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The half-life of a radioisotope can be used to measure
the age of things. The method is called radiodating.

Radiodating can be used to measure the age of rocks (see below)
and carbon dating can be used to date archaeological specimens.

Using Uranium-238 to Date Rock.

Some rocks contain uranium-238 which is radioactive and
follows a decay series until it produces a stable isotope of lead.
The amount of uranium in the rock is compared to the
amount of lead and then the age of the rock can be calculated.

For example, it is found
that there are equal amounts of uranium and lead in a rock.
The half-life of uranium-238 is 4·5 billion years.
After 4·5 billion years, half of the uranium originally present
in the rock would have decayed and become lead.
The proportion of uranium to lead would be 1 to 1 (equal amounts).
The rock could therefore be dated as 4·5 billion years old.

You can only use the ratio of uranium-238 to lead to date rock if
you are sure that there was no lead originally present in the rock and
that all the lead in the rock has come from the decay of uranium.

Using Potassium-40 to Date Rock.

Some rocks contain the radioisotope potassium-40 which
decays to form argon-40. Argon-40 is a stable isotope.
If the argon gas is unable to escape from the rock, then the
proportions of potassium-40 to argon-40 can be used to date the rock.

For example, it is found that
there is three times as much argon-40 as potassium-40 in a rock.

After 2 half-lives there is three times as much argon-40
as potassium-40 in the rock (see the boxes above).
The half-life of potassium-40 is 1·3 billion years
The rock could therefore be dated as 2 x 1·3 billion years
= 2·6 billion years old.

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