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
Section 1 of 10Alpha-particle scattering experiment
Set-up
alpha particles are fired at a thin gold foil and a detector counts how many are deflected at each angle.
(helium nuclei, positively charged)
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.
Section 2 of 10Atomic structure
Particle
Relative charge
Relative mass
Proton
+1
1
Neutron
0
1
Electron
−1
1/2000 (negligible)
Location
protons and neutrons are in the nucleus and electrons are outside it.
Neutral atom
it has equal numbers of protons and electrons.
Atom and nucleus
the atom includes the electrons, but the nucleus has only protons and neutrons.
Section 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.
Section 4 of 10Nuclide notation and isotopes
Notation
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.
Nucleon and nuclide
a nucleon is a proton or neutron, and a nuclide is a nucleus with a specific number of protons and neutrons.
Isotopes = 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.
Instability
many isotopes have an imbalance of neutrons and protons and decay to a more stable form, over times from nanoseconds to thousands of years.
Section 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 + 1 = 236 and 90 + 144 + 2 = 236.
Charge: 92 + 0 = 92 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.
Section 6 of 10Alpha, beta and gamma radiation
Radioactive nuclei
they are unstable, often because the nucleon numbers are out of balance, and emit particles or radiation to become more stable.
Alpha (α)
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.
Beta-minus (β−)
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.
Beta-plus (β+)
a positron from nuclei with too many protons.
Gamma (γ)
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.
Section 7 of 10Decay equations
Alpha decay
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.
β− decay
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.
β+ decay
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.
Neutrino
a particle with no charge and almost no mass, with antineutrinos coming with β− and neutrinos with β+.
Section 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.
Section 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, such as proton uud (+1) and neutron udd (0).
(or three antiquarks for an antibaryon)
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.
Section 10 of 10Quark model of beta decay
Weak interaction
one of the four fundamental forces, responsible for beta decay.
β− decay
a down quark changes into an up quark, so a neutron (udd) becomes a proton (uud) and an electron and antineutrino are emitted.
β+ decay
an up quark changes into a down quark, so a proton becomes a neutron and a positron and neutrino are emitted.
Charge check
for n → p + e− + ν̄ the left side is 2/3 − 1/3 − 1/3 = 0 and the right side is +1 − 1 + 0 = 0.