γ-ray spectrum of ⁶⁰Co

Cobalt-60 (⁶⁰Co) is a radioactive isotope of cobalt. Due to its short half life of 5.27 years ⁶⁰Co is not found in nature. It is produced artificially by neutron activation of ⁵⁹Co. ⁶⁰Co decays by negative beta decay to the stable isotope nickel-60 (⁶⁰Ni). The activated Ni-atom emits two gamma rays with energies of 1.17 and 1.33 MeV.

[edit] 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 Co60 it is equal to 0.35 mSv/(GBq h). It allows to calculate the equivalent dose, which depends on distance and activity.

Example: a Co60 source with an activity of 2.8 GBq, which is equivalent to 60µg of pure Co60, generates a dose of 1mSv in one meter distance within one hour. Swallowing of Co60 reduces the distance to millimeters, and the same dose is achieved within seconds.

Test sources, eg used for school experiments, have an activity <100kBq. Devices for nondestructive material testing uses sources with activities of 1TBq and more.

The high γ-energies result in a significant mass difference between Ni60 and Co60 of 0.003 u. The short life time contributes further to the high decay energy. It amounts to nearly 20W/g, nearly 30 times larger as for Pu-238.

[edit] Decay scheme

decay scheme of Co60 and Co60m.

The diagram shows a (simplified) decay scheme of Co60 and Co60m. The main β-decay transitons are shown. The probability for population of the middle energy level of 2.1MeV by β-decay is 0.0022%, with a maximum energy of 665.26 keV. Energy transfers between the 3 levels generate 6 different gamma-ray lines. In the diagram the two important ones are marked.^[2] Internal conversion energies are well below the main energy levels.

Co60m is an isomer of Co60. After a lifetime of 10.467 minutes and emission of 58.59keV γ rays it transforms into Co60:

Co60m(IT)Co60

With low probability Co60m also β-decays and populates the two "2+" levels of Ni60.

[edit] Applications

Car scanning using Co60 γ-ray device.

The β-decay energy is low and easily shielded. The both strong γ-lines are of same magnitude therefore Co60 is used as a γ-ray source with an energy around 1.3 MeV.

This is a container that contains a small amount of cobalt-60.

Main uses for ⁶⁰Co:

• As a tracer for cobalt in chemical reactions,
• Sterilization of medical equipment,
• Radiation source for medical radiotherapy,
• Radiation source for industrial radiography,
• Radioactive source for leveling devices and thickness gauges,
• As a radioactive source for food irradiation and blood irradiation, and
• As a radioactive source for laboratory use.

Co60 might be an efficient heater for a radioisotope thermoelectric generator. But in contrast to the commonly used Pu-238 its power nearly exhausted after 10 years. It is more difficult to absorb γ-ray energy of Co60 than the energy of α-particles emitted by Pu-238.

⁶⁰Co in a cobalt bomb, a theoretically feasible but extremely "dirty" form of nuclear weapon whereby a tamper of ⁵⁹Co is irradiated by neutron radiation from the fission process and transmuted to ⁶⁰Co.

The creation of ⁶⁰Co is an important step in nucleosynthesis. Without the ⁶⁰Co step, no elements from number 27 through 83 would be created in supernovas.^[3]

[edit] Occurrence

Due to the quite short lifetime there is no natural Co60. Artificial ⁶⁰Co is created by bombarding a ⁵⁹Co target with a slow neutron source, usually ²⁵²Cf moderated through water to slow the neutrons down, or in a nuclear reactor such as CANDU, where adjuster rods usually made of steel are instead made of Co-59.^[4]:

Co59(n,γ)Co60

[edit] Safety

After entering a living mammal (such as a human), most of the ⁶⁰Co gets excreted in feces. A small amount is absorbed by liver, kidneys, and bones, where the prolonged exposure to gamma radiation can cause cancer.

Cobalt is an element of steel-alloys. Uncontrolled disposal of Co60 in scrap is responsible for the radioactivity found in several iron-based products.^[5]

[edit] References

1. ^ Korea Atomic Energy Research Institute. "Nuclide Table". http://atom.kaeri.re.kr/ton/nuc7.html. Retrieved 2009-03-14. 
2. ^ Co60 energy levels
3. ^ The Formation of the Elements
4. ^ Isotope Production: Dual Use Power Plants
5. ^ radioactive contamination of steel

[edit] External links

• Cobalt-60, Environmental Protection Agency.
• National Pollutant Inventory - Cobalt fact sheet
• Cobalt-60, Oak Ridge National Laboratory.
• 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.
• Italians seize 30 tons of radioactive steel, USA Today