A cobalt bomb is a theoretical type of "salted bomb": a nuclear weapon designed to produce enhanced amounts of radioactive fallout, intended to contaminate a large area with radioactive material. The concept of a cobalt bomb was originally described in a radio program by physicist Leó Szilárd on February 26, 1950. He suggested that an arsenal of cobalt bombs would be capable of destroying all human life on Earth (though his conclusions are disputed).
As far as is publicly known, no cobalt bombs have ever been built. The Operation Antler/Round 1 test by the British at the Tadje site in the Maralinga range in Australia on September 14, 1957 tested a bomb using cobalt pellets as a radiochemical tracer for estimating yield. This was considered a failure and the experiment was not repeated.
A cobalt bomb could be made by placing a quantity of ordinary cobalt metal (59Co) inside a nuclear bomb. When the bomb explodes, the neutrons produced by the explosion would transmute the cobalt to the radioactive isotope cobalt-60 (60Co), which would be vaporized by the explosion. The cobalt would then condense and fall back to Earth with the dust and debris from the explosion, contaminating the ground.
The deposited Cobalt-60 would have a half-life of 5.27 years, decaying into 60Ni. The nickel nucleus is activated, and emits two gamma rays with energies of 1.17 and 1.33 MeV, hence the overall nuclear equation of the reaction is:
27Co + n → 60
27Co → 60
28Ni + e− + gamma rays.
Nickel-60 is a stable isotope and undergoes no further decays after emitting the gamma rays.
The 5.27 year half life of the 60Co is long enough to allow it to settle out before significant decay has occurred. The half-life is long enough for it to be impractical to wait in shelters for it to decay, yet short enough that intense radiation is produced. Many isotopes are more radioactive (gold-198, tantalum-182, zinc-65, sodium-24, and many more), but they would decay faster, possibly allowing some population to survive in shelters.
In a fission bomb, it has been suggested, the weapon's tamper could be made of cobalt. In a fusion bomb the radiation case around the weapon, normally made of uranium 238, could be made of cobalt. These changes would reduce the explosive power (yield) of the weapon somewhat.
Example of radiation levels vs. time 
Assume a cobalt bomb deposits intense fallout causing a dose rate of 10 sieverts (Sv) per hour. At this dose rate, any unsheltered person exposed to the fallout would receive a lethal dose in about 30 minutes (assuming a median lethal dose of 5 Sv). People in well-built shelters would be safe due to radiation shielding.
After one half-life of 5.27 years, only half of the Cobalt-60 will have decayed, and the dose rate in the affected area would be 5 Sv/hour. At this dose rate, a person exposed to the radiation would receive a lethal dose in 1 hour.
After 10 half-lives (about 53 years), the dose rate would have decayed to around 10 mSv/hour. At this point, a healthy person could spend 1 to 4 days exposed to the fallout with no immediate effects.
After 20 half-lives (about 105 years), the dose rate would have decayed to around 10 μSv/hour. At this stage, humans could remain unsheltered full-time since their yearly radiation dose would be about 80 mSv. However, this yearly dose rate is on the order of 30 times greater than the peacetime exposure rate of 2.5 mSv/year. As a result, the rate of cancer incidence in the survivor population would likely increase.
After 27 half-lives (about 142 years), the dose rate from Cobalt-60 would have decayed to less than 1 mSv/year and could be considered negligible.
Cultural references 
In the 1957 novel On the Beach by Nevil Shute, the death of all humanity is brought about by the detonation of cobalt bombs in the Northern Hemisphere. Cobalt bombs are mentioned in the episode "Obsession", the 13th episode of Season 2 of the original Star Trek, where two characters prepare an "antimatter bomb", and one character states that "less than one ounce of antimatter is more powerful than ten-thousand cobalt bombs." A cobalt and iodine "atomic device" is supplied by the Chinese government to Auric Goldfinger in the 1964 James Bond film Goldfinger, where he intends to detonate the bomb inside Fort Knox, rendering the USA's gold bullion reserves radioactive for 58 years. In the 1964 film Dr. Strangelove, the Soviet Union had established a secret nuclear deterrent comprising 60 buried cobalt bombs.
- Brian Clegg. Armageddon Science: The Science of Mass Destruction. St. Martins Griffin. p. 77. ISBN 978-1-250-01649-2.
- The Effects of Nuclear Weapons (Report) (3rd ed.). Washington, D.C.: United States Department of Defense and Department of Energy. 1977. http://www.alternatewars.com/WW3/WW3_Documents/Weapon_Effects/Effects_1977_09.pdf.
- "The global health effects of nuclear war". Current Affairs Bulletin 59 (7): 14–26. December 1982.
- "1.6 Cobalt Bombs and other Salted Bombs". Nuclearweaponarchive.org. Retrieved 10 February 2011.