And the longer the half-life, the less radioactive the isotope is. I recall reading that iron has a half life in the billions of years.
I thought it was even longer than that. Anyway, since astronomers agree that the universe is between thirteen and fourteen billion years old, and since they also agree that iron is not primordial, but is formed inside generation after generation of short-lived massive stars, that should mean that most of the iron that has ever existed in our universe is still around. (Although I do know that there are different isotopes of iron, containing different numbers of neutrons, and some of those isotopes may certainly be more radioactive than others.)
Gamma radiation may go forever, but intensity decreases exponentially as you get away from the source. And I'm pretty sure it can be distorted, blocked, etc.
It
can be blocked, but it is very hard to do so. You need something extremely massive and thick between yourself and the gamma rays to block them. In the universe, there exists something called gamma ray bursts. They are believed to be narrow jets of concentrated gamma radiation emitted either when a particularly massive star goes supernova, or when two neutron stars collide. Those jets of gamma rays contain a tremendous amount of energy. I once read what would happen to the Earth if there was a gamma ray burst taking place in our own galaxy (but thousands of light-years away) and one of the jets hitting the Earth. According to what I read, the side of the Earth that took the direct hit would be scorched and sterilized. However, the rays wouldn't be able to penetrate the Earth itself - after all, the Earth is made mostly of iron and nickel, so it is quite dense - and therefore the opposite side of the Earth would be spared the immediate effects of the gamma rays. (Of course, the devastation on one side of the Earth would eventually affect the opposite side, too.)
By comparison, an "ordinary" supernova that went off in our galaxy a few thousand light-years away wouldn't affect us in the slightest.
But I personally have never heard of human beings that give off so much gamma radiation that their bodies can kill others from a considerable distance. If they give off gamma rays, my guess is that the gamma radiation would quickly become diluted as you moved away from the radioactive bodies.
Ann
