Strontium-87

Main isotopes of strontium

iso NA half-life DM DE (MeV) DP 82Sr syn 25.36 d ε – 82Rb 83Sr syn 1.35 d ε – 83Rb β+ 1.23 83Rb γ 0.76, 0.36 – 84Sr 0.56% is stable with 46 neutrons 85Sr syn 64.84 d ε – 85Rb γ 0.514D – 86Sr 9.86% is stable with 48 neutrons 87Sr 7.00% is stable with 49 neutrons 88Sr 82.58% is stable with 50 neutrons 89Sr syn 50.52 d ε 1.49 89Rb β− 0.909D 89Y 90Sr trace 28.90 y β− 0.546 90Y
Standard atomic weight (Ar)	87.62(1)

The alkaline earth metal strontium (₃₈Sr) has four stable, naturally occurring isotopes: ⁸⁴Sr (0.56%), ⁸⁶Sr (9.86%), ⁸⁷Sr (7.0%) and ⁸⁸Sr (82.58%). It's standard atomic weight is 87.62(1).

Only ⁸⁷Sr is radiogenic; it is produced by decay from the radioactive alkali metal ⁸⁷Rb, which has a half-life of 4.88 × 10¹⁰ years (i.e. more than three times longer than the current age of the universe). Thus, there are two sources of ⁸⁷Sr in any material: primordial, formed during nucleosynthesis along with ⁸⁴Sr, ⁸⁶Sr and ⁸⁸Sr; and that formed by radioactive decay of ⁸⁷Rb. The ratio ⁸⁷Sr/⁸⁶Sr is the parameter typically reported in geologic investigations; ratios in minerals and rocks have values ranging from about 0.7 to greater than 4.0. Because strontium has an electron configuration similar to that of calcium, it readily substitutes for Ca in minerals.

In addition to the four stable isotopes, thirty-one unstable isotopes of Strontium are known to exist (see Table, below): the longest-lived of these are ⁹⁰Sr with a half-life of 28.9 years and ⁸⁵Sr with a half-life of 64.853 days. Of importance are strontium-89 (⁸⁹Sr) with a half-life of 50.57 days, and strontium-90 (⁹⁰Sr). They decay by emitting an electron and an antineutrino (${\displaystyle {\bar {\nu }}_{e}}$) in beta decay (β⁻ decay) to become yttrium:

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