Calcium (20Ca) has 26 known isotopes, ranging from35Ca to60Ca. There are fivestable isotopes (40Ca,42Ca,43Ca,44Ca and46Ca), plus one isotope (48Ca) with such a longhalf-life that it is for all practical purposes stable. The most abundant isotope,40Ca, as well as the rare46Ca, are theoretically unstable on energetic grounds, but their decay has not been observed. Calcium also has acosmogenic isotope,41Ca, withhalf-life 99,400 years. Unlikecosmogenic isotopes that are produced in theair,41Ca is produced byneutron activation of40Ca. 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 to41K, a critical indicator of solar system anomalies. The most stable artificial isotopes are45Ca with half-life 163 days and47Ca with half-life 4.5 days. All other calcium isotopes have half-lives of minutes or less.[4]
Stable40Ca comprises about 97% of natural calcium and is mainly created by nucleosynthesis in large stars. Similarly to40Ar, however, some atoms of40Ca are radiogenic, created through the radioactive decay of40K. WhileK–Ar dating has been used extensively in thegeological sciences, the prevalence of40Ca 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 plasmamass spectrometry (CC-MC-ICP-MS) techniques, however, have been used for successfulK–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 toRb-Sr.
Stable isotope variations of calcium (most typically44Ca/40Ca or44Ca/42Ca, denoted as 'δ44Ca' and 'δ44/42Ca' in delta notation) are also widely used across the natural sciences for a number of applications, ranging from early determination ofosteoporosis[10] to quantifyingvolcanic eruption timescales.[11] Other applications include: quantifyingcarbon sequestration efficiency in CO2 injection sites[12] and understandingocean acidification,[13] exploring both ubiquitous and rare magmatic processes, such as formation ofgranites[14] andcarbonatites,[15] tracing modern and ancienttrophic webs including in dinosaurs,[16][17][18] assessingweaning 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 viadouble beta decay with an extremely long half-life of about 6.4×1019 years, though single beta decay is also theoretically possible.[22] This decay can analyzed with thesdnuclear 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 atRiken alongside59Ca and seven isotopes of other elements,[26] its existence suggests that there are additional even-N isotopes of calcium up to at least70Ca, while59Ca is probably the last bound isotope with oddN.[27] Earlier predictions had estimated the neutron drip line to occur at60Ca, with59Ca unbound.[26]
In the neutron-rich region,N = 40 becomes amagic number, so60Ca was considered early on to be a possibly doubly magic nucleus, as is observed for the68Niisotone.[28][29] However, subsequent spectroscopic measurements of the nearby nuclides56Ca,58Ca, and62Ti instead predict that it should lie on theisland of inversion known to exist around64Cr.[29][30]
^Neufcourt, Léo; Cao, Yuchen; Nazarewicz, Witold; et al. (14 February 2019). "Neutron Drip Line in the Ca Region from Bayesian Model Averaging".Physical Review Letters.122 (6): 062502.arXiv:1901.07632.doi:10.1103/PhysRevLett.122.062502.PMID30822058.
^Gade, A.; Janssens, R. V. F.; Weisshaar, D.; et al. (21 March 2014). "Nuclear Structure TowardsN = 4060Ca: In-Beam γ -Ray Spectroscopy of58, 60Ti".Physical Review Letters.112 (11): 112503.arXiv:1402.5944.doi:10.1103/PhysRevLett.112.112503.PMID24702356.
