LucidSTEM
A-LEVEL 9702 · AS · TOPIC 11
Particle physics
The build-chain of the topic: alpha-scattering reveals a tiny dense nucleus, nucleon notation labels that nucleus, radioactive decay uncovers antiparticles and neutrinos, and beneath it all sit quarks that compose every proton and neutron. Around the hexagon are the four ideas; above is what it builds on, below is where it leads.
TOPIC 11: PARTICLE PHYSICS
CAMBRIDGE A-LEVEL PHYSICS 9702 · PATHWAYS
TheLucidSTEM · thelucidstem.com
BUILDS ON
T4 Forces: Coulomb deflection T5 Energy: conservation T9 Electricity: charge, the eV 11.1
11.1
11.1
11.2
TOPIC 11
PARTICLE
PHYSICS
1 · THE NUCLEAR ATOM
Most of the atom is empty space.
Alpha-scattering: most α pass straight through;
a few deflect, very few bounce back.
So mass and positive charge sit in a tiny nucleus.
Backscatter needs a near head-on Coulomb repulsion.
nucleus ~ 10−15 m, atom ~ 10−10 m
N+
α beam
a tiny dense nucleus turns the few back
2 · NUCLEONS, ISOTOPES & DECAY
Label the nucleus, then balance the books.
Notation: A = nucleon number, Z = proton number.
Isotopes: same Z, different number of neutrons.
Every decay conserves nucleon number and charge.
Mass is measured in the unified mass unit u.
A = Z + N 1 u = 1.66 × 10−27 kg
X
A
Z
nucleon no.
proton no.
element
A and Z balance on both sides of a decay
3 · ANTIPARTICLES & NEUTRINOS
Beta decay needs a hidden partner.
Every particle has an antiparticle: same mass,
opposite charge. The positron is the anti-electron.
α energies are discrete; β energies are continuous.
An (anti)neutrino carries off the missing energy.
n → p + β− + ν̄ p → n + β+ + ν
E max
N
energy of β particle
continuous β spectrum
4 · QUARKS, HADRONS & LEPTONS
Beneath the nucleons lie quarks.
Six flavours: up has +⅔e charge, down has −⅓e.
Hadrons: baryons (qqq) and mesons (q q̄).
In β− decay a down quark turns into an up quark.
Leptons (electron, neutrino) are fundamental.
proton = uud neutron = udd
u
u
d
+⅔, +⅔
−⅓
proton (uud): total charge +1e
LEADS TO
T22 Quantum physics: photons, the eV, line spectra T23 Nuclear physics: mass defect, E = mc², decay T24 Medical physics: radiation in imaging Each carries the idea forward: the same nucleus and its decays power binding energy, line spectra and clinical imaging.
← Builds on IGCSE: The nuclear atom