Strontium-90

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This article is about the chemical isotope. For the band, see Strontium 90 (band).
Strontium-90
General
Name, symbol Strontium-90,90Sr
Neutrons 52
Protons 38
Nuclide data
Natural abundance syn
Half-life 28.79 years
Decay products 90Y
Decay mode Decay energy
Beta decay 0.546 MeV

Strontium-90 (90Sr) is a radioactive isotope of strontium produced by nuclear fission, with a half-life of 28.8 years. It undergoes β− decay into yttrium-90, with a decay energy of 0.546 MeV.[1] Strontium-90 has applications in medicine and industry and is an isotope of concern in fallout from nuclear weapons and nuclear accidents.[2]

Radioactivity

Naturally occurring strontium is nonradioactive and nontoxic at levels normally found in the environment, but 90Sr is a radiation hazard.[3] 90Sr undergoes β− decay with a half-life of 28.79 years and a decay energy of 0.546 MeV distributed to an electron, an anti-neutrino, and the yttrium isotope 90Y, which in turn undergoes β− decay with half-life of 64 hours and decay energy 2.28 MeV distributed to an electron, an anti-neutrino, and 90Zr (zirconium), which is stable.[4] Note that 90Sr/Y is almost a pure beta particle source; the gamma photon emission from the decay of 90Y is so infrequent that it can normally be ignored.

90Sr has a specific activity of 5.21 TBq/g.[5]

Medium-lived
fission products
Prop:
Unit:		 t½
(a)		 Yield
(%)		 Q *
(keV)		 βγ *
155Eu 4.76 0.0803 252 βγ
85Kr 10.76 0.2180 687 βγ
113mCd 14.1 0.0008 316 β
90Sr 28.9 4.505 2826 β
137Cs 30.23 6.337 1176 βγ
121mSn 43.9 0.00005 390 βγ
151Sm 96.6 0.5314 77 β

Fission product

90Sr is a product of nuclear fission. It is present in significant amount in spent nuclear fuel and in radioactive waste from nuclear reactors and in nuclear fallout from nuclear tests. For thermal neutron fission as in today's nuclear power plants, the fission product yield from U-235 is 5.7%, from U-233 6.6%, but from Pu-239 only 2.0%.[6]

Biological effects

Biological activity

Strontium-90 is a "bone seeker" that exhibits biochemical behavior similar to calcium, the next lighter group 2 element.[3][7] After entering the organism, most often by ingestion with contaminated food or water, about 70–80% of the dose gets excreted.[2] Virtually all remaining strontium-90 is deposited in bones and bone marrow, with the remaining 1% remaining in blood and soft tissues.[2] Its presence in bones can cause bone cancer, cancer of nearby tissues, and leukemia. Exposure to 90Sr can be tested by a bioassay, most commonly by urinalysis.[3]

The biological half-life of strontium-90 in humans has variously been reported as from 14 to 600 days,[8][9] 1000 days,[10] 18 years,[11] 30 years[12] and, at an upper limit, 49 years.[13] The wide ranging published biological half life figures are explained by strontium's complex metabolism within the body. However, by averaging all excretion paths, the overall biological half life is estimated to be about 18 years.[14]

The elimination rate of strontium-90 is strongly affected by age and sex, due to differences in bone metabolism.[15]

Together with the caesium isotopes 134Cs, 137Cs, and iodine isotope 131I it was among the most important isotopes regarding health impacts after the Chernobyl disaster. As strontium has an affinity to the calcium-sensing receptor of parathyroid cells that is similar to that of calcium, the increased risk of liquidators of the Chernobyl power plant to suffer from primary hyperparathyroidism could be explained by binding of strontium-90.[16]

Uses

Radioisotope Thermoelectric Generators (RTGs)

The radioactive decay of strontium-90 generates a significant amount of heat, 0.536 W/g in the form of pure strontium metal or approximately 0.256 W/g as strontium titanate[17] and is cheaper than the alternative 238Pu. It is used as a heat source in many Russian/Soviet radioisotope thermoelectric generators, usually in the form of strontium titanate.[18] It was also used in the US "Sentinel" series of RTGs.[19]

Industrial applications

90Sr finds use in industry as a radioactive source for thickness gauges.[2]

Medical applications

90Sr finds extensive use in medicine as a radioactive source for superficial radiotherapy of some cancers. Controlled amounts of 90Sr and 89Sr can be used in treatment of bone cancer. It is also used as a radioactive tracer in medicine and agriculture.[2]

90Sr contamination in the environment

Strontium-90 is not quite as likely as caesium-137 to be released as a part of a nuclear reactor accident because it is much less volatile, but is probably the most dangerous component of the radioactive fallout from a nuclear weapon.[20]

A study of hundreds of thousands of deciduous teeth, collected by Dr. Louise Reiss and her colleagues as part of the Baby Tooth Survey, found a large increase in 90Sr levels in through the 1950s and early 1960s. The study's final results showed that children born in St. Louis, Missouri in 1963 had levels of 90Sr in their deciduous teeth that was 50 times higher than that found in children born in 1950, before the advent of large-scale atomic testing. Commentators on the study said that the fallout was likely to cause increased cases of diseases in those who absorb strontium-90 into their bones.[21]

An article with the study's initial findings was circulated to U.S. President John F. Kennedy in 1961, and helped convince him to sign the Partial Nuclear Test Ban Treaty with the United Kingdom and Soviet Union, ending the above-ground nuclear weapons testing that placed the greatest amounts of nuclear fallout into the atmosphere.[22]

The Chernobyl disaster released roughly 10 PBq, or about 5% of the core inventory, of strontium-90 into the environment.[23] The Fukushima Daiichi disaster released 0.1-1 PBq of strontium-90 in the form of contaminated cooling water into the Pacific Ocean.[24]

External links

References

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  4. ↑ Decay data from National Nuclear Data Center at the Brookhaven National Laboratory in the US.
  5. ↑ Lua error in package.lua at line 80: module 'strict' not found.
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  20. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  21. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  22. ↑ Hevesi, Dennis. "Dr. Louise Reiss, Who Helped Ban Atomic Testing, Dies at 90", The New York Times, January 10, 2011. Accessed January 10, 2011.
  23. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  24. ↑ Lua error in package.lua at line 80: module 'strict' not found.
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