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Learning Outcomes <ul><li>You will be successful if you can: </li></ul><ul><li>Recall your knowledge of the properties of alpha, beta and gamma radiation . </li></ul><ul><li>Explain why the ACTIVITY of a radioactive source decreases. </li></ul><ul><li>Show that, despite the RANDOM nature of decay, there is a simple PATTERN to decay . </li></ul><ul><li>Use these patterns and properties to help us in many different ways, including medical, engineering and archaeological applications . </li></ul>
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Activity 1 <ul><li>This is a model for radioactive decay . </li></ul><ul><li>Roll 100 dice into a tray. </li></ul><ul><li>Remove the dice with the 6 face pointing upwards. </li></ul><ul><li>Count the dice removed and deduct this from the total. </li></ul><ul><li>Repeat, with the remainder, removing the 6s each time. </li></ul><ul><li>Keep counting until there are only a few left </li></ul>Throw Number 0 1 2 3 Number removed 0 23 14 Number remaining 100 77 63
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Half life The decay of radioisotopes can be used to measure the material’s age. The HALF-LIFE of an atom is the time taken for HALF of the radioisotopes in a sample to decay… At start there are 16 radioisotopes After 1 half life half have decayed (that’s 8) After 3 half lives another 2 have decayed (14 altogether) After 2 half lives another half have decayed (12 altogether) = radioisotope = new atom formed
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A radioactive decay graph Time Count 1 half life 2 half lives 3 half lives 4 half lives
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Dating materials using half-lives Question: Uranium decays into lead. The half life of uranium is 4,000,000,000 years. A sample of radioactive rock contains 7 times as much lead as it does uranium. Calculate the age of the sample. Answer : The sample was originally completely uranium… … of the sample was uranium Now only 4/8 of the uranium remains – the other 4/8 is lead Now only 2/8 of uranium remains – the other 6/8 is lead Now only 1/8 of uranium remains – the other 7/8 is lead So it must have taken 3 half lives for the sample to decay until only 1/8 remained (which means that there is 7 times as much lead). Each half life is 4,000,000,000 years so the sample is 12,000,000,000 years old. 8 8 4 8 2 8 1 8 1 half life later… 1 half life later… 1 half life later…
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How old is our source? Imagine that the school has a highly dangerous radioactive source called Barhamium which decays into the much safer and more stable Urquartium ! We know that the half-life of Barhamium is five years . When the source was bought it was pure Barhamium and gave out 500 units of radiation per minute. Recently the source has been tested and found to be giving out only 62.5 radiations per minute. Can you work out how old the source is?
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How old is our source? Time (Years) Activity (Rads/minute) 0 500 5 250 10 125 15 62.5
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An exam question… (Higher Paper) Potassium decays into argon. The half life of potassium is 1.3 billion years. A sample of rock from Mars is found to contain three argon atoms for every atom of potassium. How old is the rock? (3 marks) The rock must be 2 half lives old – 2.6 billion years
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