Radioisotope Dating

What is radioactivity?

• Radioactivity is the spontaneous emission of energy from unstable atoms.

• There are stable atoms, which remain the same forever, and unstable atoms, which break down or 'decay' into new atoms.

• These unstable atoms are said to be 'radioactive', because they emit radioactivity from the nucleus as they decay.

What is radioactivity?

• Radioactivity is a random process that happens naturally as the isotopes in particular elements decay.

• The isotopes continue to break down over time. – The length of time that is taken for half of the

nuclei in an element to decay is called its 'half-life'.

What is radioactivity good for?

• Radioisotopes are commonly used in medicine.– Example: Radioactive Iodine-131 can be

used to study the function of the thyroid gland assisting in detecting disease.

• Nuclear power stations use uranium in fission reactions as a fuel to produce energy.

Who discovered radioactivity?

• Antoine Henri Becquerel (1852-1908)

– For his discovery of radioactivity, Becquerel was awarded the 1903 Nobel Prize for physics.

• Marie Curie (1867-1934) & Pierre Curie (1859-1906)

– For their work on radioactivity,the Curies were awarded the 1903 Nobel Prize in physics.

What is the connection with exponential decay?

• The radioactive half-life for a given radioisotope is the time for half the radioactive nuclei in any sample to undergo radioactive decay.

• After two half-lives, there will be one fourth the original sample, after three half-lives one eight the original sample, and so forth.

How does a nuclear fission reactor work?

• Nuclear fission

Fission Chain Reaction

History of Nuclear Energy

• 1942 Enrico Fermi at U. of Chicago successfully demonstrated the first controlled chain reaction

Radioisotope Dating• Radioisotopes have characteristic decay probabilities, commonly

expressed as halflives, which render them more or less useful in dating the objects that contain them.

• Carbon-14, for instance, has a relatively short half-life of 5730 years. • Although the C-14 on the surface of the Earth is constantly decaying

away, it is also being produced. The net effect is that the C-14 is produced at the same rate as it decays, so the level of C-14 stays constant. Its level is one part in a trillion .

• Plants absorb this carbon when they breathe in carbon-dioxide. So the carbon in plants consists of one part in a trillion C-14. We eat plants and the result is that the carbon in our bodies is also one part in a trillion C-14. As long as we eat and breathe, our carbon is one trillionth C-14.

• When we die, the C-14 decays (with its 6 thousand year half-life) but it is no longer replaced. After you are buried for 6 kyr, the amount of C-14 in your body is reduced by half. In another 6 kyr, it is cut in half again. By measuring the ratio of C-14 to ordinary carbon, we know when you died (or the tree, or the fossil, or whatever).

• For example, if we measure that a bone does not have one part in a trillion of carbon, but only 1/8 that much, then we know it has been buried for 3 half lives. (It is three half lives, because the amount is reduced by 1/2 three time, and 1/2 x 1/2 x 1/2 = 1/8.) Three half lives means it is 18 kyr old.

Dating the Dead Sea Scrolls• We date the Dead Sea Scrolls which have

about 78% of the normally occurring amount of Carbon 14 in them.

• Carbon 14 decays at a rate of about 1.202% per 100 years

Years after death

% Carbon remaining

0 100

100 =B2-0.01202*B2

200

.

.

Dating the Dead Sea Scrolls

• Do excel example

Dating the Dead Sea Scrolls

• We find that the Dead Sea Scrolls would date from between 2100 to 2000 years ago.

• Current estimates are that a 95% confidence interval for their date is 150 BC to 5 BC.