| Cobalt-60 | |
|---|---|
| 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 |
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 the radio-isotope 59
26Fe. 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
26Fe + n → 60
27Co → 60
28Ni + e− + gamma rays.
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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) at one meter from the source. 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 use 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.
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:
With a low probability 60mCo also β-decays and populates the two "2+" levels of 60
28Ni.
The β-decay energy is low and easily shielded. Two gamma-ray emission lines have energies around 1.3 MeV, and are highly penetrating.
Main uses for 60
27Co:
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]
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]
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. This is ironic because it has been used for the treatment of cancer since 1951.[6]
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.[7][8]
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.[9]