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[1]
Isotope mass	59.9338222 u
Spin	5+
Decay mode	Decay energy
β-, γ	2.824 [2] MeV

γ-ray spectrum of cobalt-60

Cobalt-60, 60
27Co, is a synthetic radioactive isotope of cobalt. Due to its half-life of 5.27 years, 60
27Co is not found in nature. It is produced artificially by neutron activation of 59
27Co. 60
27Co decays by beta decay to the stable isotope nickel-60 (60
28Ni). 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⁻ + gamma rays.

Activity

Corresponding to its half-life the radioactive activity of one gram of 60
27Co is 44 TBq (about 1100 curies). The absorbed dose constant is related to the decay energy and time. For 60
27Co it is equal to 0.35 mSv/(GBq h). This allows calculation of the equivalent dose, which depends on distance and activity.

Example: a 60
27Co source with an activity of 2.8 GBq, which is equivalent to 60 µg of pure 60
27Co, generates a dose of 1 mSv in one meter distance within one hour. The swallowing of 60
27Co 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 <100 kBq. Devices for nondestructive material testing uses sources with activities of 1 TBq and more.

The high γ-energies result in a significant mass difference between 60
28Ni and 60
27Co of 0.003 u. The short lifetime contributes further to the high decay energy. This amounts to nearly 20 watts per gram, nearly 30 times larger than that of 238
94Pu.

Decay scheme

decay scheme of ⁶⁰Co and ^60mCo.

The diagram shows a (simplified) decay scheme of 60
27Co 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.^[3] Internal conversion energies are well below the main energy levels.

^60mCo is a nuclear isomer of 60
27Co. After a half-life of 10.467 minutes and emission of 58.59 keV gamma rays it transforms into 60
27Co:

Co60m(IT)Co60

With a low probability ^60mCo also β-decays and populates the two "2+" levels of 60
28Ni.

Applications

Car scanning using Co-60 gamma-ray device.

The β-decay energy is low and easily shielded. Two gamma-ray emission lines have energies around 1.3 MeV, and are highly penetrating.

A container containing a small amount of cobalt-60.

Main uses for 60
27Co:

• 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.

60
27Co might be an efficient heater for a radioisotope thermoelectric generator. However, in contrast to the commonly-used plutonium-238 (238
94Pu), its power is nearly exhausted after 10 years. It is also more difficult to absorb the γ-ray power of 60
27Co than the power of α-particles emitted by 238
94Pu.

60
27Co could be used to "salt" a cobalt bomb. This a hypothetically-feasible but extremely "dirty" form of nuclear weapon that has never been produced. Hypothetically, a nuclear bomb with a tamper of 59
27Co would irradiate the cobalt with excess neutrons from the nuclear fission and transmute it into 60
27Co.

The creation of 60
27Co is an important step in nucleosynthesis. Without the step through 60
27Co, no elements with atomic numbers from 27 through 83 could be created in supernovas.^[4]

Occurrence

Due to its quite short half-life, there is no natural 60
27Co in existence. Synthetic 60
27Co is created by bombarding a 59
27Co target with a slow neutron source, usually californium-252 moderated through water to slow the neutrons down, or in a nuclear reactor such as a CANDU reactor, where the control rods usually made of steel are instead made of 59
27Co ^[5]

59
27Co + n → 60
27Co

Safety

After entering a living mammal (such as a human being), most of the 60
27Co gets excreted in feces. A small amount is absorbed by the liver, the kidneys, and the bones, where the prolonged exposure to gamma radiation can cause cancer.^{[citation needed]}

Cobalt is an element of steel alloys. Uncontrolled disposal of 60
27Co in scrap metal is responsible for the radioactivity found in several iron-based products.^[6][7]

In 2000, a disused radiotherapy head containing a 60
27Co source was stored at an unsecured location in Bangkok, Thailand and then accidentally was 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 Sv (sievert) . The source was safely recovered by the Thai authorities.^[8]

See also

• Cobalt bomb
• Harold E. Johns

References

1. ^ National Institute of Standards and Technology. "Radionuclide Half-Life Measurements". http://www.nist.gov/pml/data/halflife-html.cfm. Retrieved 2011-11-07. 
2. ^ Korea Atomic Energy Research Institute. "Nuclide Table". http://atom.kaeri.re.kr/ton/nuc7.html. Retrieved 2009-03-14. 
3. ^ Co60 energy levels
4. ^ The Formation of the Elements
5. ^ Isotope Production: Dual Use Power Plants
6. ^ radioactive contamination of steel
7. ^ "Lessons Learned The Hard Way". IAEA Bulletin 47-2. International Atomic Energy Agency. http://www.iaea.org/Publications/Magazines/Bulletin/Bull472/htmls/lessons_learned.html. Retrieved 16 April 2010. 
8. ^ "The Radiological Accident in Samut Prakarn"

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.
• Cobalt-60 radiation from scrap-metal yards in New Delhi, India [1]