Cobalt-60

This article is about the nuclide cobalt-60. For other uses, see Cobalt-60 (disambiguation).
Cobalt-60
General
Name, symbol Cobalt-60,60Co
Neutrons 33
Protons 27
Nuclide data
Natural abundance trace
Half-life 1925.20 d ± 0.25 d[1]
Isotope mass 59.9338222 u
Spin 5+
Decay mode Decay energy
β−, γ 2.824 [2] MeV
γ-ray spectrum of cobalt-60
A container containing cobalt-60.

Cobalt-60, 60Co, is a synthetic radioactive isotope of cobalt with a half-life of 5.2714 years. It is produced artificially in nuclear reactors. Deliberate industrial production depends on neutron activation of bulk samples of the monoisotopic and mononuclidic cobalt isotope 59Co.[3] Measurable quantities are also produced as a by-product of typical nuclear power plant operation and may be detected externally when leaks occur. In the latter case (in the absence of added cobalt) the incidentally produced 60Co is largely the result of multiple stages of neutron activation of iron isotopes in the reactor's steel structures[4] via the creation of 59Co precursor. The simplest case of the latter would result from the activation of 58Fe. 60Co decays by beta decay to the stable isotope nickel-60 (60Ni). The activated nickel nucleus emits two gamma rays with energies of 1.17 and 1.33 MeV, hence the overall nuclear equation of the reaction is

59
27Co + n → 60
27Co → 60
28Ni + e− + ν
e + gamma rays.

Activity

Corresponding to its half-life the radioactive activity of one gram of 60Co is 44 TBq (about 1100 curies). The absorbed dose constant is related to the decay energy and time. For 60Co it is equal to 0.35 mSv/(GBq h) at one meter from the source. This allows calculation of the equivalent dose, which depends on distance and activity.

Example: a 60Co source with an activity of 2.8 GBq, which is equivalent to 60 µg of pure 60Co, generates a dose of 1 mSv at one meter distance within one hour. The swallowing of 60Co reduces the distance to a few millimeters, and the same dose is achieved within seconds.

Test sources, such as those used for school experiments, have an activity of <100 kBq . Devices for nondestructive material testing use sources with activities of 1 TBq and more.

The high γ-energies result in a significant mass difference between 60Ni and 60Co of 0.003 u. This amounts to nearly 20 watts per gram, nearly 30 times larger than that of 238Pu.

Decay

The decay scheme of 60Co and 60mCo.

The diagram shows a (simplified) decay scheme of 60Co and 60mCo. The main β-decay transitions are shown. The probability for population of the middle energy level of 2.1 MeV by β-decay is 0.0022%, with a maximum energy of 665.26 keV. Energy transfers between the three levels generate six different gamma-ray frequencies.[5] In the diagram the two important ones are marked. Internal conversion energies are well below the main energy levels.

60mCo is a nuclear isomer of 60Co with a half-life of 10.467 minutes. It decays by internal transition to 60Co, emitting 58.6 keV gamma rays, or with a low probability (0.22%) by β-decay into 60Ni.[6]

Applications

Security screening of cars at the Super Bowl using 60Co gamma-ray scanner
Prototype irradiator for food irradiation to prevent spoilage, 1984. The 60Co is in the central pipes

The main advantage of 60Co is that it is a high intensity gamma-ray emitter with a relatively long half-life, 5.27 years, compared to other gamma ray sources of similar intensity. The β-decay energy is low and easily shielded; however, the gamma-ray emission lines have energies around 1.3 MeV, and are highly penetrating. The main uses for 60Co are:

Cobalt has been discussed as a "salting" element to add to nuclear weapons, to produce a cobalt bomb, an extremely "dirty" weapon which would contaminate large areas with 60Co nuclear fallout, rendering them uninhabitable. In one hypothetical design, the tamper of the weapon would be made of 59Co. When the bomb exploded, the excess neutrons from the nuclear fission would irradiate the cobalt and transmute it into 60Co. No nation is known to have done any serious development of this type of weapon.

60Co needle implanted in tumors for radiotherapy, around 1955.
60Co teletherapy machine for cancer radiotherapy, early 1950s.
Brookhaven plant mutation experiment using 60Co source in the pipe, center.
60Co source for sterilizing screwflies in the 1959 Screwworm Eradication Program.

Occurrence

There is no natural 60Co in existence; thus, synthetic 60Co is created by bombarding a 59Co target with a slow neutron source. Californium-252, moderated through water, can be used for this purpose, as can the neutron flux in a nuclear reactor. The CANDU reactors can be used to activate 59Co, by substituting the stainless steel control rods with cobalt rods.[10] In the United States, it is now being produced in a BWR at Hope Creek Nuclear Generating Station. The cobalt targets are substituted here for a small number of fuel assemblies.[11]

59Co + n → 60Co

Safety

After entering a living mammal (such as a human being), some of the 60Co is excreted in feces. The remainder is taken up by tissues, mainly the liver, kidneys, and bones, where the prolonged exposure to gamma radiation can cause cancer. Over time, the absorbed cobalt is eliminated in urine.[8]

Cobalt is an element of steel alloys. Uncontrolled disposal of 60Co in scrap metal is responsible for the radioactivity found in several iron-based products.[12][13]

In 2000, a disused radiotherapy head containing a 60Co source was stored at an unsecured location in Bangkok, Thailand and then accidentally sold to scrap collectors. Unaware of the dangers, a junkyard employee dismantled the head and extracted the source, which remained unprotected for a period of days at the junkyard. Ten people, including the scrap collectors and workers at the junkyard, were exposed to high levels of radiation and became ill. Three of the junkyard workers subsequently died as a result of their exposure, which was estimated to be over 6 Gy. Afterward, the source was safely recovered by the Thai authorities.[14]

In August 2012, Petco recalled several models of steel pet food bowls after US Customs and Border Protection determined that they were emitting low levels of radiation. The source of the radiation was determined to be 60Co that had contaminated the steel.[15]

In May 2013 a batch of metal-studded belts sold by online retailer Asos were confiscated and held in a US radioactive storage facility after testing positive for cobalt-60.[16]

In December 2013, a truck carrying a disused 111 TBq 60Co teletherapy source from a hospital in Tijuana to a radioactive waste storage center was hijacked at a gas station near Mexico City.[17][18][19] The truck was recovered shortly after, but it was discovered that the thieves had removed the source from its shielding. The source was found abandoned and intact in a field close by, and authorities stated that the thieves were "likely doomed".[19][20]

Parity

Main article: Wu experiment

In 1957 Chien-Shiung Wu et al. discovered the β-decay process violated parity, implying nature has a handedness. [21]

In the Wu experiment her group aligned radioactive 60Co nuclei by cooling the source to low temperatures in a magnetic field. Wu's observation was that more β-rays were emitted in the opposite direction to the nuclear spin. This asymmetry violates parity conservation.

Suppliers

Argentina, Canada and Russia are the largest suppliers of cobalt-60 in the world.

See also

References

  1. ↑ National Institute of Standards and Technology. "Radionuclide Half-Life Measurements". Retrieved 2011-11-07.
  2. ↑ Korea Atomic Energy Research Institute. "Nuclide Table". Retrieved 2009-03-14.
  3. ↑ Malkoske, G. R. Cobalt-60 production in CANDU power reactors
  4. ↑ US EPA [http://www.epa.gov/radiation/radionuclides/cobalt.html#wheredoes Radiation Protection: Cobalt
  5. ↑ "Table of Isotopes decay data". Retrieved April 16, 2012.
  6. ↑ "Table of Isotopes decay data". Retrieved April 16, 2012.
  7. 1 2 Gamma Irradiators For Radiation Processing (PDF). IAEA. 2005.
  8. 1 2 3 4 "Cobalt | Radiation Protection | US EPA". EPA. Retrieved April 16, 2012.
  9. ↑ Croatia fruit farmers fight flies
  10. ↑ Isotope Production: Dual Use Power Plants
  11. ↑ PSEG Nuclear's Hope Creek reactor back on line, begins production of Cobalt-60
  12. ↑ radioactive contamination of steel
  13. ↑ "Lessons Learned The Hard Way". IAEA Bulletin 47-2. International Atomic Energy Agency. Retrieved 16 April 2010.
  14. ↑ The Radiological Accident in Samut Prakarn (PDF). IAEA. 2002. Retrieved 2012-04-14.
  15. ↑ "Petco Recalls Some Stainless Steel Pet Bowls Due to Cobalt-60 Contamination". Retrieved 21 August 2012.
  16. ↑ "Asos Belts Seized Over Radioactive Studs". Sky News. 2013-05-28. Retrieved 2013-12-05.
  17. ↑ "Truck with dangerous radioactive materials hijacked in Mexico - IAEA — RT News". RT. 2013-12-04. Retrieved 2013-12-05.
  18. ↑ "Mexico Informs IAEA of Theft of Dangerous Radioactive Source". IAEA. Retrieved 2013-12-05.
  19. 1 2 "Mexico Says Stolen Radioactive Source Found in Field". IAEA. 2013-12-05. Retrieved 2013-12-05.
  20. ↑ Will Grant (2013-12-05). "BBC News - Mexico radioactive material found, thieves' lives 'in danger'". BBC. Retrieved 2013-12-05.
  21. ↑ Wu, C. S.; Ambler, E; Hayward, R. W.; Hoppes, D. D.; Hudson, R. P. (1957). "Experimental Test of Parity Conservation in Beta Decay". Physical Review 105 (4): 1413–1415. Bibcode:1957PhRv..105.1413W. doi:10.1103/PhysRev.105.1413.

External links

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