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]
 | This article needs additional citations for verification. (May 2018) |
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| Standard atomic weight Ar°(Ca) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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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 'δ44/42Ca' 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.
List of isotopes
| Nuclide |
Z | N | Isotopic mass (Da)[20] [n 1] |
Half-life[1] [n 2] |
Decay mode[1] [n 3] |
Daughter isotope [n 4] |
Spin and parity[1] [n 5][n 6] |
Natural abundance (mole fraction) | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Normal proportion[1] | Range of variation | ||||||||||||||||||
| 35Ca | 20 | 15 | 35.00557(22)# | 25.7(2) ms | β+, p (95.8%) | 34Ar | 1/2+# | ||||||||||||
| β+, 2p (4.2%) | 33Cl | ||||||||||||||||||
| β+ (rare) | 35K | ||||||||||||||||||
| 36Ca | 20 | 16 | 35.993074(43) | 100.9(13) ms | β+, p (51.2%) | 35Ar | 0+ | ||||||||||||
| β+ (48.8%) | 36K | ||||||||||||||||||
| 37Ca | 20 | 17 | 36.98589785(68) | 181.0(9) ms | β+, p (76.8%) | 36Ar | 3/2+ | ||||||||||||
| β+ (23.2%) | 37K | ||||||||||||||||||
| 38Ca | 20 | 18 | 37.97631922(21) | 443.70(25) ms | β+ | 38K | 0+ | ||||||||||||
| 39Ca | 20 | 19 | 38.97071081(64) | 860.3(8) ms | β+ | 39K | 3/2+ | ||||||||||||
| 40Ca[n 7] | 20 | 20 | 39.962590850(22) | Observationally stable[n 8] | 0+ | 0.9694(16) | 0.96933–0.96947 | ||||||||||||
| 41Ca | 20 | 21 | 40.96227791(15) | 9.94(15)×104 y | EC | 41K | 7/2− | Trace[n 9] | |||||||||||
| 42Ca | 20 | 22 | 41.95861778(16) | Stable | 0+ | 0.00647(23) | 0.00646–0.00648 | ||||||||||||
| 43Ca | 20 | 23 | 42.95876638(24) | Stable | 7/2− | 0.00135(10) | 0.00135–0.00135 | ||||||||||||
| 44Ca | 20 | 24 | 43.95548149(35) | Stable | 0+ | 0.0209(11) | 0.02082–0.02092 | ||||||||||||
| 45Ca | 20 | 25 | 44.95618627(39) | 162.61(9) d | β− | 45Sc | 7/2− | ||||||||||||
| 46Ca | 20 | 26 | 45.9536877(24) | Observationally stable[n 10] | 0+ | 4×10−5 | 4×10−5–4×10−5 | ||||||||||||
| 47Ca | 20 | 27 | 46.9545411(24) | 4.536(3) d | β− | 47Sc | 7/2− | ||||||||||||
| 48Ca[n 11][n 12] | 20 | 28 | 47.952522654(18) | 5.6(10)×1019 y | β−β−[n 13][n 14] | 48Ti | 0+ | 0.00187(21) | 0.00186–0.00188 | ||||||||||
| 49Ca | 20 | 29 | 48.95566263(19) | 8.718(6) min | β− | 49Sc | 3/2− | ||||||||||||
| 50Ca | 20 | 30 | 49.9574992(17) | 13.45(5) s | β− | 50Sc | 0+ | ||||||||||||
| 51Ca | 20 | 31 | 50.96099566(56) | 10.0(8) s | β− | 51Sc | 3/2− | ||||||||||||
| β−, n? | 50Sc | ||||||||||||||||||
| 52Ca | 20 | 32 | 51.96321365(72) | 4.6(3) s | β− (>98%) | 52Sc | 0+ | ||||||||||||
| β−, n (<2%) | 51Sc | ||||||||||||||||||
| 53Ca | 20 | 33 | 52.968451(47) | 461(90) ms | β− (60%) | 53Sc | 1/2−# | ||||||||||||
| β−, n (40%) | 52Sc | ||||||||||||||||||
| 54Ca | 20 | 34 | 53.972989(52) | 90(6) ms | β− | 54Sc | 0+ | ||||||||||||
| β−, n? | 53Sc | ||||||||||||||||||
| β−, 2n? | 52Sc | ||||||||||||||||||
| 55Ca | 20 | 35 | 54.97998(17) | 22(2) ms | β− | 55Sc | 5/2−# | ||||||||||||
| β−, n? | 54Sc | ||||||||||||||||||
| β−, 2n? | 53Sc | ||||||||||||||||||
| 56Ca | 20 | 36 | 55.98550(27) | 11(2) ms | β− | 56Sc | 0+ | ||||||||||||
| β−, n? | 55Sc | ||||||||||||||||||
| β−, 2n? | 54Sc | ||||||||||||||||||
| 57Ca | 20 | 37 | 56.99296(43)# | 8# ms [>620 ns] | β−? | 57Sc | 5/2−# | ||||||||||||
| β−, n? | 56Sc | ||||||||||||||||||
| β−, 2n? | 55Sc | ||||||||||||||||||
| 58Ca | 20 | 38 | 57.99836(54)# | 4# ms [>620 ns] | β−? | 58Sc | 0+ | ||||||||||||
| β−, n? | 57Sc | ||||||||||||||||||
| β−, 2n? | 56Sc | ||||||||||||||||||
| 59Ca | 20 | 39 | 59.00624(64)# | 5# ms [>400 ns] | β−? | 59Sc | 5/2−# | ||||||||||||
| β−, n? | 58Sc | ||||||||||||||||||
| β−, 2n? | 57Sc | ||||||||||||||||||
| 60Ca | 20 | 40 | 60.01181(75)# | 2# ms [>400 ns] | β−? | 60Sc | 0+ | ||||||||||||
| β−, n? | 59Sc | ||||||||||||||||||
| β−, 2n? | 58Sc | ||||||||||||||||||
| This table header & footer: | |||||||||||||||||||
- ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
- Bold half-life – nearly stable, half-life longer than age of universe.
- Modes of decay:
EC: Electron capture n: Neutron emission p: Proton emission - Bold symbol as daughter – Daughter product is stable.
- ( ) spin value – Indicates spin with weak assignment arguments.
- # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
- Heaviest observationally stable nuclide with equal numbers of protons and neutrons
- Believed to undergo double electron capture to 40Ar with a half-life no less than 9.9×1021 y
- Believed to undergo β−β− decay to 46Ti
- Believed to be capable of undergoing triple beta decay with very long partial half-life
- Lightest nuclide known to undergo double beta decay
- Theorized to also undergo β− decay to 48Sc with a partial half-life exceeding 1.1+0.8
−0.6×1021 years[21]
Calcium-48
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 6.4×1019 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
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]
References
Further reading
External links
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