Cobalt bomb

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This is an article about the weapon of mass destruction; for the cancer treatment sometimes known by this name [1], see radiation therapy

A cobalt bomb, a type of salted bomb, is a form of nuclear weapon originally proposed by physicist Leó Szilárd, in which the weapon's tamper is made of ordinary cobalt metal, rather than a second fissionable material like ²³⁵U. This would be transmuted into the isotope cobalt-60 upon initiation and bombardment by neutron radiation. ⁶⁰Co is a very strong emitter of gamma rays as it undergoes beta decay, and is currently used for beneficial purposes in nuclear medicine.

The fallout would have a half-life of 5.27 years and is intensely radioactive, a combination which caused Szilárd to suggest that such bombs could wipe out all life on the planet. One gram of ⁶⁰Co contains approximately fifty curies (1.85 terabecquerels) of radioactivity. Held at close range, this amount of cobalt-60 would irradiate a person with approximately 0.5 gray of ionizing radiation per minute (also 0.5 sievert per minute, since for this type of ionizing radiation on a whole body, the gray and sievert values are roughly equivalent). A full body dose of approximately three to four sieverts will kill 50% of the population in thirty days, and could be accumulated in just a few minutes of exposure to a gram of ⁶⁰Co. Smaller amounts of ⁶⁰Co would take longer to kill, but would be effective over a large area.

What is unusual about this type of bomb is the half-life (5.27 years) is long enough to settle out before significant decay has occurred, and to make it impractical to wait out in shelters, yet is short enough that intense radiation is produced. The fallout of other nuclear weapons has the appearance of sand or ground pumice, which falls back to the ground in short time, and can be filtered by even a handkerchief, unlike ⁶⁰Co ^{[citation needed]}. After fifteen to twenty years, the ⁶⁰Co radiation would decrease by a factor of eight to sixteen, presumably making the area habitable again. The ⁶⁰Co would have decayed to harmless ⁶⁰Ni.

The United Kingdom reputedly conducted a nuclear experiment involving cobalt as a radioactive tracer in 1957, at the Tadje site, Maralinga range, Australia, but it was announced to be a failure.

Inspired by Szilárd's warnings, science fiction authors have occasionally made cobalt bombs the doomsday weapons in their works. On the Beach by Nevil Shute is one of the most well known of the fictional stories dealing with cobalt bombs. Also, the movie Dr. Strangelove (release Jan, 29, 1964) by Stanley Kubrick describes the Soviet Union building a cobalt-thorium G-bomb. Perhaps following the Strangelove reference, the movie Goldfinger (first released September 1964) features a Chinese "nuclear device" which is "of a particularly dirty kind." James Bond surmises this device will release radioactive "cobalt and iodine" to keep the gold at Fort Knox radioactive for "57 years" (Goldfinger corrects him to 58 years). China had yet to explode its first nuclear weapon (October 1964) when this movie was written, so the reference may be a deliberately obscure reference to a dirty bomb (see below).

In the twenty-first century, new attention came to ⁶⁰Co as a weapon of mass destruction, as the possibility of creating a dirty bomb to disperse this material might produce a swath of death downwind from it, over a significant area, as a terrorist attack. This is simpler than an actual nuclear weapon cobalt bomb, with a smaller range, though it is suggested that it could kill millions of people in a dense urban area [2] (although to reach a death toll this high would require exceedingly large and impractical amounts of material).