Alpha (α)
Nuclear & Particle Physics - OCR A-Level Physics
- Alpha (α)Alpha radiation — helium nuclei emitted during radioactive decay. Stopped by paper or a few cm of air.: helium-4 nucleus (2 protons + 2 neutrons). ChargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). +2e, mass 4u. Strongly ionising, range ~5 cm in air. Stopped by paper or skin.
- Beta-minus (β⁻)A high-speed electron emitted from the nucleus when a neutron decays into a proton. Stopped by a few mm of aluminium.: electron emitted when a neutron decays into a proton. ChargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). -e, negligible mass. Moderately ionising, range ~1 m in air. Stopped by a few mm of aluminium.
- Beta-plus (β⁺)A positron (anti-electron) emitted from the nucleus when a proton decays into a neutron. Annihilates on contact with an electron.: positron emitted when a proton decays into a neutron. ChargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). +e, negligible mass. Annihilates with an electron on contact, producing two gamma photons.
- Gamma (γ)High-energy electromagnetic radiation emitted from the nucleus. Very penetrating — reduced by thick lead or concrete.: high-energyThe capacity to do work. Measured in joules (J). electromagnetic radiation. No charge, no mass. Weakly ionising, very penetrating. IntensityThe powerThe rate of energy transfer. Measured in watts (W). transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared. reduced by lead or thick concrete; follows an inverse-square law.
- Alpha decay: $_{Z}^{A}$$X \rightarrow _{Z-2}^{A-4}$Y + _2^$4\alpha.$
- Beta-minus decay: $_{Z}^{A}$$X \rightarrow _{Z+1}^{A}$Y + $_{-1}^{0}\beta$^- + $\bar{\nu}$_e (antineutrino emitted).
- Beta-plus decay: $_{Z}^{A}$$X \rightarrow _{Z-1}^{A}$Y + $_{+1}^{0}\beta$^+ + $\nu$_e (neutrino emitted).
- Gamma emission does not change A or Z - it is the release of excess energyThe capacity to do work. Measured in joules (J). from an excited nucleus after alpha or beta decay.
Common Mistake
MEDIUM
Wrong: Beta decay does not conserve lepton numberA quantum number conserved in all interactions. Leptons (electrons, neutrinos) have lepton number +1, antileptons have -1, and hadrons have 0. because only an electron is emitted.
Right: An antineutrino is also emitted in beta-minus decay (and a neutrino in beta-plus decay). This conserves lepton numberA quantum number conserved in all interactions. Leptons (electrons, neutrinos) have lepton number +1, antileptons have -1, and hadrons have 0.. Pauli postulated the neutrino specifically to explain the continuous beta energyThe capacity to do work. Measured in joules (J). spectrum.
Right: An antineutrino is also emitted in beta-minus decay (and a neutrino in beta-plus decay). This conserves lepton numberA quantum number conserved in all interactions. Leptons (electrons, neutrinos) have lepton number +1, antileptons have -1, and hadrons have 0.. Pauli postulated the neutrino specifically to explain the continuous beta energyThe capacity to do work. Measured in joules (J). spectrum.