Particle physics

6. Particle physics

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6. Particle physics — AS PHYSICS 9702 notes: Describe the structure of the atom, nuclear decay, hadrons, leptons and quarks and apply conservation laws to particle reactions.Structure — an atom has a tiny, dense, positively charged nucleus of protons and neutrons (nucleons) surro…

Rutherford experiment
Nuclide notation
Attractive
Unpredictable
Unaffected
γ emission

Section 1 of 12The nuclear atom

The nuclear atom

Structure

Structure an atom has a tiny, dense, positively charged nucleus of protons and neutrons (nucleons) surrounded by electrons.

Size

Size the nuclear diameter is about 10⁻¹⁵ m, compared with about 10⁻¹⁰ m for the atom.

Rutherford experiment

Rutherford experiment most α-particles pass through undeviated, so the atom is mostly empty space.
A small number are deflected through large angles.
A very few, about 1 in 8000, are scattered back through more than 90°, showing a small, dense, positively charged nucleus holding most of the mass.

Section 2 of 12Nuclides and isotopes

Nuclide notation

Nuclide notation written as ^A_ZX, where Z is the proton number, A is the nucleon number and the neutron number is N = A − Z.

Isotopes

Isotopes nuclei with the same Z but different N, with the same chemical properties but different masses.

Charge

Charge the proton has +e (+1.60 × 10⁻¹⁹ C), the neutron 0 and the electron −e.

Mass

Mass proton and neutron are each about 1.67 × 10⁻²⁷ kg, with 1 u = 1.66 × 10⁻²⁷ kg, and the electron is about 9.11 × 10⁻³¹ kg.

Section 3 of 12Strong nuclear force

Role

Role it acts between nucleons and binds them against the electrostatic repulsion between protons.

Attractive

Attractive at separations up to about 3 fm.

Repulsive

Repulsive below about 0.5 fm, which prevents collapse.

Range

Range negligible beyond several fm.

Section 4 of 12Radioactive decay

Nature

Nature unstable nuclei emit radiation randomly and spontaneously.

Unpredictable

Unpredictable the decay of a given nucleus cannot be predicted.

Unaffected

Unaffected it is not affected by temperature or pressure.

Radiation	Nature	Charge	Penetration	Ionising power
Alpha (α)	Helium nucleus	+2e	Stopped by paper or a few cm of air	Strong
Beta-minus (β−)	Fast electron	−e	Stopped by a few mm of aluminium	Medium
Gamma (γ)	Electromagnetic photon	0	Reduced by thick lead or concrete	Weak

Section 5 of 12Decay equations

α decay

α decay the nucleus loses 2 protons and 2 neutrons, A decreases by 4 and Z by 2.

β− decay

β− decay a neutron changes to a proton, with emission of an electron and an antineutrino; Z rises by 1.

β+ decay

β+ decay a proton changes to a neutron, with emission of a positron and a neutrino; Z falls by 1.

γ emission

γ emission it follows decay and leaves the proton and neutron numbers unchanged.

Neutrino

Neutrino it was proposed because β particles have a continuous range of energies and the neutrino carries away the remainder.

Conserved

Conserved charge, nucleon number and mass–energy are conserved in every decay.

Section 6 of 12Fundamental particles

Fundamental particles

Classification

Classification particles divide into hadrons and leptons.

Hadrons

Hadrons they feel the strong force and are made of quarks.
Baryons, such as the proton and neutron, contain three quarks.
Mesons, such as the pion, contain a quark and an antiquark.

Leptons

Leptons electron, muon, tau and their neutrinos are fundamental and do not feel the strong force.

Antiparticles

Antiparticles every particle has an antiparticle with the same mass but opposite charge, for example the positron.

Section 7 of 12Quark charges

Quark	Charge	Baryon number
up (u)	+2/3 e	+1/3
down (d)	−1/3 e	+1/3
strange (s)	−1/3 e	+1/3

Proton

Proton uud, with charge 2/3 + 2/3 − 1/3 = +1.

Neutron

Neutron udd, with charge 2/3 − 1/3 − 1/3 = 0.

Antiquarks

Antiquarks they have the opposite charge and baryon number.

Confinement

Confinement quarks are never observed alone.

Section 8 of 12Fundamental interactions

Strong force

Strong force between hadrons, mediated by gluons.

Electromagnetic force

Electromagnetic force between charges, mediated by photons.

Weak force

Weak force responsible for β decay.

Gravity

Gravity acts between masses.

β− in quark terms

β− in quark terms d → u + e⁻ + antineutrino.

β+ in quark terms

β+ in quark terms u → d + e⁺ + neutrino.

Section 9 of 12Conservation laws in reactions

Conserved quantities

Conserved quantities charge, baryon number, lepton number and mass–energy, including momentum.

Strangeness

Strangeness it may change in weak interactions but not in strong interactions.

Testing a reaction

Testing a reaction add each quantity on both sides.

Pair production

Pair production a high-energy photon can produce a particle and antiparticle if its energy is at least 2mc².

Annihilation

Annihilation a particle–antiparticle pair converts into photons.

Energy units

1 eV = 1.60 × 10⁻¹⁹ J, and the energy equivalent of an electron’s mass is 0.511 MeV.

Section 10 of 12More from the specification

Section 11 of 12Key terms

Term	Meaning
Nucleon	A proton or a neutron.
Isotope	Nuclei with the same proton number but different neutron numbers.
Hadron	Particle made of quarks that experiences the strong force.
Baryon	Hadron made of three quarks.
Meson	Hadron made of a quark and an antiquark.
Lepton	Fundamental particle that does not feel the strong force.
Quark	Fundamental building block of hadrons.
Weak interaction	Force responsible for β decay.

Section 12 of 12Exam technique

Saying the nucleus contains electrons — it contains only protons and neutrons.

Writing the wrong sign for the down quark charge — it is −1/3 e.

Forgetting the antineutrino in β− decay.

Saying γ emission changes the proton number — it changes neither A nor Z.