Calcium-40

Main isotopes of calcium

iso NA half-life DM DE (MeV) DP 40Ca 96.941% is stable with 20 neutrons 41Ca trace 1.03×105 y ε – 41K 42Ca 0.647% is stable with 22 neutrons 43Ca 0.135% is stable with 23 neutrons 44Ca 2.086% is stable with 24 neutrons 45Ca syn 162.7 d β− 0.258 45Sc 46Ca 0.004% is stable with 26 neutrons 47Ca syn 4.5 d β− 0.694, 1.99 47Sc γ 1.297 – 48Ca 0.187% 4.3×1019 y β−β− 4.274 48Ti
Standard atomic weight (Ar)	40.078(4)

Calcium (₂₀Ca) has 24 isotopes, from ³⁴Ca to ⁵⁷Ca. There are five stable isotopes (⁴⁰Ca, ⁴²Ca, ⁴³Ca, ⁴⁴Ca and ⁴⁶Ca), plus one isotope (⁴⁸Ca) with such a long half-life that for all practical purposes it can be considered 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, radioactive ⁴¹Ca, which has a half-life of 102,000 years. Unlike cosmogenic isotopes that are produced in the atmosphere, ⁴¹Ca is produced by neutron activation of ⁴⁰Ca. Most of its production is in the upper metre or so of the soil column, where the cosmogenic neutron flux is still sufficiently strong. ⁴¹Ca has received much attention in stellar studies because it decays to ⁴¹K, a critical indicator of solar-system anomalies. The most stable artificial radioisotope is ⁴⁵Ca, with a half-life of 163 days.

All other isotopes have half-lives of 163 days or less, most under a minute. The least stable is ³⁴Ca with a half-life shorter than 35 nanoseconds.

⁴⁰Ca comprises about 97% of naturally occurring calcium. ⁴⁰Ca is also one of the daughter products of ⁴⁰K decay, along with ⁴⁰Ar. While K-Ar dating has been used extensively in the geological sciences, the prevalence of ⁴⁰Ca in nature has impeded its use in dating. Techniques using mass spectrometry and a double spike isotope dilution have been used for K–Ca age dating.

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