Radioactivity: Half-life
 
 

 

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The concept of "half-life" does not appear in some foundation GCSE exams.
Check with your teacher if you're not sure which level of exam you'll be taking.


 Radioacitvity decreases with time

If you point a Geiger counter at a radioactive substance for a period of time, you'll notice that the reading on the meter decreases as you watch. This is shown on the graph.

The radioactivity from some substances dies away very fast - perhaps in a few microseconds. Others take thousands of years before you'll notice that the radioactivity had decreased at all.

 

In theory, every radioactive substance should stay slightly radioactive for ever - the graph should never actually fall to zero.

This means that we can't usefully talk about the "life" of a radioactive source.

Instead, we use the idea of "half-life".

This is the time it takes for the radioactivity to fall by half.

 

This sequence shows what would happen with an imaginary radioactive substance.

Use the buttons to step through it >>

Notice that the radioactivity in this example falls by half every 2 hours.

So we say that this imaginary substance has a half-life of 2 hours.

 

The count rate coming from a radioactive source depends on how many unstable atoms it contains.

That's the number of un-decayed atoms.

If the count rate has fallen by half, it means the number of unstable atoms has fallen by half.


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A typical exam question may be "A radioactive substance has a half-life of 2 hours. How much of the substance will remain after 6 hours have passed?"

 count on your fingers

Here's how to do it - simply count on your fingers.

*First finger: after one half-life (2 hours), half of the substance is left.

*Second finger: after two half-lives (another 2 hours), half of that is left, so we're down to a quarter.

*Third finger: after three half lives (that's 6 hours) half of that is left, so it's down to an eighth of what we started with.

So the answer to the exam question is: for a substance with a half-life of 2 hours, 1/8 of the original atoms will remain after 6 hours.


Note that this also means that 7/8 of the atoms will have decayed in that time.


Take another look at the graph sequence above, you'll see that after 6 hours the activity has fallen from 8,000 to 1,000; i.e. it's fallen to 1/8 of the starting value.

That's the key to solving half-life questions - count on your fingers, saying "half", "quarter", "eighth", "sixteenth", "thirty-second", "sixty-fourth"... etc.



At GCSE level you're unlikely to meet a question that needs more than 5 half-lives, and any questions you're asked will always involve whole numbers of half-lives (which makes life much easier!)

Let's see how much you've learned:


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