Transcript Radioisotope Dating

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
200
.
.
100
=B2-0.01202*B2
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.