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 first-order
process to half its starting is known as the "half-life"
for the decay.
The faster the decay the shorter the half-life.
For carbon-14 the half-life is 5570 years, which means that if an
experiment is begun with one gram of pure carbon-14, 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 half-lives: uranium-238
has a half-life of 4,510,000,000 years, whereas the elusive polonium-213
nucleus has a half-life of only 4.2 millionths of a second.
Since the half-life, t�
is the time required for the ratio [C-14]/[C-14]o
to decrease to 0.5 , the half-life and the decay rate
constant, k, are related by the expression
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