Nuclear and Particle Physics — American System Physics Revision Notes & Topical Questions

The nucleon number is A = Z + N (proton number plus neutron number). Proton number (atomic number Z) identifies the element and equals the electrons in a neutral atom.

The strong nuclear force is attractive between nucleons, much stronger than electrostatic repulsion but acting only over a very short range (about 3 × 10⁻¹⁵ m). Protons repel electrostatically; all nucleons attract by the strong force; neutrons add attraction without extra repulsion; where attraction outweighs repulsion the nucleus is stable, and energy equal to the binding energy is needed to separate it.

To calculate binding energy

• add the masses of the separate protons (Z × 1.007276 u) and neutrons (N × 1.008665 u);
  • subtract the measured nuclear mass to get the mass defect Δm (u);
  • binding energy (MeV) = Δm × 931.5 MeV/u, or convert Δm to kg (× 1.66 × 10⁻²⁷) and use E = Δmc²;
  • divide by A for binding energy per nucleon.
• The higher the binding energy per nucleon, the more stable the nucleus.

Decay equations balance A and Z: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He. Activity falls by half every half-life. A typical fission is ¹₀n + ²³⁵₉₂U → ¹⁴¹₅₆Ba + ^{92₃₆Kr + 3 1₀n + energy (about 200 MeV). Fusion is light nuclei combining and releasing energy.}

Ionising radiation has enough energy to remove electrons from atoms, making ions. It damages or kills cells and can cause mutations and cancer. Alpha is most dangerous inside the body; beta and gamma are most dangerous outside it. Precautions: keep sources in lead-lined boxes, handle with tongs, keep distance, minimise exposure time and wear dosimeter badges.

The Standard Model describes all known matter as made from 6 quarks and 6 leptons, with forces carried by exchange particles (bosons), plus the Higgs boson. An elementary particle is not made of smaller particles.

Quark charges

• up, charm, top +2/3 e;
  • down, strange, bottom −1/3 e.