A cobalt bomb is a theoretical type of "salted bomb": a nuclear weapon intended to contaminate an area by radioactive material, with relatively little blast.
Originally proposed by physicist Leó Szilárd, who suggested that it would be capable of destroying all life on Earth, the weapon's tamper would be of ordinary cobalt metal, which the explosion then would transmute to the radioactive isotope cobalt-60 (60Co), which would produce deadly nuclear fallout.
The cobalt tamper would be transmuted into the isotope 60Co upon initiation and bombardment by neutron radiation. 60Co decays into an excited 60Ni by beta decay. The excited 60Ni then transitions to a ground state 60Ni, releasing gamma radiation.
Other isotopes could also be used for salted bombs, including gold-197, tantalum-181, and zinc-64.
Weapon of global destruction
The fallout would have a half-life of 5.27 years and would be intensely radioactive, a combination which caused Szilárd to suggest that such bombs could wipe out all life on the planet. Others argue that the mass needed would still be unreasonably large: 1 gram of 60Co per square kilometer of Earth's surface is 510 tonnes, and fallout does not reach all areas in equal amounts. While the sheer size and cost of such a weapon makes it unlikely to be built, it is technically possible because there is no maximum size limit for a thermonuclear bomb.
However, the effects of nuclear weapons, including blast, physical damage and fallout, do not scale up linearly with weapon size or yield; the magnitude of these effects increases more gradually than the energy released by the nuclear detonation. Stratospheric scientist and Australian peace activist Brian Martin has published analyses demonstrating this crucial weakness in the idea that the planetary nuclear arsenal is several times as large as that required to destroy all life on Earth; this analysis would have implications for the real versus the assumed lethality of the cobalt bomb to all life on Earth.
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