Cobalt-60 (60Co) is a synthetic radioactive isotope of cobalt with a half-life of 5.2714 years.[3][4]: 39 It is produced artificially in nuclear reactors. Deliberate industrial production depends on neutron activation of bulk samples of the monoisotopic and mononuclidic cobalt isotope59 Co .[5] 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 60 Co is largely the result of multiple stages of neutron activation of iron isotopes in the reactor's steel structures[6] via the creation of its 59 Co precursor. The simplest case of the latter would result from the activation of 58 Fe . 60 Co undergoes beta decay to the stable isotope nickel-60 (60 Ni ). The activated cobalt nucleus emits two gamma rays with energies of 1.17 and 1.33 MeV, hence the overall equation of the nuclear reaction (activation and decay) is: 59 27Co + n → 60 27Co → 60 28Ni + e− + 2 γ
Activity
Given its half-life, the radioactive activity of a gram of 60Co is close to 42 TBq (1,100 Ci). 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.
For example, 2.8 GBq or 60 μg of 60Co, generates a dose of 1 mSv at 1 meter away, within an 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 correspond to a significant mass difference between 60Ni and 60Co: 0.003 u. This amounts to nearly 20 watts per gram, nearly 30 times larger than that of 238Pu.
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.[7] 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.[4] 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.[7]
Applications
Security screening of cars at Super Bowl XLI using 60Co gamma-ray scanner (2007)
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 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 as caesium-137.
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 design, the tamper of the weapon would be made of 59Co. When the bomb explodes, neutrons from the nuclear fission would irradiate the cobalt and transmute it to 60Co. No country 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.
Exposure to 60Co is lethal for humans, and can cause death (potentially in less than an hour from acute exposure).[14]
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 the 60Co is excreted in feces. The rest 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.[9]
Steel contamination
Cobalt is found in steel. Uncontrolled disposal of 60Co in scrap metal is responsible for the radioactivity in some iron products.[15][16]
Circa 1983, construction was finished of 1700 apartments in Taiwan 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 the linear no-threshold model would predict, but suffered a lower cancer mortality than the general Taiwan public. These observations support the radiation hormesis model,[17] however other studies have found health impacts that confound the results.
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, which was determined to be from 60Co that had contaminated the steel.[18]
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 60Co.[19]
Incidents involving medical radiation sources
A radioactive contamination incident occurred in 1984 in Ciudad Juárez, Chihuahua, Mexico, originating from a radiation 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 contaminated rebar.[20] 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.[20]
In the Samut Prakan radiation accident 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 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.[21]
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.[22][23] The truck was soon recovered, but the thieves had removed the source from its shielding. It was found intact in a nearby field.[23][24] Despite early reports with lurid headlines asserting that the thieves were "likely doomed",[25] the radiation sickness was mild enough that the suspects were quickly released to police custody,[26] and no one is known to have died from the incident.[27]
Other incidents
On 13 September 1999, six people tried to steal 60Co rods from a chemical plant in the city of Grozny, Chechen Republic.[28] 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.[29]
In 1957, Chien-Shiung Wu et al. discovered that β-decay violated parity, implying nature has a handedness.[30] In the Wu experiment, researchers aligned 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, India and Russia are the largest suppliers of 60Co in the world.[31] Both Argentina and Canada have (as of 2022) an all-heavy-water reactor fleet for power generation. Canada has CANDU in numerous locations throughout Ontario as well as Point Lepreau Nuclear Generating Station in New Brunswick, while Argentina has two German-supplied heavy water reactors at Atucha nuclear power plant and a Canadian-built CANDU at Embalse Nuclear Power Station. India has a number of CANDU reactors at the Rajasthan Atomic Power Station used for producing 60Co.[32] India had a capacity of more than 6 MCi of 60Co production in 2021; this capacity is slated to increase with more CANDU reactors being commissioned at the Rajasthan Atomic Power Station.[33] Heavy-water reactors are particularly well suited for production of 60Co because of their excellent neutron economy and because their capacity for online refueling allows targets to be inserted into the reactor core and removed after a predetermined time without the need for cold shutdown. Also, the heavy water used as a moderator is commonly held at lower temperatures than is the coolant in light water reactors, allowing for a lower speed of neutrons, which increases the neutron cross section and decreases the rate of unwanted (n,2n) "knockout" reactions.
^"Criminal Dies Stealing Radioactive Material". James Martin Center for Nonproliferation Studies at the Monterey Institute of International Studies. 14 September 1999. Archived from the original on 6 October 2021. Retrieved 6 October 2021.{{cite web}}: CS1 maint: bot: original URL status unknown (link)
Dr. Henry Kelly (March 6, 2002). "Cobalt-60 as a Dirty Bomb". Federation of American Scientists. Archived from the original on April 5, 2002. Retrieved November 26, 2005.
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