Isotopes
Strontium has four stable, naturally occurring isotopes: 84Sr (0.56%), 86Sr (9.86%), 87Sr (7.0%) and 88Sr (82.58%). 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. Thus, there are two sources of 87Sr in any material: first the portion formed in stars along with the isotopes 84Sr, 86Sr, and 88Sr; and second the portion 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 atomic radius similar to that of calcium, it readily substitutes for Ca in minerals.
Sixteen unstable isotopes are known to exist. Of greatest importance are 90Sr with a half-life of 28.78 years and 89Sr with a half-life of 50.5 days. 90Sr is a by-product of nuclear fission found in 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 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 Chernobyl nuclear accident contaminated a vast area with 90Sr. 90Sr confined inside a concave silver plaque is also used for the medical treatment of a resected pterygium.
89Sr is a short-lived artificial radioisotope that is used in the treatment of bone cancer. In circumstances where cancer patients have widespread and painful bony metastases (secondaries), the administration of 89Sr results in the delivery of radioactive emissions (beta particles in this case) directly to the area of bony problem (where calcium turnover is greatest). The 89Sr is manufactured as the chloride salt (which is soluble), and when dissolved in normal saline can be injected intravenously. Typically, cancer patients will be treated with a dose of 150 MBq. Patients must take precautions following this because their urine becomes contaminated with radioactivity, so they must sit to urinate and double-flush the toilet. The beta particles travel about 3.5 mm in bone (energy 0.583 MeV) and 6.5 mm in tissue, so there is no requirement to isolate patients having been treated, except to say they should not have any one (especially young children) sitting in their laps for 10–40 days. The variation in time results from the variable clearing time for 89Sr, which depends on renal function and the number of bony metastases. With a lot of bony metastases, the entire 89Sr dose can be taken up into bone and so the radioactivity is retained to decay over a 50.5-day half-life. It takes about 10 half-lives or about 500 days for 99.9% of the radioactive strontium to decay. However, where there are few bony metastases, the large proportion of 89Sr not taken up by the bone will be filtered by the kidney, so that the effective half-life (a combination of the physical and biological half-life) will be much shorter.
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