Radioactivity

What these notes cover

These notes cover radioactivity for Cambridge O Level Physics (5054) and Cambridge IGCSE Physics (0625): the structure of the atom and nuclide notation, Rutherford’s scattering experiment, isotopes, the properties of alpha, beta and gamma radiation, and radioactive decay equations.

Key formulas and definitions

• Nuclide notation: the nucleon (mass) number A is written at the top and the proton (atomic) number Z at the bottom; the number of neutrons = A − Z.
• Rutherford’s alpha-scattering experiment showed that an atom has a tiny, dense, positively charged nucleus and is mostly empty space.
• Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
• Alpha (α): a helium nucleus (2 protons + 2 neutrons). Most strongly ionising and least penetrating; stopped by paper or a few centimetres of air.
• Beta (β⁻): a fast electron emitted from the nucleus when a neutron changes into a proton. Moderately ionising; stopped by a few millimetres of aluminium.
• Gamma (γ): electromagnetic radiation. Weakly ionising and most penetrating; reduced by thick lead or concrete.
• In electric and magnetic fields, alpha and beta are deflected in opposite directions (beta more strongly); gamma is not deflected.
• Decay equations: in alpha decay the nucleon number falls by 4 and the proton number by 2; in beta decay the nucleon number is unchanged and the proton number rises by 1.