Particle physics

What is an isotope?

Nuclei of the same element that have the same number of protons but different numbers of neutrons

What is an atomic mass unit?

One atomic mass unit is one-twelfth the mass of a neutral carbon-12 atom

What is the symbol for the atomic mass unit?

u

What graph shows how the strong nuclear force varies with the separation of nucleons? Graph should have a scale for the separation.

1 fm = 1×10^-15 m

Explain the shape of the graph showing how the strong nuclear force varies with the separation of nucleons

• The strong force can only act over very short distances < ~3 fm. When r is 3 fm the nucleons are too far apart for the strong nuclear force to act, so the force is 0

• As the seperation r decreases, the nucleons become close enough for the gluons to carry the strong force, so the nucleons experience an attractive force, which is negative.

• The force becomes more negative (attraction becomes stronger) as the particles get closer, down to a minimum.

• As the nucleons get closer (r decreases), the strong nuclear force eventually becomes repulsive and so positive (distances < ~0.5 fm)

What is the proton number?

The number of protons in a nucleus

What is the atomic number?

The number of protons in a nucleus

What is the nucleon number?

The total number of protons and neutrons in a nucleus

What is the equation for the radius of a nucleus, and what is the meaning of each term?

R = r₀ A^1/3

• R is the radius of the nucleus

• r₀ is a constant and is roughly 1.2 fm = 1.2×10^-15 m

• A is the nucleon number

Why do large nuclei have more neutrons than protons?

• In larger nuclei, the range of the strong nuclear force for each nucleon is smaller than the size of the nucleus, as the strong force is short ranged.

• The electrostatic repulsion from all protons in the nucleus reaches all the others, as it is long ranged.

• If large nuclei had the same number of protons and neutrons, the electrostatic repulsion would add to overcome the strong nuclear attraction.

• So more neutrons than protons are need to provide the extra strong force needed for equilibrium

What particle is the force carrier for the strong force?

gluons

What is the mean density of nuclei?

10¹⁷ kg m^-3

What is the mean density of atoms?

10³ kg m^-3 - same as water

What are the principal features of the nuclear model of the atom suggested by Rutherford.

• nucleus is very small compared with atom

• nucleus contains most of mass of atom

• nucleus has positive charge

• electrons surround the nucleus

• electrons are negative and have small mass

What are the observations and conclusions of Rutherford’s alpha-scattering experiment?

• Most alpha particles went through without deflection, hence the atom is mostly empty space

• Some alpha particles were scattered through large angles, hence there must be a nucleus at the centre of the atom

• Nucleus must have positive charge to repel the alpha particles

• Nucleus must be small and dense as it most of atom is empty

What is r₀ in the equation R = r₀ A^1/3?

The radius of a proton which ≈ 1.2 fm

What is the radius of a proton?

~1.2 fm = 1.2 × 10^-15 m

Derive an equation for the density of the nucleus

radius of nucleus R = r₀ A^1/3. If we assume nucleus is a sphere, V = 4/3 π r³ = 4/3 π (r₀ A^1/3)³ = 4/3 π r₀³ A = (4 π r₀³ A) / 3

density ρ = m / v = m / [ (4 π r₀³ A) / 3 ] = 3m / (4 π r₀³ A)

Describe the nature of the strong nuclear force

• One of the four fundamental forces in nature

• Acts on hadrons and holds nuclei together

• Very short ranged - only acts at distances < ~3 fm

• Attractive at distances > ~0.5 fm and < ~3 fm

• Repulsive at distances < ~0.5 fm

What is a fundamental particle?

A particle that has no internal structure and hence cannot be split into smaller particles

What is a hadron?

A particle or antiparticle that is affected by the strong nuclear force

What are the two types of hadron?

• Baryon

• Meson

What is a baryon?

A hadron made with a combination of three quarks

What is a meson?

Any hadron comprising of a combination of a quark and an anti-quark

What is a lepton?

A fundamental particle or antiparticle that is not affected by the strong nuclear force

How do you write an electron in a decay equation?

How do write a positron in a decay equation?