a) what importance do 1/2 lives have to society?

Answer 1

A good rule of thumb is that it takes about 10 half lives for a material to decay to the point that it isn't dangerous (from the radiation). How would you like to live in a house that is built above a tank holding an isotope with a half life of 10 seconds? How about one with a half live of 1 million years. Does that tell you something about the importance of half life.