What is the limitations if carbon dating and what is a example of the materials for which the technique cannot be used?
"Carbon dating" is the technique used to calculate how long ago
a living organism died.
Here's how it works. Cosmic rays from space hit atoms in the
atmosphere, and in some cases will generate thermal neutrons.
Nitrogen-14 atoms in the upper atmosphere absorb a few of those
neutron, become unstable, and then emit a proton. This changes the
Nitrogen-14 (atomic number 7) to carbon-14 (atomic number 6). This
process has been going on for a very long time, so there's a
relatively constant - but low - percentage of carbon 14 in the
atmosphere. Living things breathe in the carbon-14 (along with
oxygen and ordinary nitrogen) and the biological processes
incorporate this carbon-14.
Carbon-14 is slightly radioactive, and decays with a half-life
of 5760 years. As long as you are alive, you keep replenishing your
supply of carbon-14. But when you (or any other living thing) dies,
it's no longer getting fresh carbon-14, and the existing carbon-14
continues to decay.
By measuring the amount of carbon-14 remaining in any organic
material, we can calculate approximately how long ago the organism
died. It's fairly accurate back to about 10,000 years, and sort-of
accurate back to about 60,000 years. After that, the proportion of
carbon-14 that remains is too tiny for accurate analysis.
This only works with living things, and only within the past
60,000 years.
This also assumes that there have been no discontinuities in the
formation of carbon-14 in the atmosphere. If there had been a
supernova explosion within about 50 light years, a spike in cosmic
rays might have caused a spike in carbon-14 production, and
carbon-dating analysis might indicate that the sample was younger
than it really was.