(Note that this doesn't mean the half-life of an element is a constant.
Thus this essay, which is my attempt at producing such a source.
Contents: The half-life of a radioactive isotope is defined as the time it takes half of a sample of the element to decay.
A mathematical formula can be used to calculate the half-life from the number of breakdowns per second in a sample of the isotope.
When I first got involved in the creationism/evolution controversy, back in early 1995, I looked around for an article or book that explained radiometric dating in a way that nonscientists could understand. Young-Earth creationists -- that is, creationists who believe that Earth is no more than 10,000 years old -- are fond of attacking radiometric dating methods as being full of inaccuracies and riddled with sources of error.
All these methods point to Earth being very, very old -- several billions of years old.
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.
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.
Some isotopes can break down in more than one way -- in these cases, each different breakdown type has its own half-life.
The decay rate and therefore the half-life are fixed characteristics of an isotope. That's the first axiom of radiometric dating techniques: the half-life of a given isotope is a constant.