Fundamental particles: (h) beta-minus (β⁻) decay; beta-plus (β⁺) decay

The idea

Beta decay is a weak interaction process in which a neutron or proton changes quark flavour. In beta-minus (β⁻) decay, a neutron converts to a proton: one down quark changes to an up quark, emitting an electron and an anti-neutrino. The quark change is d → u, so the nucleon changes from udd to uud. In beta-plus (β⁺) decay, a proton converts to a neutron: one up quark changes to a down quark, emitting a positron and a neutrino. The quark change is u → d, so the nucleon changes from uud to udd. In both cases charge, baryon number and lepton number are conserved. Beta decay explains how unstable nuclei adjust their neutron-to-proton ratio.

What earns the marks

• Beta-minus decay: a neutron changes into a proton.
• In β⁻ decay, a down quark changes to an up quark (d → u).
• β⁻ decay emits an electron and an anti-neutrino.
• Beta-plus decay: a proton changes into a neutron.
• In β⁺ decay, an up quark changes to a down quark (u → d).
• β⁺ decay emits a positron and a neutrino.
• Charge, baryon number and lepton number are conserved in both decays.

Common mistakes and how to put them right

• Saying beta-minus decay emits a positron.

  Put it right by noting β⁻ emits an electron and an anti-neutrino.

• Confusing the quark change direction.

  Put it right by remembering β⁻ is d → u and β⁺ is u → d.

• Forgetting the neutrino or anti-neutrino.

  Put it right by including it to conserve lepton number.

• Thinking beta decay involves the strong interaction.

  Put it right by identifying it as a weak interaction.

Exam tips

• Write the quark-level change first, then the emitted particles.
• Check conservation of charge, baryon number and lepton number in each decay.
• Remember β⁻ increases proton number by 1 and β⁺ decreases it by 1.
• Use the mnemonic: beta-minus makes a proton, beta-plus makes a neutron.