Carbon-14

In the previous experiment, you knew the elapsed time, measured the activity and determined the half-life. In this paragraph, you will learn to use the half-life and activity to calculate the elapsed time.

New isotopes and carbon-dating

Even though unstable isotopes decay all the time, new unstable isotopes are also made all the time. The sun’s radiation hitting the atmosphere is turning carbon-12 into carbon-14 and deep underground, radioactive isotopes are thrust to the surface by volcanos. Carbon-dating takes advantage of this; About 1 in every 1012 atoms of carbon is carbon-14, an unstable isotope. Because every living thing on the planet is carbon based, 1 out of every 1012 of the carbon atoms in their bodies is carbon-14. The half-life of carbon-14 is 5730 years so quite a few of them will decay while inside of that organism. Luckily, plants inhale CO2 from the atmosphere and the rest of the organisms eat other organisms so new carbon-14 is ingested, keeping the amount about the same.

This process of renewing the carbon-14 stops when the organism dies. From then on, the number of carbon-14 atoms will decline as will the measured activity caused by their decay. If you know the activity of when the organism died and measure the activity now, you can work out the time passed between these two events.

Deep underground quite a lot of radioactive isotopes are stored since the formation of the earth itself. When a blob of lava is expelled onto the surface, the activity in this rock will slowly go down as well.

Play the fourth tab, “dating game”

In this game, you can use the GM-counter to measure the Carbon-14 or Uranium-238 level of certain objects, by moving the blue line in the graph, you can read out how long that object has been sitting there.

(if a certain object doesn’t give a reading using Carbon-14 or Uranium-238, use custom and set your own half-life)

86. How do you decide which to use: Carbon-14 or Uranium-238?

87. Why can’t you use this method to estimate the age of the living tree?

88. If you were a forensic scientist and found a freshly dead body, could you use one of the isotopes in the simulation to figure out how long ago the person died? Explain.

On the test, you need to be able to determine the age of an object using the activity. We’ll only do this with a whole number of half-lives. The exercises below are a good indication of what you can expect. You can use your calculator of course.

Assignments

90. Carbon-14 has a half-live of 5730 years. In a frozen mammoth, just 25% of the expected activity is measured. Calculate how long ago this mammoth died.

91. A piece of carbon-10 (half-live 19,2 seconds) is 250 000 Bq. Calculate what the activity in 96 seconds will be.

92. A piece of nickel contains 50 000 000 atoms nickel-65 (half-live 2,5 hours). Calculate how many of these atoms remain after 10 hours.