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.
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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.
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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. |
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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?"
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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. |
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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.
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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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