Some isotopes have very long half-lives, measured in billions or even trillions of years.Others have extremely short half-lives, measured in tenths or hundredths of a second.
I found several good sources, but none that seemed both complete enough to stand alone and simple enough for a nongeologist to understand them.
Thus this essay, which is my attempt at producing such a source.
The fourth one is that we know what the concentration of atmospheric C14 was when the organism lived and died.
The story of radiocarbon dating shows science at its finest.
Presented with a new method that gave answers different than existing methods, the scientists involved did not simply assume that either the old method or the new one was wrong.
They viewed the problem as a challenge, dug into it with all their energy, and didn't stop until they understood exactly why their C14 dates disagreed with traditional dates, what was wrong with their C14 procedures, and how to compensate for the problems in the future. When Professor William Libby developed the C14 dating system in 1949, he assumed that the amount of C14 in the atmosphere was a constant.
Radiometric dating methods are the strongest direct evidence that geologists have for the age of the Earth.
When I first became interested in the creation-evolution debate, in late 1994, I looked around for sources that clearly and simply explained what radiometric dating is and why young-Earth creationists are driven to discredit it.
So the dates derived from C14 decay had to be revised.