Isotopes of calcium

Calcium (₂₀Ca) has 26 known isotopes, ranging from ³⁵Ca to ⁶⁰Ca. There are five stable isotopes (⁴⁰Ca, ⁴²Ca, ⁴³Ca, ⁴⁴Ca and ⁴⁶Ca), plus one isotope (⁴⁸Ca) with such a long half-life that it is for all practical purposes stable. The most abundant isotope, ⁴⁰Ca, as well as the rare ⁴⁶Ca, are theoretically unstable on energetic grounds, but their decay has not been observed. Calcium also has a cosmogenic isotope, ⁴¹Ca, with half-life 99,400 years. Unlike cosmogenic isotopes that are produced in the air, ⁴¹Ca is produced by neutron activation of ⁴⁰Ca. Most of its production is in the upper metre of the soil column, where the cosmogenic neutron flux is still strong enough. ⁴¹Ca has received much attention in stellar studies because it decays to ⁴¹K, a critical indicator of solar system anomalies. The most stable artificial isotopes are ⁴⁵Ca with half-life 163 days and ⁴⁷Ca with half-life 4.5 days. All other calcium isotopes have half-lives of minutes or less.^[4]

Stable ⁴⁰Ca comprises about 97% of natural calcium and is mainly created by nucleosynthesis in large stars. Similarly to ⁴⁰Ar, however, some atoms of ⁴⁰Ca are radiogenic, created through the radioactive decay of ⁴⁰K. While K–Ar dating has been used extensively in the geological sciences, the prevalence of ⁴⁰Ca in nature initially impeded the proliferation of K-Ca dating in early studies, with only a handful of studies in the 20th century. Modern techniques using increasingly precise Thermal-Ionization (TIMS) and Collision-Cell Multi-Collector Inductively-coupled plasma mass spectrometry (CC-MC-ICP-MS) techniques, however, have been used for successful K–Ca age dating,^{[5] [6]} as well as determining K losses from the lower continental crust^[7] and for source-tracing calcium contributions from various geologic reservoirs^{[8] [9]} similar to Rb-Sr.

Stable isotope variations of calcium (most typically ⁴⁴Ca/⁴⁰Ca or ⁴⁴Ca/⁴²Ca, denoted as 'δ⁴⁴Ca' and 'δ^44/42Ca' in delta notation) are also widely used across the natural sciences for a number of applications, ranging from early determination of osteoporosis ^[10] to quantifying volcanic eruption timescales.^[11] Other applications include: quantifying carbon sequestration efficiency in CO₂ injection sites^[12] and understanding ocean acidification,^[13] exploring both ubiquitous and rare magmatic processes, such as formation of granites ^[14] and carbonatites,^[15] tracing modern and ancient trophic webs including in dinosaurs,^{[16] [17] [18]} assessing weaning practices in ancient humans,^[19] and a plethora of other emerging applications.

Calcium-48 is a doubly magic nucleus with 28 neutrons; unusually neutron-rich for a light primordial nucleus. It decays via double beta decay with an extremely long half-life of about ×ばつ10¹⁹ years, though single beta decay is also theoretically possible.^[22] This decay can analyzed with the sd nuclear shell model, and it is more energetic (4.27 MeV) than any other double beta decay.^[23] It can also be used as a precursor for neutron-rich and superheavy nuclei.^{[24] [25]}

Calcium-60 is the heaviest known isotope as of 2020^[update] .^[1] First observed in 2018 at Riken alongside ⁵⁹Ca and seven isotopes of other elements,^[26] its existence suggests that there are additional even-N isotopes of calcium up to at least ⁷⁰Ca, while ⁵⁹Ca is probably the last bound isotope with odd N.^[27] Earlier predictions had estimated the neutron drip line to occur at ⁶⁰Ca, with ⁵⁹Ca unbound.^[26]

In the neutron-rich region, N = 40 becomes a magic number, so ⁶⁰Ca was considered early on to be a possibly doubly magic nucleus, as is observed for the ⁶⁸Ni isotone.^{[28] [29]} However, subsequent spectroscopic measurements of the nearby nuclides ⁵⁶Ca, ⁵⁸Ca, and ⁶²Ti instead predict that it should lie on the island of inversion known to exist around ⁶⁴Cr.^{[29] [30]}

• C. Michael Hogan. 2010. Calcium. ed. A. Jorgenson and C. Cleveland. Encyclopedia of Earth, National Council for Science and the Environment, Washington, D.C.

• National Isotope Development Center Official website ^{[permanent dead link ‍]}
• Calcium isotopes data from The Berkeley Laboratory Isotopes Project's

• Isotopes of calcium
• Calcium
• Lists of isotopes by element

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