Alpha & Beta Decay (AQA A Level Physics)

• When nuclei are unstable, they can become more stable through the process of radioactive decay
• Three of the most common decay mechanisms are:
  • Alpha decay
  • Beta-minus decay
  • Beta-plus decay

Alpha Decay

• Alpha decay is common in large, unstable nuclei with too many nucleons (protons and neutrons)
• The decay involves a nucleus emitting an alpha particle and decaying into a different nucleus
• An alpha particle consists of 2 protons and 2 neutrons
  • This is equivalent to a helium nucleus

During alpha decay, a parent nucleus becomes a daughter nucleus by emitting an alpha particle (helium nucleus)

• When an unstable nucleus (the parent nucleus) emits radiation, the constitution of its nucleus changes
• As a result, the isotope will change into a different element (the daughter nucleus)
• Alpha decay can be represented by the following radioactive decay equation:

• When an alpha particle is emitted from a nucleus:
  • The nucleus loses 2 protons: proton number decreases by 2
  • The nucleus loses 4 nucleons: nucleon number decreases by 4

Beta-Minus Decay

• A beta-minus, β^-, particle is a high energy electron emitted from the nucleus
• β^- decay is when a neutron turns into a proton emitting an electron and an anti-electron neutrino

During beta-minus decay, a neutron in a parent nucleus becomes a proton in a daughter nucleus by emitting a beta-minus particle (an electron) and an anti-electron neutrino

• When a β^- particle is emitted from a nucleus:
  • The number of protons increases by 1: proton number increases by 1
  • The total number of nucleons stays the same: nucleon number remains the same

• The new nucleus formed from the decay is called the “daughter” nucleus (nitrogen in the example above)

Beta-Plus Decay

• A beta-plus, β⁺, particle is a high energy positron emitted from the nucleus
• β⁺ decay is when a proton turns into a neutron emitting a positron (anti-electron) and an electron neutrino

During beta-plus decay, a proton in a parent nucleus becomes a neutron in a daughter nucleus by emitting a beta-plus particle (a positron) and an electron neutrino

• When a β⁺ particle is emitted from a nucleus:
  • The number of protons decreases by 1: proton number decreases by 1
  • The total number of nucleons stays the same: nucleon number remains the same

• An electron neutrino is a type of subatomic particle with no charge and negligible mass which is also emitted from the nucleus
• The anti-neutrino is the antiparticle of a neutrino
  • Electron anti-neutrinos are produced during β– decay
  • Electron neutrinos are produced during β+ decay

• Although the neutrino has no charge and negligible mass, its existence was hypothesised to account for the conservation of energy in beta decay

• When the number of α particles is plotted against kinetic energy, there are clear spikes that appear on the graph
• This demonstrates that α-particles have discrete energies (only certain values)

Alpha particles have discrete energy levels whilst beta particles have a continuous range of energies

• When the number of β particles is plotted against kinetic energy, the graph shows a curve
• This demonstrates that beta particles (electrons or positrons) have a continuous range of energies
• This is because the energy released in beta decay is shared between the beta particles (electrons or positrons) and neutrinos (or anti-neutrinos)
• This was one of the first clues of the neutrino’s existence
• The principle of conservation of momentum and energy applies in both alpha and beta emission