· Smoke detectors use 241Am to emit alpha particles which pass through a small air gap before being detected. If smoke particles are present they interrupt the beam of alpha particles and this triggers the alarm to go off
· Tomorrow, will the 241Am still be as radioactive?
· Next year, will the 241Am still be as radioactive?
· In a thousand years, will the 241Am still be as radioactive?
Answers
· To answer the questions, we need to know the half life of Americium-241 which is 432 years
· Tomorrow and even next year its activity will hardly have changed at all (sensible for a smoke detector - you don't want it to suddenly stop working!)
· In a thousand years its activity will have dropped to about a quarter
7.10 to 7.12
12 January 2012
10:24
· 7.10 understand that the activity of a radioactive source decreases over a period of time and is measured in becquerels
· 7.11 recall the term ‘half-life’ and understand that it is different for different radioactive isotopes
· 7.12 use the concept of half-life to carry out simple calculations on activity
<<Half life.pptx>>
<<interactive simple half life calculations.swf>>
<<Decay of Balonium - exponential graph.swf>>
7.10 to 7.12 questions
01 February 2012
09:46
1. What happens to the amount of ‘mother’ nuclei as time passes?
2. What sort of radioisotope will decay the fastest - one with a long half life or one with a short half life?
3. Does half life tell us exactly when a particular nucleus in a radioisotope will decay?
4. What are the two definitions of half life?
5. What does the activity of a source mean?
6. What is the unit of activity?
7. What will happen to the number of ‘mother’ nuclei after two half lives?
8. What will happen to the activity of a source after two half lives?
7.10 to 7.12 calculation questions
12 January 2012
10:24
1. A radioisotope has a half life of 12 years. What fraction of the radioisotope will be left after 60 years?
2. If the activity of a sample falls to 1/64th of its original level after 2 hours, what is the half life of the sample?
3. The background radiation in a laboratory is 7 Bq. The count rate from a radioisotope is measured and it has a reading of 119 Bq. If the half life of the radioisotope is 10 minutes, what will be the reading 20 minutes later?
4. 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?
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