The Dish on Dirty Bombs

The normal atomic bomb is insidious enough.  They're easier to build than dirty bombs.  They wipe out entire cities and everything nearby.  Spontaneous tornadoes come up afterwards and if people don't die from the initial bomb, they will shortly afterwards.  Dirty bombs are a whole other story.  Normal nuclear bombs use the heat of the explosion to toast everything in its range. They emit a bit of radiation, but not as much as dirty bombs.  Dirty bombs use gamma rays.  Gamma rays have extremely high wavelengths.  Think of them as X-rays that tear you apart when you are exposed to large amounts of them.  They burn people to a crisp, but they also cause damage more subtle to people who aren't right underneath them.  They bore into bone marrow and mix up white blood cells' chromosomes.  They either die, become cancerous, or grow continuously.  They then end up deformed and cannot fight infections.  They are from hell.

Cobalt is the main villain when it comes to dirty bombs.   The warhead (main thing inside the bomb) is surrounded by a jacket of cobalt-59.  In the warhead, plutonium undergoes fission with hydrogen.  The cobalt takes neutrons from these reactions, changing it from stable cobalt-59 to loose-cannon cobalt-60, which would float down onto the victims as ash, coating the area it's detonated in.  A 10th of an ounce of cobalt-60 covering every surface of the Earth would be enough to completely exterminate the human race . . . or every race, for that matter.  Even cockroaches.  Normal atomic bombs can be waited out in underground shelters for a few days.  Dirty cobalt bombs, however, cannot be waited out.  It would take an entire lifetime for the land hit by one to become survivable again.  The cobalt ash on the ground keeps it radiated, gamma rays continuing to flow out for decades.