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 through neutron activation of ⁵⁹
Co (of which natural cobalt consists entirely). 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 produced ⁶⁰
Co is largely the result of multiple stages of neutron activation of iron isotopes in the reactor's steel structures 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 an excited state of the stable isotope nickel-60 (⁶⁰
Ni), which then emits two gamma rays with energies of 1.17 MeV and 1.33 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, the radioactive activity of a gram of ⁶⁰Co is close to 42 TBq (1,100 Ci). The absorbed dose constant, used in calculations of gamma-ray exposure, 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.

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 26 watts per gram, about 40 times larger (by mass) than that of e.g. ²³⁸Pu but still not very significant for practical sources.

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 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.

The half-value and 1/10th value layer thickness for shielding against this isotope's radiations has been determined for different materials:

The main advantage of ⁶⁰Co 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 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 for a decade or more (multiple half-lives of cobalt-60) due to the gamma radiation field. In one design, the tamper of the weapon would be made of ⁵⁹Co (natural cobalt). 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.