§ 11.1 Atoms, nuclei and radiation
Key ideas
- Alpha scattering: most alphas pass straight through the foil, a few deflect sharply, a very few bounce back: the atom is mostly empty space around a tiny, dense, positive nucleus.
- Nuclide notation: nucleon number A above, proton number Z below; isotopes share Z but differ in A.
- α is a helium nucleus (charge +2e); β− is a fast electron; β+ is a positron; γ is a high-energy photon with no charge or mass.
- β− decay emits an antineutrino (a neutron becomes a proton); β+ decay emits a neutrino (a proton becomes a neutron).
- In every nuclear equation, nucleon number A and charge Z balance across the arrow.
- The unified atomic mass unit u is one twelfth of the mass of a carbon-12 atom.
Equations
α decay: Z−2, A−4the nucleus sheds two protons and two neutronsno unit
β−: Z+1, A same, + ν̄neutron → proton + electron + antineutrinono unit
β+: Z−1, A same, + νproton → neutron + positron + neutrinono unit
Watch out: match the neutrino to the decay: β− pairs with the antineutrino, β+ with the neutrino. Check the lepton balance the same way you check A and Z.
§ 11.2 Fundamental particles
Key ideas
- Quarks: the up quark carries +⅔e, the down quark −⅓e. A proton is uud (+e); a neutron is udd (0).
- Hadrons are built from quarks: baryons from three quarks, mesons from a quark and an antiquark.
- Leptons (electron, neutrino) are fundamental; so are quarks. Protons and neutrons are not.
- Every particle has an antiparticle of the same mass and opposite charge; the positron is the electron's.
- Beta decay at quark level: β− is d → u (plus e− and ν̄); β+ is u → d (plus e+ and ν).
Equations
u = +⅔e, d = −⅓equark charges; uud = +e, udd = 0C
Fig. 1 · Counting charge: uud gives ⅔ + ⅔ − ⅓ = +1; udd gives ⅔ − ⅓ − ⅓ = 0. Three quarks make each of them a baryon.
Fig. 2 · β− decay seen at quark level: the highlighted down quark becomes an up quark, the neutron becomes a proton, and the electron and antineutrino carry away charge and lepton number.
Watch out: protons and neutrons are not fundamental: they are baryons built from quarks. The fundamental particles on this syllabus are the quarks and the leptons (electron, neutrino).