Radioactive decay is a random and spontaneous process. Explain what is meant by the terms 'random' and 'spontaneous' in this context, and describe how the count rate of a radioactive source changes over time.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A technician monitors a radioactive source over several days and notices that the count rate detected gradually decreases.
Model answer (5 marks)
Random means that it is impossible to predict which nucleus will decay next or when any particular nucleus will decay.
Spontaneous means that the decay occurs without being triggered or affected by external conditions such as temperature, pressure or chemical environment.
The count rate of a radioactive source decreases over time.
This decrease occurs because the number of unstable nuclei remaining in the source gets smaller.
The count rate never reaches exactly zero; it decreases by the same fraction in equal time intervals – the activity halves every half‑life.
Spontaneous means that the decay occurs without being triggered or affected by external conditions such as temperature, pressure or chemical environment.
The count rate of a radioactive source decreases over time.
This decrease occurs because the number of unstable nuclei remaining in the source gets smaller.
The count rate never reaches exactly zero; it decreases by the same fraction in equal time intervals – the activity halves every half‑life.
Examiner tips
- Use the exact terms ‘random’ and ‘spontaneous’ as defined in the scheme.
- Show the link between decreasing nuclei and decreasing count rate.
- Mention that the activity follows an exponential decay and never becomes exactly zero.
- Keep the answer concise and directly address each point.
Mark scheme (5 marks)
- Random means it is impossible to predict which nucleus will decay next (or when any particular nucleus will decay)
- Spontaneous means the decay is not triggered or affected by external conditions (e.g. temperature, pressure, chemical environment)
- The count rate decreases over time
- The count rate decreases because the number of unstable (radioactive) nuclei remaining in the source decreases
- The count rate never reaches exactly zero / the source always retains some radioactive nuclei (or: the count rate decreases by the same fraction in equal time intervals — halving in each half-life)
Key terms in this question
random · spontaneous · radioactive decay · count rate
Related
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