Rubidium-85

Main isotopes of rubidium

iso NA half-life DM DE (MeV) DP 83Rb syn 86.2 d ε – 83Kr γ 0.52, 0.53, 0.55 – 84Rb syn 32.9 d ε – 84Kr β+ 1.66, 0.78 84Kr γ 0.881 – β− 0.892 84Sr 85Rb 72.17% is stable with 48 neutrons 86Rb syn 18.7 d β− 1.775 86Sr γ 1.0767 – 87Rb 27.83% 4.9×1010 y β− 0.283 87Sr
Standard atomic weight (Ar)	85.4678(3)

Rubidium (₃₇Rb) has 32 isotopes, with naturally occurring rubidium being composed of just two isotopes; ⁸⁵Rb (72.2%) and the radioactive ⁸⁷Rb (27.8%). Normal mixes of rubidium are radioactive enough to fog photographic film in approximately 30 to 60 days.

⁸⁷Rb has a half-life of 4.92×10¹⁰ years. It readily substitutes for potassium in minerals, and is therefore fairly widespread. ⁸⁷Rb has been used extensively in dating rocks; ⁸⁷Rb decays to stable strontium-87 by emission of a negative beta particle, i.e. an electron ejected from the nucleus. During fractional crystallization, Sr tends to become concentrated in plagioclase, leaving Rb in the liquid phase. Hence, the Rb/Sr ratio in residual magma may increase over time, resulting in rocks with increasing Rb/Sr ratios with increasing differentiation. Highest ratios (10 or higher) occur in pegmatites. If the initial amount of Sr is known or can be extrapolated, the age can be determined by measurement of the Rb and Sr concentrations and the ⁸⁷Sr/⁸⁶Sr ratio. The dates indicate the true age of the minerals only if the rocks have not been subsequently altered. See rubidium-strontium dating for a more detailed discussion.

Other than ⁸⁷Rb, the longest-lived radioisotopes are ⁸³Rb with a half-life of 86.2 days, ⁸⁴Rb with a half-life of 33.1 days and ⁸⁶Rb with a half-life of 18.642 days. All other radioisotopes have half-lives less than a day.

⁸²Rb is used in some cardiac PET scans to assess myocardial perfusion. It has a half-life of 1.273 minutes. It does not exist naturally, but can be made from the decay of ⁸²Sr.

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