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Strontium-90

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This article is about the chemical isotope. For the band, see Strontium 90 (band).
Strontium-90, 90Sr
General
Symbol90Sr
Namesstrontium-90, 90Sr, Sr-90
Protons (Z)38
Neutrons (N)52
Nuclide data
Natural abundancesyn
Half-life (t1/2)28.79 years
Decay products90Y
Decay modes
Decay modeDecay energy (MeV)
Beta decay0.546
Isotopes of strontium
Complete table of nuclides

Strontium-90 (90
Sr
) 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

Natural strontium is nonradioactive and nontoxic, but 90Sr is a radioactivity hazard. 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.[3] 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.

Medium-lived
fission products
t½
(year) 		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 88.8  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 5.8%, from U-233 6.8%, but from Pu-239 only 2.1%.

Biological effects

Biological activity

Strontium-90 is a "bone seeker" that exhibits biochemical behavior similar to calcium, the next lighter group 2 element. After entering the organism, most often by ingestion with contaminated food or water, about 70–80% of the dose gets excreted. Virtually all remaining strontium-90 is deposited in bones and bone marrow, with the remaining 1% remaining in blood and soft tissues. 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. Strontium-90 is probably the most dangerous component of the radioactive fallout from a nuclear weapon.

The biological half life of strontium-90 in humans has variously been reported as from 14 to 600 days,[4][5] 1000 days,[6] 18 years,[7] 30 years[8] and finally at an upper limit, 49 years.[9] The wide ranging published biological half life figures are explained by the isotope's complex metabolism within the body, but by averaging over all excretion paths the biological half life is about 18 years.[10]

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.[11]

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.

90Sr in fallout

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.[1]

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 through the 1950s and early 1960s. The study's final results showed that children born 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.[12]

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.[13]

Industrial and aerospace applications

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

Heat source for Radioisotope thermoelectric generators

The radioactive decay of strontium-90 generates significant amount of heat (0.921 W/g) of the isotope or approximately 0.536 W/g of the compound.[14] 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 fluoride. It was also used in the US Sentinel series of RTGs in the form of strontium titanate.[15]

Dispersal hazards

Accidental mixing of radioactive sources containing strontium with metal scrap can result in production of radioactive steel. Discarded radioisotope thermoelectric generators are a major source of 90Sr contamination in the area of the former Soviet Union.

References

  1. ^ a b "Nuclear Fission Fragments". Retrieved 18 June 2012.
  2. ^ "Strontium | Radiation Protection | US EPA". EPA. 24 April 2012. Retrieved 18 June 2012.
  3. ^ Decay data from National Nuclear Data Center at the Brookhaven National Laboratory in the US.
  4. ^ http://hanford-site.pnnl.gov/envreport/2001/env01_45.pdf
  5. ^ http://www.osti.gov/bridge/servlets/purl/10136486-6sLptZ/native/10136486.pdf
  6. ^ http://www.areaivenvirothon.org/freshwaterecology.htm
  7. ^ http://www.epi.alaska.gov/eh/radiation/RadioisotopesInFood.pdf
  8. ^ http://www.gsseser.com/FactSheet/Strontium.pdf
  9. ^ http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/biohalf.html
  10. ^ http://www.fourmilab.ch/etexts/www/effects/eonw_12.pdf#zoom=100 Glasstone and Dolan The effects of Nuclear Weapons 1977 page 605
  11. ^ Boehm, Bernhard O. (2011). "The Parathyroid as a Target for Radiation Damage". New England Journal of Medicine. 365 (7): 676–678. doi:10.1056/NEJMc1104982. PMID 21848480. Retrieved 19 August 2011. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
  12. ^ Schneir, Walter (April 25, 1959). "Strontium-90 in U.S. Children". The Nation. 188 (17): 355–357.
  13. ^ Hevesi, Dennis. "Dr. Louise Reiss, Who Helped Ban Atomic Testing, Dies at 90", The New York Times, January 10, 2011. Accessed January 10, 2011.
  14. ^ [Properties of Selected Radioisotopes http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19680020487_1968020487.pdf]
  15. ^ "Power Sources for Remote Arctic Applications" (PDF). Washington, DC: U.S. Congress, Office of Technology Assessment. June 1994. OTA-BP-ETI-129.
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