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Cobalt-60
General
Name, symbol Cobalt-60,60Co
Neutrons 33
Protons 27
Nuclide data
Natural abundance 0 (artificial element)
Half-life 1925.1 d ± 0.1 d
Isotope mass 59.9338222 u
Spin 5+
Decay mode Decay energy
Beta 2.824 [1] MeV
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.

Contents

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). It allows to calculate 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. Swallowing of 60Co reduces the distance to 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 60Ni and 60Co of 0.003 u. The short life time contributes further to the high decay energy. It amounts to nearly 20 W/g, nearly 30 times larger than 238Pu.

Decay scheme

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 2.1MeV 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.

60mCo is a nuclear isomer of 60Co. After a lifetime of 10.467 minutes and emission of 58.59 keV γ rays it transforms into 60Co:

Co60m(IT)Co60

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

Applications

Car scanning using Co60 γ-ray device.

The β-decay energy is low and easily shielded. Both strong γ-lines are of same magnitude therefore 60Co 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.

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

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]

Occurrence

Due to the quite short lifetime there is no natural 60Co. 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 59Co.[4]:

Co59(n,γ)Co60

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 60Co in scrap is responsible for the radioactivity found in several iron-based products.[5]

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

External links

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