Radioactive decay and half-life
Watch a sample of unstable nuclei decay at random and measure the half-life from the decay curve.
- Random decay
- Half-life
- Decay curves
Time elapsed
0.0min
Measured count rate
820cpm
Source activity
800cpm
Half-lives passed
0.00
Predict
A source has a count rate of 800 cpm and a half-life of 15 minutes. What is the count rate after 45 minutes, ignoring background?
Experiment
- 1Set background to 0 and run the decay. Check the curve halves every half-life.
- 2Set background to 60 and run again. Notice the curve now flattens at 60, not 0.
- 3Work out the half-life from the graph with background included — remember to subtract it first.
Explain
Radioactive decay is random: you cannot say which nucleus will decay next, only that a fixed fraction decays in a given time. That fixed fraction is what produces the exponential curve.
The half-life is the time for half the undecayed nuclei to decay, or equivalently for the activity to halve. It is constant for a given isotope.
In a real experiment the detector also picks up background radiation. Subtract the background count rate from every reading before halving, or your half-life will come out too long.