Random & Spontaneous Decay
The average behaviour of a large sample is predictable, but one nucleus is not. Random and spontaneous describe two different limits on what can start or predict a decay.
Random and spontaneous describe different facts
Definition
1 markWhat is meant by random radioactive decay?
Model answer: The time at which a particular nucleus decays cannot be predicted.
Definition
1 markWhat is meant by spontaneous radioactive decay?
Model answer: The decay occurs without an external cause and is not affected by external environmental conditions.
(9702/42/O/N/24 Q10(a))
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| Word | What it tells you | Evidence or consequence |
|---|---|---|
| Random | You cannot predict which nucleus decays next or when it decays | Measured count rate fluctuates |
| Spontaneous | No external trigger starts the decay | Ordinary changes in temperature, pressure or chemical state do not change the decay |
Random decay trial
Change the time or repeat the trial. The individual pattern changes, but the large-sample prediction stays the same.
This trial
38 nuclei remain
Large-sample model
40.0 expected
Current activity from A = λN
6.6 Bq
The model predicts the average behaviour of many trials. It does not predict the exact nucleus that decays next.
A source gives 19, 25 and 21 counts in three equal intervals. Does this prove that the source strength changed between intervals?
Show worked answer
No. Different readings are expected because decay is random. A conclusion about the underlying mean needs many readings with the apparatus kept fixed.