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We are going to start with 2 coins each and throw them all and then count how many had landed with heads facing up and record this. We will then remove those coins that have landed heads up and throw the remaining coins. How many of these will land heads up? Can you predict what would happen to an individual coin? Number of coins Number of throws
Half-Life The half life is the time taken for half of the radioactive nuclei present to decay.
Go After one half-life Time zero: Number of nuclei, N
After two half-lives Go Time 1: Number of nuclei, N/2
After three half-lives Go Number of nuclei, N/4
After four half-lives Go Number of nuclei, N/8
After five half-lives Go Number of nuclei, N/16
Number of nuclei, N/32 After five half-lives
Key points • Each nucleus in a sample of an isotope decays at random, regardless of what the other nuclei are doing. • Radioactive decay is not effected by: -Chemical state of the isotope, i.e. whether its in a compound or as its element -Temperature or pressure -Whether it is solid, liquid, gas or in solution
Key points • Half life is: ‘time taken for half the radioactive nuclei to decay’. • It is the same regardless of how many radioactive nuclei we start with. • The sample never completely disappears, it just halves each time, this is why radioactive isotopes can still be detected today, 4 billion years after the earth was formed.
Half Life radioactivity decreases The _________ of a sample always _______ over time. Each time a decay happens ______, ______ or ______ radiation is emitted. This means a radioactive _______ has decayed. The problem with trying to measure the time for all the atoms to decay is that the activity never reaches _____. The half life is the _____ taken for ____ of the radioactive ______ now present to decay. An isotope with a ____ half life decays more quickly than an isotope with a _____ half life. alpha beta gamma nucleus zero time half nuclei short long
Radioactive Dating • C-14 makes up 1/10,000,000 of the carbon in the air. This same proportion is found in living things. When they die the C-14 is trapped inside and gradually decays with a half life of 5600 yrs • By measuring the proportion of C-14 left and knowing the half life you can calculate how long ago something was living. PRACTICE QUESTION: An axe handle was found to contain 1 part in 40,000,000 C-14. How old is the axe? The C-14 was originally 1 part in 10,000,000. After 1 half life it would be down to 1 part in 20,000,000 and after 2 half lives it would be down to 1 part in 40,000,000. So the axe handle is 2 half lives old, i.e. 2 x 5600 years = 11,200 years.
Radioactive Dating Question The half life of C-14 is 5,600 years and C-14 makes up 1 part in 10,000,000 of the carbon in the air. Calculate how long ago each of the following was living material: • A fossil containing 1 part in 320,000,000 C-14 • A spear handle containing 1 part in 80,000,000 C-14 • An axe handle containing 1 part in 20,000,000 C-14
Remember… • For these radioisotopes to work as geological clocks the following must be met: -Half-life must be known accurately -No movement of parent or daughter isotopes out of or into mineral since crystallisation -no resetting of geological clock through heating and deformation of the rocks (metamorphism)
Dating rocks The proportions of the parent radioisotope potassium-40 and its daughter decay product argon-40 can also be used to date igneous rocks from which the gaseous argon has been unable to escape.
Dating Rocks Igneous rocks can be dated if you measure the ratios of uranium-235 and its decay product lead. The half life of uranium-235 is 4.5 billion years. Assuming no lead was present when the rock was formed find the ages of the rocks: a) uranium: lead 1:1 b) uranium: lead 75:525 c) uranium: lead 1:0 d) uranium: lead 75:225
Uses of Carbon-14 dating • Read the green boxes on pages 9-10 for more examples of where carbon-14 dating has been used.
Radioactive tracers • As we have learnt more about isotopes a new field of medicine has begun to develop, ‘Nuclear Medicine’. • Radioisotopes can now be injected into the body to enable detection and treatment for a range of problems. • The half-life of the isotopes is important so as not to cause unnecessary damage to the cells. • Examples include iodine-131 to detect thyroid problems and technetium-99 to detect tumours.
