Leo Szilard calculates that if 2 grams of Cobalt-60 were released per square kilometer onto the planet, the resulting gamma radiation would wipe out the human race. The weapon could be a multi-stage warhead: for example, a first stage of plutonium, which triggers a hydrogen device, which fuses an outer layer of Cobalt-59, which absorbs neutrons and transforms into the unstable Cobalt-60. In slang, this is called a salted bomb.
