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What is half li


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Second decay Third decay Fourth decay Fifth decay Half-Life As a material decays it transmutes, forming a new element. There is less of the original material left Imagine 100 atoms each has a 50% chance of decaying in any given time period. First decay Half decay, leaving 50. Second decay Third decay Half decay, leaving 25. Half decay, leaving 12. Half decay, leaving 6. Half decay, leaving 3. Fourth decay Fifth decay Start with 100 atoms.

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1 2 3 4 5 Draw line of best fit Half-Life 1 2 3 4 5 time Number of particles 0- 10- 20- 30- 40- 50- 60- 70- 80- 90- 100- 0 1 2 3 4 5 x x x x x x Draw line of best fit We can plot this decay on a graph. Click the time periods…

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Half-Life The shape of this graph is characteristic of radioactive decay. The graph starts steep and then slopes less and less as it approaches the X-axis. The graph only looks this smooth if we sample huge numbers of atoms. Since radioactive decay is totally random, we can only achieve an average by sampling millions or billions of decays.

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Half-Life The gradient of this graph tells us a lot. Gradient = y =Change in number of particles = N x time t So the decay tells us the number of decays per unit time or… …decays per second. Which is activity! (in Becquerel) Gradient = y = 40-6 = 34 = 17 Bq x 2 2 34 2

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Half-Life This means that the steeper the line the greater the activity. The activity starts off high and then quickly lowers. Gradient = y = -34 = 17 Bq x 2 34 2 So a radioactive substance becomes less active with time, though its rate of decay slows quite quickly. So if we leave a radioactive material long enough, it becomes safe… Gradient = y = -36 = 36 Bq x 1

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Half-Life Different materials decay at different rates. We compare the rate of decay by looking at a materials half-life. The half life is the time it takes for half of the sample to decay. In our sample, it takes one second for the number of particles to halve. It’s half life is one second. 100 = 1 50 2 24 = 1 12 2

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Half-Life So to find the half life… 80 = 1 40 2 Pick a value of N Find where N is half of this Mark the time axis I’ll pick 80 Half of 80 is 40 Measure the time Calculate the time difference. 1s Note that it doesn’t matter where you measure the half-life, it will always be the same. Clever eh?

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Real half-Life values The half life values of elements varies widely. Bismuth-209 has a half life of twenty billion billion years; so long that you wouldn’t really consider it radioactive at all! Polonium-214 has a half life of 150 microseconds…faster than a blink! Many radioactive nuclei decay through several unstable states before achieving a stable nuclear configuration. Such elements move down a decay ‘chain’ as shown for Uranium-238 on the right.

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Real half-Life values What you need to know: - Radioactive decay is totally random. We can find patterns in decay by sampling huge numbers of atoms Different radioactive elements decay at different rates. The gradient of a graph of decay tells us the activity. The activity of a sample drops with time. We can calculate the ‘half-life’ of a sample from a decay graph. The half-life is the time taken for half the material to decay or… …the half life is the time taken for the activity to fall by half.

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