Cobalt-60 1

Cobalt-60
Cobalt-60

Full table

General

Name, symbol Cobalt-60,60Co

Neutrons 33

Protons 27

Nuclide data

Natural abundance trace

Half-life [1]
1925.20 d ± 0.25 d

Isotope mass 59.9338222 u

Spin 5+

Decay mode Decay energy

[2]
β-, γ 2.824 MeV

Cobalt-60, 60Co, is a synthetic radioactive isotope of cobalt with a

half-life of 5.27 years. It is produced artificially by neutron activation
of the isotope 59Co.[3] 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 γ-ray spectrum of cobalt-60
−
28Ni + 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 in 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 <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. 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 238Pu.
Cobalt-60 2

Decay
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.[4] In the diagram
the two important ones are marked. Internal conversion energies are
well below the main energy levels. decay scheme of 60Co and 60mCo.

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.[5]

Applications
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.

Car scanning using Co-60 gamma-ray device.

The main uses for 60Co are:

• As a tracer for cobalt in chemical reactions
• Sterilization of medical equipment.[6]
• Radiation source for medical radiotherapy.[7]
• Radiation source for industrial radiography.[7]
• Radiation source for leveling devices and thickness gauges.[7]
• Radiation source for pest insect sterilization.[8]
• As a radiation source for food irradiation and blood irradiation.[6]
• As a radiation source for laboratory use.
Cobalt has been discussed as a "salting" element to add to nuclear
weapons, to produce a cobalt bomb, an extremely "dirty" weapon A container containing a small amount of
which would contaminate large areas with 60Co nuclear fallout, cobalt-60.

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.

The creation of 60Co is an important step in nucleosynthesis. Without the step through 60Co, no elements with
atomic numbers from 27 through 83 could be created in supernovas.[9]
Cobalt-60 3

Occurrence
There is no natural 60Co in existence; thus, synthetic 60Co is created by bombarding a 59Co 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 59Co [10]
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.[7]
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.[11][12]
In 2000, a disused radiotherapy head containing a 60Co 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 Gy. The source was safely recovered by the Thai authorities.[13]
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.[14]

Parity
In 1957 Madam Wu et al. discovered the β-decay process violated parity – nature has a handedness.
Her group aligned radioactive 60Co nuclei by cooling the source to low temperatures in a magnetic field. Wu's
observation: more β-rays were emitted in the opposite direction to the nuclear spin. This forwards-backwards
asymmetry violates parity conservation.

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] Malkoske, G. R. Cobalt-60 production in CANDU power reactors (http:/ / www. nuclearfaq. ca/ malkoskie_cobalt_paper. pdf)
[4] "Table of Isotopes decay data" (http:/ / nucleardata. nuclear. lu. se/ NuclearData/ toi/ nuclide. asp?iZA=270060). . Retrieved April 16, 2012.
[5] "Table of Isotopes decay data" (http:/ / nucleardata. nuclear. lu. se/ NuclearData/ toi/ nuclide. asp?iZA=270360). . Retrieved April 16, 2012.
[6] Gamma Irradiators For Radiation Processing (http:/ / www-naweb. iaea. org/ napc/ iachem/ Brochgammairradd. pdf). IAEA. 2005. .
[7] "Cobalt | Radiation Protection | US EPA" (http:/ / www. epa. gov/ rpdweb00/ radionuclides/ cobalt. html). EPA. . Retrieved April 16, 2012.
[8] Croatia fruit farmers fight flies (http:/ / www. reuters. com/ article/ 2012/ 10/ 02/ us-croatia-farming-nuclear-idUSBRE8910M020121002)
[9] The Formation of the Elements (http:/ / physics. fortlewis. edu/ Astronomy/ astronomy today/ CHAISSON/ AT321/ HTML/ AT32104. HTM)
[10] Isotope Production: Dual Use Power Plants (http:/ / www. atomicinsights. com/ jun96/ isotope. html)
[11] radioactive contamination of steel (http:/ / www. nrc. gov/ reading-rm/ doc-collections/ gen-comm/ info-notices/ 1983/ in83016. html)
[12] "Lessons Learned The Hard Way" (http:/ / www. iaea. org/ Publications/ Magazines/ Bulletin/ Bull472/ htmls/ lessons_learned. html). IAEA
Bulletin 47-2. International Atomic Energy Agency. . Retrieved 16 April 2010.
[13] The Radiological Accident in Samut Prakarn (http:/ / www-pub. iaea. org/ MTCD/ publications/ PDF/ Pub1124_scr. pdf). IAEA. 2002. .
Retrieved 2012-04-14.
[14] "Petco Recalls Some Stainless Steel Pet Bowls Due to Cobalt-60 Contamination" (https:/ / petpitchusa. wordpress. com/ 2012/ 08/ 10/
petco-recalls-some-stainless-steel-pet-bowls-due-to-cobalt-60-contamination/ ). . Retrieved 21 August 2012.
Cobalt-60 4

External links
• Cobalt-60 (http://www.bt.cdc.gov/radiation/isotopes/cobalt.asp), Centers for Disease Control and
Prevention.
• NLM Hazardous Substances Databank – Cobalt, Radioactive (http://toxnet.nlm.nih.gov/cgi-bin/sis/search/
r?dbs+hsdb:@term+@na+@rel+cobalt,+radioactive)
• Beta decay of Cobalt-60 (http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/betaex.html), HyperPhysics,
Georgia State University.
• Dr. Henry Kelly. Cobalt-60 as a Dirty Bomb (http://www.fas.org/ssp/docs/030602-kellytestimony.htm),
Federation of American Scientists, March 6, 2002.
Article Sources and Contributors 5

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