 | RADIOACTIVE AGE DATING |
Some atomic nuclei are not stable. If allowed to sit,
they spontaneously break apart into smaller nuclei releasing
particles and energy. For example, the aluminum isotope
13Al26
is not stable. It
decays
to magnesium as shown below
13Al26 ---> 12Mg26
+ positron + energy
This process has a half-life of
720,000 years. The
half-life measures
the amount of time it takes for 1/2 of the Al nuclei in a box
to change into Mg nuclei. This property of radioactive decay lends
itself well to the age dating of rocks and other materials.
Age Dating
- Imagine a rock which contains 32
13Al26 nuclei and 0 magnesium
nuclei
- After a time equal to one half-life of the aluminum, 720,000 y, 1/2 of the
aluminum will have decayed to 12Mg26. That is, the rock
will now be made of
16 aluminum nuclei and 16 nuclei
magnesium nuclei
- If we wait for a second another half-life, then one-half of the remaining
aluminum nuclei will decay to magnesium.
After 2 half-lives, the rock will then be composed
of
8 aluminum nuclei and 24 nuclei
- If we wait for a third half-life, then a further one-half of the remaining
aluminum nuclei will decay to magnesium. After 3 half-lives, the
rock will then be composed of
4 aluminum nuclei and 28 nuclei
- If we wait for a fourth half-life, the composition of the rock will be
? aluminum nuclei and ? nuclei
We see that by comparing the relative amounts of aluminum and magnesium in the
rock, we can infer the age of the rock. This important technique is called
Radioactive Age Dating
Question: What are some things which could cause errors in the method?
