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Sr-84 (0.56%), Sr-86 (9.86%), Sr-87 (7.0%) and Sr-88 (82.58%). Only Sr-87 is [[radiogenic]]; it is produced by decay from the [[radioactive]] alkali metal [[rubidium]]-87, which has a [[half-life]] of 48,800,000,000 years. Thus, there are two sources of Sr-87 in any material: that formed during primordial nucleo-synthesis along with Sr-84, Sr-86 and Sr-88, as well as that formed by radioactive decay of Rb-87. The ratio Sr-87/Sr-86 is the parameter typically reported in [[geology|geologic]] investigations. Because strontium has an [[atomic radius]] similar to that of [[calcium]], it readily substitutes for Ca in [[mineral]]s. Sr-87/Sr-86 ratios in minerals and [[Rock (geology)|rock]]s have values ranging from about 0.7 to greater than 4.0.
Sixteen unstable isotopes are known to exist. Of greatest importance is Sr-90 with a [[half-life]] of 28.78 years. It is a by-product of [[nuclear fallout]] and presents a health problem since it substitutes for calcium in [[bone]], preventing expulsion from the body. This isotope is one of the best long-lived high-energy [[beta ray|beta]] emitters known, and is used in SNAP ([[Systems for Nuclear Auxiliary Power]]) devices. These devices hold promise for use in [[spacecraft]], remote weather stations, navigational buoys, etc, where a lightweight, long-lived, nuclear-electric power source is required. The 1986 [http://en.wikipedia.org/wiki/Chernobyl_accident/ Chernobyl nuclear accident] contaminated a vast area with Sr-90.
== Precautions ==
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