It is therefore said to be 'fissile' and we use the expression 'nuclear fission'.
Meanwhile, like all radioactive isotopes, they decay.
Approximately 50 of these are found in nature; the rest are produced artificially as the direct products of .
Understanding the ages of related fossil species helps scientists piece together the evolutionary history of a group of organisms.
A fossil can be studied to determine what kind of organism it represents, how the organism lived, and how it was preserved.
However, by itself a fossil has little meaning unless it is placed within some context.
U-238 decays very slowly, its half-life being about the same as the age of the Earth (4500 million years).
This means that it is barely radioactive, less so than many other isotopes in rocks and sand.