement to calculate an age.
An alternate relationship technique—extra generally used these days—is to transmute a small quantity of the potassium-39 in a rock into argon-39. This argon-39 acts as a proxy for the quantity of potassium and by extension for the quantity of potassium-40. Geologists can subsequently use the ratio of argon-39 to argon-40 to find out the age of the rock. This argon–argon relationship method provides the benefit that the mass spectrometry measurements goal isotopes of the identical ingredient, which might be achieved extra rapidly and exactly than comparisons of various parts. The potassium–argon transmutation happens by way of neutron activation in a reactor, a considerably messy course of that imparts a slew of extra reactions and corrections on the age dedication.
To transform argon and potassium abundances from each strategies into an age, one should quantify the general decay price of potassium-40, in addition to the relative decay charges to every descendant (branching ratios). This may be surprisingly troublesome, because it requires precisely measuring each the mum or dad isotope and a enough variety of extraordinarily uncommon decays. The work by the KDK Collaboration offers with a uncommon subset of the roughly 10% of the potassium-40 that decays to argon-40 by electron seize. About 99% of this 10% goes to an excited state of argon-40, which is a helpful function as a result of the next (practically instant) decay to the bottom state of argon-40 emits a attribute gamma ray. Researchers can measure that gamma ray to assist quantify the speed of this course of and likewise to appropriate for its presence in different conditions, resembling in darkish matter observatories the place radioactive decays are a big interference.
Nevertheless, a really small subset of electron-capture decays of potassium-40 go on to the bottom state of argon-40, that means there isn’t any gamma ray, simply low-energy x-rays which can be troublesome to isolate. The results of every electron seize is similar so far as geochronology is worried—each decays produce a secure argon-40 nucleus—however the price of the direct-to-ground-state subset is way more durable to measure. Lengthy predicted, it has been estimated to be as a lot as 2% of the decays to argon-40 [3, 4] however has been omitted solely from some generally used decay fashions [5]. The KDK work, utilizing cautious measurement of the x-ray and gamma-ray spectra produced by an enriched potassium supply (described in [1] and in additional element in [2]), exhibits it’s the truth is nearer to half that worth. This consequence represents the primary direct measurement of the decay price of potassium-40 to the argon-40 floor state, and it additionally implies a necessity for a redetermination of different associated decay charges. As a consequence, some potassium–argon ages could require corrections of near 1%, affecting the age of some outdated meteorites and rocks by tens of tens of millions of years.
The instant implications for argon–argon relationship, as identified by the researchers, shall be restricted. The explanation for that is that argon–argon relationship is a relative method; requirements of identified age are positioned within the nuclear reactor together with the rock samples in order that the identical proportion of potassium-39 is transmuted to argon-39 in each. One of many benefits of this method is that the uncertainties in lots of bodily constants—such because the decay charges—partially cancel out as a result of they apply to the age-determining components of each the requirements and the coirradiated samples. Absolutely the ages of many frequent requirements may even not be affected, as a result of for essentially the most half they’re based mostly on different chronometers—using both different decay schemes (primarily uranium–lead [6]) or methods resembling calibrating a number of dated layers in a sedimentary sequence utilizing astronomical cycles [7]. This, nevertheless, is a weak spot of the argon–argon technique, because it pins all dates to the systematic biases inherent to those different strategies. A medium-term purpose for the sphere is to enhance direct calibration of the argon–argon technique utilizing potassium–argon relationship to the purpose that this calibration can be utilized for impartial comparability with methods like uranium–lead ones. This can require correct and exact accounting of all bodily constants concerned within the decay of potassium-40 to argon-40 and its incorporation into minerals, together with uncommon decay modes that have an effect on the general decay fixed and branching ratio of potassium-40. As progress within the discipline of high-precision geochronology continues, corrections just like the one thought of right here will solely develop in significance.
References
- M. Stukel et al. (KDK Collaboration), “Uncommon 40Okay decay with implications for basic physics and geochronology,” Phys. Rev. Lett. 131, 052503 (2023).
- L. Hariasz et al. (KDK Collaboration), “Proof for ground-state electron seize of 40Okay,” Phys. Rev. C 108, 014327 (2023).
- D. W. Engelkemeir et al., “Positron emission within the decay of Okay40,” Phys. Rev. 126, 1818 (1962).
- J. Carter et al., “Manufacturing of 40Ar by an missed mode of 40Okay decay with implications for Okay-Ar geochronology,” Geochronology 2, 355 (2020).
- Okay. Min et al., “A check for systematic errors in 40Ar/39Ar geochronology by way of comparability with U/Pb evaluation of a 1.1-Ga rhyolite,” Geochim. Cosmochim. Acta 64, 73 (2000).
- P. R. Renne et al., “Joint dedication of 40Okay decay constants and 40Ar/40Okay for the Fish Canyon sanidine commonplace, and improved accuracy for 40Ar/39Okay geochronology,” Geochim. Cosmochim. Acta 74, 5349 (2010).
- Okay. F. Kuiper et al., “Synchronizing rock clocks of Earth historical past,” Science 320, 500 (2008).
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