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Slide 2

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Activity
(decays/sec) Take any point on the curve
Read its value - 80 decays/sec
80 1
Half that value - 40 decays/sec
60 Find the point on your curve that equals
this halved value
2 Find the time taken to go from point 1 to
40
point 2 from the horizontal axis
20 2-0 = 2s - Half life ­ time taken for
. activity to half
2 4 6 8
Time (sec)…read more

Slide 3

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Note that we chose the start of the graph as our
first point ­ we can choose any starting point
Activity
(decays/sec) Take any point on the curve
Read its value - 40 decays/sec
80
Half that value - 20 decays/sec
60 Find the point on your curve that equals
this halved value
1 Find the time taken to go from point 1 to
40
point 2 from the horizontal axis
20 2 4-2 = 2s - Half life ­ time taken for
. activity to half
2 4 6 8
Time (sec)…read more

Slide 4

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Note that this method works for all the in
between values too
Activity
(decays/sec) Take any point on the curve
Read its value - 54 decays/sec
80
Half that value - 27 decays/sec
60 Find the point on your curve that equals
1 this halved value
Find the time taken to go from point 1 to
40
point 2 from the horizontal axis
2
20 2.8-0.8 = 2s - Half life ­ time taken for
. activity to half
2 4 6 8 Note ­ figures may
Time (sec) have been slightly
bodged…read more

Slide 5

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Additional points
· In reality these 3 half life values may not be
exactly the same
· This will be because your curve is not perfectly
smooth ­ we don't expect it to be ­
radioactivity is random!
· A good way of getting a more accurate value
of half-life is to take the 3 readings (as we
have just done) and average them…read more

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