A level Physics · Edexcel 9PH0 · Topic 11A

Radioactive decay

Key points

Alpha (helium nucleus): strongly ionising, stopped by paper or a few cm of air. Beta-minus (electron): moderately ionising, stopped by a few mm of aluminium. Gamma (photon): weakly ionising, reduced by thick lead.
Decay equations must balance nucleon number and proton number, e.g. alpha decay reduces $A$ by 4 and $Z$ by 2; $\beta^-$ decay increases $Z$ by 1 and also emits an antineutrino.
Decay is random (cannot predict which nucleus) and spontaneous (not affected by conditions).
Activity $A = -\frac{dN}{dt} = \lambda N$ (Bq); $N = N_0e^{-\lambda t}$ and $A = A_0e^{-\lambda t}$ .
Half-life $t_{1/2} = \frac{\ln 2}{\lambda}$ is the time for half the nuclei to decay. A graph of $\ln A$ against $t$ is a straight line with gradient $-\lambda$ .
Always subtract background count (e.g. from rocks, cosmic rays, medical sources).
Core practical: investigate the absorption of gamma radiation by lead, keeping the source–detector distance fixed and handling the source safely.

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An unstable nucleus becomes more stable by emitting an alpha particle, a beta particle or a gamma ray. Which nucleus decays next is pure chance, yet a large sample decays in a completely predictable exponential way.

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