Edexcel A Level (IAL) Physics-5.17 The Random Nature of Nuclear Decay- Study Notes- New Syllabus

The Spontaneous and Random Nature of Nuclear Decay

Nuclear decay is the process by which an unstable nucleus emits radiation in order to become more stable. This process is both spontaneous and random.

Spontaneous Nature of Nuclear Decay

Nuclear decay is described as spontaneous because:

• Decay occurs without any external trigger.
• It does not depend on temperature, pressure, or chemical state.
• It cannot be started, stopped, or altered by physical conditions.

Key idea: An unstable nucleus will decay naturally when conditions are right within the nucleus itself.

 Random Nature of Nuclear Decay

Nuclear decay is random because:

• It is impossible to predict when a particular nucleus will decay.
• All unstable nuclei have the same probability of decaying per unit time.
• One nucleus may decay immediately, while another identical one may last millions of years.

Important: Although individual decays are unpredictable, the behaviour of a large number of nuclei is predictable.

 Probability of Decay

The probability that a nucleus decays in a short time interval is constant.

• This probability does not depend on the age of the nucleus.
• Nuclei do not “age” or become more likely to decay over time.

This leads to an exponential decrease in the number of undecayed nuclei.

 Consequences of Random Decay

• Radioactive decay curves are smooth only for large numbers of nuclei.
• Small samples show large random fluctuations.
• Measured count rates vary randomly about a mean value.

Therefore: Long counting times and repeated measurements are needed to reduce uncertainty.

Link to Half-Life

The random nature of decay leads to the concept of half-life.

• Half-life is the time taken for half the unstable nuclei to decay.
• It is a constant property of a radioactive isotope.
• It is independent of the initial number of nuclei.

Important: Half-life does not mean all nuclei decay after a fixed time — only half do.

 Common Misconceptions

•  Decay cannot be predicted for a single nucleus.
•  Older nuclei are not more likely to decay.
•  External conditions do not affect decay rate.

Only the probability of decay per unit time is fixed.