Particle Physics
11. Particle Physics
5 min read 10 parts Lesson 1 of 1
Lesson summary
11. Particle Physics — Cambridge AS & A Level Physics 9702 — CAMBRIDGE AS & A LEVEL PHYSICS 9702 — COMPLETE NOTES notes: Describe the structure of the atom and nucleus, write balanced decay equations, and classify fundamental particles using the quark and lepton models.Set-up — a…
- Neutral atom
- Unified atomic mass unit (u)
- Antimatter
- Nucleon and nuclide
- Instability
- Nuclear equation
Part 1 of 10Alpha-particle scattering experiment
- Set-up — alpha particles (helium nuclei, positively charged) are fired at a thin gold foil and a detector counts how many are deflected at each angle.
- Evidence — each observation gives a conclusion about the atom.
| Observation | Conclusion |
|---|---|
| Most alpha particles pass straight through | The atom is mostly empty space |
| A few are deflected through small angles (below 10°) | There is a positively charged nucleus, since like charges repel |
| A very small number rebound at angles over 90° | The nucleus is tiny and holds the mass and positive charge |
- Model — an atom has a small, dense, positive nucleus surrounded by negative electrons.
- The atom is roughly 100 000 times wider than its nucleus.
- Graph — the count rate is greatest at small angles and falls rapidly as the deflection angle increases.
Part 2 of 10Atomic structure
| Particle | Relative charge | Relative mass |
|---|---|---|
| Proton | +1 | 1 |
| Neutron | 0 | 1 |
| Electron | −1 | 1/2000 (negligible) |
- Locationprotons and neutrons are in the nucleus and electrons are outside it.
- Neutral atomit has equal numbers of protons and electrons.
- Atom and nucleusthe atom includes the electrons, but the nucleus has only protons and neutrons.
Part 3 of 10Atomic mass unit and antimatter
- Unified atomic mass unit (u) = 1/12 of the mass of a carbon-12 atom, where 1 u = 1.66 × 10−27 kg (given in the data booklet).
- A proton and a neutron are each about 1 u, an electron about 0.0005 u and an alpha particle about 4 u.
- The mass of an atom in u is roughly its nucleon number, so uranium-235 is about 235 u.
- Antimatter — every particle has an antiparticle with the same mass but opposite charge.
| Particle | Charge | Antiparticle | Charge |
|---|---|---|---|
| Electron e− | −1 | Positron e+ | +1 |
| Proton p | +1 | Antiproton | −1 |
| Neutron n | 0 | Antineutron | 0 |
- Naming — apart from the electron, the antiparticle has the prefix "anti-" and a line above the matter particle symbol.
Part 4 of 10Nuclide notation and isotopes
1Notation2 points
— AZX, where X is the chemical symbol, A the nucleon (mass) number and Z the proton (atomic) number.
- Protons = Z, electrons in a neutral atom = Z and neutrons = A − Z.
- 73Li has 3 protons, 3 electrons and 4 neutrons.
2Nucleon and nuclide
— a nucleon is a proton or neutron, and a nuclide is a nucleus with a specific number of protons and neutrons.
3Isotopes1 point
= atoms of the same element with the same proton number but different neutron numbers, and so different nucleon numbers.
- Hydrogen has the isotopes deuterium and tritium.
4Instability
— many isotopes have an imbalance of neutrons and protons and decay to a more stable form, over times from nanoseconds to thousands of years.
Part 5 of 10Conserving nucleon number and charge
- Nuclear equation — in every nuclear reaction the total nucleon number and the total charge (proton number) are the same on both sides.
- Fission example — 23592U + 10n → 9038Sr + 14454Xe + 210n.
- Nucleons: 235 + 1236 and 90 + 144 + 2 = 236.
- Charge: 92 + 092 and 38 + 54 + 0 = 92.
- Finding an unknown — add the top numbers on the side where every value is known, equate the other side including the unknown, then repeat for the bottom numbers.
Part 6 of 10Alpha, beta and gamma radiation
1Radioactive nuclei
— they are unstable, often because the nucleon numbers are out of balance, and emit particles or radiation to become more stable.
2Alpha (α)1 point
— 2 protons and 2 neutrons, emitted by nuclei that are too large.
- It is highly ionising and weakly penetrating, with a range of a few cm in air, stopped by paper or skin.
3Beta-minus (β−)1 point
— a high-energy electron from nuclei with too many neutrons.
- It is moderately ionising and penetrating, with a range of about 20 cm to 3 m in air, stopped by a few mm of aluminium.
4Beta-plus (β+)
— a positron from nuclei with too many protons.
5Gamma (γ)1 point
— a high-energy electromagnetic wave from nuclei that need to lose energy.
- It is weakly ionising and very penetrating.
| Radiation | Composition | Mass / u | Charge / e | Speed / c |
|---|---|---|---|---|
| Alpha | 2 protons + 2 neutrons | 4 | +2 | 0.05 |
| Beta-minus | Electron | 0.0005 | −1 | above 0.99 |
| Beta-plus | Positron | 0.0005 | +1 | above 0.99 |
| Gamma | Electromagnetic wave | 0 | 0 | 1 |
- Ionisation — radiation knocks electrons from atoms, which changes materials chemically and can damage or kill living cells.
Part 7 of 10Decay equations
1Alpha decay2 points
— AZX → A−4Z−2Y + 42α.
- The nucleon number falls by 4 and the proton number by 2, so the neutron number falls by 2.
- 21284Po → 20882Pb + 42α.
2β− decay2 points
— a neutron changes into a proton, an electron and an electron antineutrino: AZX → AZ+1Y + 0−1e + ν̄.
- The proton number rises by 1 and the nucleon number is unchanged.
- 146C → 147N + e− + ν̄.
3β+ decay1 point
— a proton changes into a neutron, a positron and an electron neutrino: AZX → AZ−1Y + 0+1e + ν.
- The proton number falls by 1 and the nucleon number is unchanged.
4Neutrino
— a particle with no charge and almost no mass, with antineutrinos coming with β− and neutrinos with β+.
Part 8 of 10Energy of alpha and beta particles
| Radiation | Energy spectrum | Reason |
|---|---|---|
| Alpha | Discrete energies (sharp spikes on a number against kinetic energy graph) | The energy released is fixed by the two-body decay |
| Beta | Continuous range of energies | The energy is shared between the beta particle and a neutrino or antineutrino |
- Neutrino clue — the continuous beta spectrum was an early clue that neutrinos exist.
- Conservation — energy and momentum are conserved in both alpha and beta emission.
Part 9 of 10Quarks, hadrons and leptons
- Fundamental particle = a particle that is not made of anything smaller.
- Quarks — fundamental particles that make up hadrons and are never seen alone.
- The six flavours are up, down, charm, strange, top and bottom.
- Up, charm and top have charge +2/3 e, and down, strange and bottom have −1/3 e.
- An antiquark has the same mass but opposite charge.
- Hadron = any particle made of quarks, which feels the strong force.
- Its charge is the sum of the charges of its quarks.
- Baryons — three quarks (or three antiquarks for an antibaryon), such as proton uud (+1) and neutron udd (0).
- Mesons — a quark and an antiquark, such as the pion; the antiparticle of a meson is again a quark–antiquark pair.
- Whole-number charge — hadrons have integer charges, so a baryon is never a mixture of quarks and antiquarks.
- Leptons — six fundamental particles with no quark structure: electron, muon and tau, each with its own neutrino.
- Electron, muon and tau carry −1e, and the muon and tau are heavier than the electron.
- Neutrinos have no charge and almost no mass, and are the most abundant leptons in the universe.
- Leptons feel the weak, electromagnetic and gravitational forces, but not the strong force.
- A pion is a meson, so it is not a lepton, and the antimuon is an antilepton.
Part 10 of 10Quark model of beta decay
- Weak interactionone of the four fundamental forces, responsible for beta decay.
- β− decaya down quark changes into an up quark, so a neutron (udd) becomes a proton (uud) and an electron and antineutrino are emitted.
- β+ decayan up quark changes into a down quark, so a proton becomes a neutron and a positron and neutrino are emitted.
- Charge checkfor n → p + e− + ν̄ the left side is 2/3 − 1/3 − 1/3 = 0 and the right side is +1 − 1 + 0 = 0.
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