Edexcel IAL Physics: Nuclear and Particle Physics — Revision Notes

Condensed for the final weeks. For the full explanation, use the Nuclear and Particle Physics study guide.

Scope note: this page is filed under Unit 4 sub-topic 4.5. The “Atomic structure and accelerators”, “The standard model”, “Fundamental forces” and “Particle accelerators” sections below are genuine Unit 4.5 material. The “Mass–energy equivalence” section’s binding-energy/fission/fusion content sits substantially in Unit 5 sub-topic 5.4, and is included here as useful overlap/context.

Atomic structure and accelerators

Nucleon number (mass number, A) is the total protons + neutrons; proton number (atomic number, Z) is protons only. Thermionic emission releases electrons from a heated material, which are then accelerated by electric and magnetic fields to feed accelerators like the linac and cyclotron.

Why high energies are needed: resolving very small distances inside a nucleon requires very short associated wavelengths (via de Broglie), which correspond to high momentum and hence high particle energy.

Relativistic time dilation can significantly increase a short-lived particle’s observed lifetime as its speed approaches c — this is why some particles that should decay almost instantly are observed travelling measurable distances in accelerator experiments.

The standard model

Quarks (up, down, strange, charm, top, bottom) and leptons (electron, muon, tau and their neutrinos) are fundamental. Protons and neutrons are not.

proton  = uud      charge  2(+2/3) + (-1/3) = +1
neutron = udd      charge  (+2/3) + 2(-1/3) = 0

Hadrons feel the strong force: baryons (three quarks) and mesons (quark + antiquark). Leptons do not feel the strong force.

Fundamental forces

Force	Exchange particle	Acts on
Strong	Gluon	Quarks and hadrons
Electromagnetic	Photon	Charged particles
Weak	W⁺, W⁻, Z⁰	All quarks and leptons (i.e. all fermions)
Gravitational	Graviton (hypothetical)	All mass

Only the weak force changes quark flavour, which is why it governs beta decay.

Antiparticles. Every particle has a corresponding antiparticle whose properties — such as charge — are the exact opposite, deducible from (or used to deduce) the particle’s own properties. The symmetry of the standard model predicted the top quark’s existence before it was experimentally observed, a strong piece of evidence for the model itself.

Conservation rules

Charge, baryon number, lepton number (per family), energy and momentum must all balance. Strangeness is conserved in strong and electromagnetic interactions but may change by ±1 in weak ones.

When rejecting a proposed interaction, you must name which rule is violated — saying “it’s impossible” without the reason scores nothing.

Mass–energy equivalence

E = mc^2         1 u = 931.5 MeV/c^2
binding energy = mass defect x c^2

Mass defect — the mass of a nucleus is less than the sum of its separate nucleons, because energy was released when they bound together.

Binding energy per nucleon peaks around iron-56. This single graph explains both processes:

• Fusion of light nuclei moves towards the peak → energy released.
• Fission of heavy nuclei moves towards the peak → energy released.

Both release energy because both increase binding energy per nucleon. Being able to say why from the graph is the high-value answer.

Particle accelerators

Linear accelerator — alternating p.d. across successive tubes, timed so the particle is always accelerated as it crosses each gap. Tubes get longer because the particle moves faster.

Cyclotron — two dees in a magnetic field, with an alternating p.d. across the gap. The magnetic field keeps the particle circling; radius increases as it speeds up, since r = mv/BQ.

The cyclotron’s limitation is relativistic: as the speed approaches c, the particle’s mass increases, so the time per semicircle is no longer constant and it falls out of step with the alternating field. That is why synchrotrons vary the field or frequency.

Detection: in a magnetic field, the radius of curvature gives momentum (r = p/BQ) and the direction of curvature gives the sign of the charge. A particle and its antiparticle curve in opposite directions.

Exam traps

• Calling protons or neutrons fundamental.
• Saying leptons feel the strong force.
• Not naming the violated conservation rule.
• Saying the nucleus is heavier than its constituent nucleons.
• Explaining fission and fusion without reference to binding energy per nucleon.
• Forgetting the relativistic limit of the cyclotron.

Self-test

1. Which particles are fundamental?
2. Which force changes quark flavour, and what does that explain?
3. What is the mass defect, and why does it exist?
4. Why do both fission and fusion release energy?
5. What limits a cyclotron at high speeds?
6. Distinguish nucleon number from proton number.
7. Why does relativistic time dilation matter for detecting short-lived particles?
8. What did the symmetry of the standard model successfully predict, before it was confirmed experimentally?

Answers: 1. Quarks and leptons; protons and neutrons are composite. 2. The weak force; it explains beta decay, where a down quark becomes an up quark or vice versa. 3. The difference between the mass of a nucleus and the total mass of its separate nucleons; it exists because energy was released when the nucleons bound together, and that energy came from mass. 4. Both move nuclei towards the peak of the binding energy per nucleon curve at around iron-56, increasing binding energy per nucleon and releasing the difference. 5. As the particle approaches the speed of light its mass increases, so the time to complete each semicircle is no longer constant and it loses synchronisation with the alternating supply. 6. Nucleon number (mass number) is the total number of protons and neutrons in a nucleus; proton number (atomic number) is the number of protons only. 7. It increases a fast-moving particle’s observed lifetime, so particles that would otherwise decay almost instantly can travel measurable distances and be detected in accelerator experiments. 8. The existence of the top quark, before it was experimentally confirmed.