Why are Hiroshima and Nagasaki safe to live while Marie Curie’s notebook won’t be safe to handle for at least another millennium?
In: Physics
There is the physical aspect and the radiological aspect.
Physically, the bombs in Hiroshima and Nagasaki were airburst above the ground. Most of the radioactive material was launched into the air. Some went up into clouds and were spread over great distance, some settled out in dust particles over a few kilometers, and some rained down near the explosion. Natural weather (wind & rain) spread a lot of the radioactive materials over a large distance, diluting the concentration to less harmful levels.
Marie Curie studied radioactive elements for decades in a few small laboratories. Her work was ground breaking, and the negative health effects of radiation were not well understood, so strict radiation control measures were not used. This lead to a concentration of radioactive material in certain locations.
Radiologically, each element has a half life – the time it takes to release half of the radioactivity. Different elements have different half lives, from a few seconds to millions of years.
Elements that have a very short half life (such as less than a day) release a lot of radiation quickly and decay away to safe levels. If an element has a half life of 1 day, it will be 1/2 as radioactive tomorrow, 1/4 as radioactive the next day, then 1/8, 1/16, etc… after 20 days, it will be a million times less radioactive, so even a terribly high level would be safe after a month.
Elements that have a very long half life (like a million years or more) release radiation so slowly that it is not harmful unless you are in close proximity to a lot of it for a long time.
In the middle are nasty elements that can cause harm over several months to several centuries. One of the worst from the atomic bomb was Iodine-131, which has a half life of 8 days. A concentration that is 500 times too high would take 9 half lives to get to safe levels. That is 72 days – people would be exposed for months without knowing. Marie Curie studied Polonium (half life of 140 days) and Radium (1600 years). From a 500x concentration, Polonium would take 34 years to get to a safe level, and Radium would take 14,400 years.
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