 | |
| General | |
|---|---|
| Symbol | 60Co |
| Names | cobalt-60 |
| Protons(Z) | 27 |
| Neutrons(N) | 33 |
| Nuclide data | |
| Natural abundance | trace |
| Half-life(t1/2) | 5.2714 years[1] |
| Isotope mass | 59.9338222[2]Da |
| Spin | 5+ |
| Decay modes | |
| Decay mode | Decay energy (MeV) |
| β- (beta decay) | 2.823[3] |
| Isotopes of cobalt Complete table of nuclides | |
Cobalt-60 (60Co) is asynthetic radioactiveisotope of cobalt with ahalf-life of 5.2714 years.[4][5]: 39 It is produced artificially innuclear reactors throughneutron activation of59
Co (of which natural cobalt consists entirely).[6] 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, the incidentally produced60
Co is largely the result of multiple stages of neutron activation ofiron isotopes in the reactor's steel structures[7] via the creation of its59
Co precursor. The simplest case of the latter would result from the activation of58
Fe.60
Co undergoesbeta decay to an excited state of the stable isotopenickel-60 (60
Ni), which then emits twogamma rays with energies of1.17 MeV and1.33 MeV. The overall equation of the nuclear reaction (activation and decay) is:59
27Co + n →60
27Co →60
28Ni + e− +ν
e + 2γ
Given its half-life, theradioactive activity of agram of60Co is close to 42 TBq (1,100 Ci). Theabsorbed dose constant, used in calculations of gamma-ray exposure, is related to the decay energy and time. For60Co it is equal to 0.35 mSv/(GBq h) at one meter from the source. This allows calculation of theequivalent dose, which depends on distance and activity - for example, 2.8 GBq or 60 μg of60Co, generates a dose of 1 mSv at 1 meter away, within an hour.
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.[citation needed]
The decay energy of cobalt-60 amounts to about 26watts per gram,[8] about 40 times larger (by mass) than that of e.g.238Pu but still not very significant for practical sources.
The diagram shows a simplifieddecay scheme of60Co and60mCo. The main β-decay transitions are shown. The probability for population of the middle energy level of 2.1 MeV by β-decay is only 0.0022%, with a β-energy of 0.67 MeV. Transitions between the three levels generate six different gamma-ray frequencies. In the diagram the two important ones are marked.Internal conversion is not significant.
Thehalf-value and 1/10th value layer thickness for shielding against this isotope's radiations has been determined for different materials:
| Absorber Material | Co-60 HVL (cm) | Co-60 1/10 VL (cm) |
|---|---|---|
| water (soft tissue) | 13 | — |
| plastic (acrylic) | 11 | — |
| steel | 2.1 | 6.9 |
| lead | 1.0 | 4.0 |
The main advantage of60Co is that it is a high-intensity gamma-ray emitter with a relatively long half-life (over 5 years) compared to similar gamma-ray sources. The β-radiation is low-energy and easily shielded; however, the gamma rays are highly penetrating. The physical properties of cobalt such as resistance to bulk oxidation and low solubility in water give some advantages in safety in the case of a containment breach over some other gamma sources such ascaesium-137.The main uses for60Co are:
Cobalt has been discussed as a "salting" element to add tonuclear weapons, to produce acobalt bomb, an extremely "dirty" weapon which would contaminate large areas with60Conuclear fallout, rendering them uninhabitable for a decade or more (multiple half-lives of cobalt-60) due to the gamma radiation field. In one design, thetamper of the weapon would be made of59Co (natural cobalt). When the bomb explodes, neutrons from thenuclear fission would irradiate the cobalt and transmute it to60Co. No country is known to have done any serious development of this type of weapon.
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60Co does not occur naturally on Earth in significant amounts, so60Co is synthesized by bombarding a59Co target with athermal neutron source - in a commercial or industrial context, that means a nuclear reactor. TheCANDU reactors can be used to activate59Co, by substituting thecontrol rods with cobalt rods.[15] In the United States, as of 2010, it is being produced in aboiling water reactor atHope Creek Nuclear Generating Station. The cobalt targets are substituted here for a small number of fuel assemblies.[16] Still, over 40% of allsingle-use medical devices are sterilized using60
Co fromBruce nuclear generating station.[17] The reaction in all cases is
Because of the activity of manufactured60Co sources, their radioactivity can present a severe hazard to humans, and can cause death (potentially in less than an hour from acute exposure).[18]
After entering a living mammal (such as a human), assuming that the subject does not die shortly after exposure (as may happen in acute exposure incidents), some of the60Co is excreted infeces. The rest is taken up by tissues, mainly theliver,kidneys, andbones, where the prolonged exposure to gamma radiation can cause cancer. Over time, the absorbed cobalt is eliminated in urine.[11]
Cobalt is found insteel. Uncontrolled disposal of60Co inscrap metal is responsible for the radioactivity in some iron products.[19][20]
Circa 1983, construction was finished of 1700 apartments inTaiwan which were built with steel contaminated with cobalt-60. About 10,000 people occupied these buildings during a 9–20 year period. On average, these people unknowingly received a radiation dose of 0.4 Sv. Some studies have found that this large group did not suffer a higher incidence of cancer mortality, as thelinear no-threshold model would predict, but suffered a lower cancer mortality than the general Taiwan public. These observations support theradiation hormesis model,[21] however other studies have found health impactsthat confound the results.[citation needed]
In August 2012,Petco recalled several models of steel pet food bowls afterUS Customs and Border Protection determined that they were emitting low levels of radiation, which was determined to be from60Co that had contaminated the steel.[22]
In May 2013, a batch of metal-studded belts sold by online retailerASOS were confiscated and held in a US radioactive storage facility after testing positive for60Co.[23]
Aradioactive contamination incident occurred in 1984 inCiudad Juárez,Chihuahua, Mexico, originating from aradiation therapy unit illegally purchased by a private medical company and subsequently dismantled for lack of personnel to operate it. The radioactive material,60Co, ended up in a junkyard, where it was sold to foundries that inadvertently smelted it with other metals and produced about 6,000 tons of contaminatedrebar.[24] These were distributed in 17 Mexican states and several cities in the United States. It is estimated that 4,000 people were exposed to radiation as a result of this incident.[24]
In theSamut Prakan radiation accident in 2000, a disused radiotherapy head containing a60Co source was stored at an unsecured location inBangkok, Thailand and then accidentally sold to scrap collectors. Unaware of the danger, 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 junkyard workers later died of their exposure, which was estimated to be over 6 Gy. Afterward, the source was safely recovered by Thai authorities.[25]
In December 2013, a truck carrying a disused 111 TBq60Coteletherapy source from a hospital inTijuana to aradioactive waste storage center was hijacked at a gas station nearMexico City.[26][27] The truck was soon recovered, but the thieves had removed the source from its shielding. It was found intact in a nearby field.[27][28] Despite early reports with lurid headlines asserting that the thieves were "likely doomed",[29] theradiation sickness was mild enough that the suspects were quickly released to police custody,[30] and no one is known to have died from the incident.[31]
On 13 September 1999, six people tried to steal60Co rods from a chemical plant in the city ofGrozny, Chechen Republic.[32] During the theft, the suspects opened the radioactive material container and handled it, resulting in the deaths of three of the suspects and injury of the remaining three. The suspect who held the material directly in his hands died of radiation exposure 30 minutes later. This incident is described as an attempted theft, but some of the rods are reportedly still missing.[33]
In 1957,Chien-Shiung Wu et al. discovered that β-decay violatedparity, implying nature (theweak force) sees handedness.[34] In theWu experiment, researchers aligned60Co 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 asymmetryviolates parity conservation.
Argentina, Canada, India and Russia are the largest suppliers of60Co in the world.[35] Both Argentina and Canada have (as of 2022) an all-heavy-water reactor fleet for power generation. Canada hasCANDU in numerous locations throughout Ontario as well asPoint Lepreau Nuclear Generating Station in New Brunswick, while Argentina has two German-supplied heavy water reactors atAtucha nuclear power plant and a Canadian-built CANDU atEmbalse Nuclear Power Station. India has CANDU reactors at theRajasthan Atomic Power Station used for producing60Co.[36] India had a capacity of more than 6 MCi of60Co production in 2021; this capacity is slated to increase with more CANDU reactors being commissioned at the Rajasthan Atomic Power Station.[37] Heavy-water reactors are particularly well suited for production of60Co because of their excellentneutron economy and because their capacity foronline refueling allows targets to be inserted into the reactor core and removed after a predetermined time without the need forcold shutdown.
60Co is the material encasing a massive nuclear warhead attached to a missile called the Alpha-Omega Doomsday Bomb in the filmBeneath the Planet of the Apes (1970).
In an episode of9-1-1 (TV series), a truck illegally transporting60Co causes a hazardous emergency for a team of firefighters.[38]
In the 1959 film,City of Fear,60Co is central to the plot in which an escaped convict obtains the material, believing it to beheroin and endangering the city ofLos Angeles.
{{cite book}}:|journal= ignored (help){{cite web}}: CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link){{cite web}}: CS1 maint: bot: original URL status unknown (link)| Lighter: cobalt-59 | Cobalt-60 is an isotope ofcobalt | Heavier: cobalt-61 |
| Decay product of: iron-60 | Decay chain of cobalt-60 | Decays to: nickel-60 |