Cobalt-60

This article is about the nuclide cobalt-60. For other uses, see Cobalt-60 (disambiguation).

Cobalt-60 Full table
General
Name, symbol	Cobalt-60,60Co
Neutrons	33
Protons	27
Nuclide data
Natural abundance	trace
Half-life	1925.20 d ± 0.25 d
Isotope mass	59.9338222 u
Spin	5+
Decay mode	Decay energy
β−, γ	2.824 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. 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 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. 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.

Applications

Security screening of cars at the Superbowl 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:

• As a tracer for cobalt in chemical reactions
• Sterilization of medical equipment.
• Radiation source for medical radiotherapy. Cobalt therapy, using beams of gamma rays from 60Co teletherapy machines to treat cancer, has been widely used since the 1950s.
• Radiation source for industrial radiography.
• Radiation source for leveling devices and thickness gauges.
• Radiation source for pest insect sterilization.
• As a radiation source for food irradiation and blood irradiation.
• As a radiation source for laboratory mutagenesis use.

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. 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.

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.

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.

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.

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.

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.

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. 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".

Parity

Main article: Wu experiment

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

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

• Cobalt bomb
• Harold E. Johns

External links

• Cobalt-60, Centers for Disease Control and Prevention.
• NLM Hazardous Substances Databank – Cobalt, Radioactive
• Beta decay of Cobalt-60, HyperPhysics, Georgia State University.
• Dr. Henry Kelly. Cobalt-60 as a Dirty Bomb, Federation of American Scientists, March 6, 2002.