After another equal time interval the
concentration will be at an eighth its starting value, then a sixteenth,
and so on. The time required for any beginning quantity of material
to decay by a firstorder
process to half its starting is known as the "halflife"
for the decay.
The faster the decay the shorter the halflife.
For carbon14 the halflife is 5570 years, which means that if an
experiment is begun with one gram of pure carbon14, only a half
gram will be left after 5570 years.
In 11,140 years only a quarter gram will
remain, and after 16,710 years, one eighth gram will remain. Unstable
nuclei vary widely in their decay rates or halflives: uranium238
has a halflife of 4,510,000,000 years, whereas the elusive polonium213
nucleus has a halflife of only 4.2 millionths of a second.
Since the halflife, t_{½}
is the time required for the ratio [C14]/[C14]_{o}
to decrease to 0.5 , the halflife and the decay rate
constant, k, are related by the expression

