TAP 516-3: Radioactive decay used as a clock
Because of the predictability of the random behaviour of large numbers of atoms, activity can
be used as a clock.
Clocking radioactive decay
Activity
Half-life
N0
number N of
nuclei halves
every time t
increases by
half-life t1/2
N0 /2
slope = activity =
dN
dt
halves every half-life
N0 /4
N0 /8
t1/2
t1/2
t1/2
t1/2
t1/2
time t
time t
Radioactive clock
In any time t the number N is reduced by a constant factor
Measure activity. Activity proportional to number N left
In one half-life t1/2 the number N is reduced by a factor 2
Find factor F by which activity has been reduced
In L half-lives the number N is reduced by a factor 2
L
(e.g. in 3 half-lives N is reduced by the factor 23 = 8)
Calculate L so that 2L = F
L = log2F
age = t1/2 L
Practical advice
This diagram is reproduced here so that you can talk through it, or adapt it to your own
purposes.
External reference
This activity is taken from Advancing Physics chapter 10, Display material 20O