Cobalt-60

Cobalt-60 (⁶⁰Co) is a

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 

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 half-life 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:

• As a tracer for cobalt in chemical reactions
• Sterilization of medical equipment.^[8]
• Radiation source for medical
  teletherapy
  machines to treat cancer.
• Radiation source for industrial radiography.^[9]
• Radiation source for leveling devices and thickness gauges.^[9]
• Radiation source for pest insect sterilization.^[10]
• As a radiation source for food irradiation and blood irradiation.^[8]

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.

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.

After entering a living mammal (such as a human), 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.^[14][15]

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. 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.^{[dubious – discuss]} These observations appear to be compatible with the radiation hormesis model.^[16]

In August 2012,

A radioactive contamination incident occurred in 1984 in Ciudad Juárez, 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.^[19] 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.^[19]

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.^[20]

In December 2013, a truck carrying a disused 111 TBq ⁶⁰Co

On 13 September 1999, six people tried to steal ⁶⁰Co rods from a chemical plant in the city of Grozny, Chechen Republic.^[27] 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.^[28]

Parity

In 1957,

Suppliers

Argentina, Canada, India and Russia are the largest suppliers of ⁶⁰Co in the world.

⁶⁰Co is the material encasing a missile nuclear warhead in the 1970 film Beneath the Planet of the Apes.

See also

• Cobalt bomb
• Harold E. Johns

References