PARTICLE PHYSICS

Narrow beam of alpha particles, all of the same E_k, from a radioactive source

Targeted at a piece of gold foil only a few atomic layers thick

Particles scattered by the foil

Detected on a flurescent screen made of zinc sulfide mounted in front of a microscope

Produces tiny specks of light

Microscope moved around

Counts the number of particles scattered through different values of the angle per minute

Most of the particles passed straight through the foil with little scattering

1/2000 scattered

Most of the atom is empty space

Most of the mass of the atom is concentrated in the nucleus
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Few of the particles were deflected through angles > 90^o

1/10000 deflected

The nucleus repelled the few particles that came near it

The nucleus has a positive charge

The charge is quantised and given by +Ze

Head-on collision

• Rebounds back
• Scattering angle = 180^o
• Minimum distance between the particle and nucleus = d
• Infrequent because of the tiny diameter of the nucleus

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Oblique collision

• Scattering angle = Θ

At d, the alpha particle momentarily stops

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Initial E_k of the alpha particle = electric potential energy at d

1.2 x 10^-12 = Qq / 4πε_od

1.2 x 10^-12 = (79 x 2 x (1.6 x 10^-19)²) / (4 x π x (8.85 x 10^-12) x d)

d = 3.0 x 10^-14 m

≈ 10^-14 m

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↑ E_k = ↓ d

• Nuclei of the same element

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• Same atomic number (number of protons)

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• Different nucleon numbers (number of protons and neutrons)

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• Undergo the same chemical reactions

One atomic mass unit (1 u) = 1/12 mass of a neutral carbon-12 atom

= 1.661 x 10^-27 kg

R = r_oA^1/3

• r_o = 1.2 fm = radius of a proton
• A = nucleon number

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↑ mass and ↓ volume

∴ ↑ density of 10¹⁷ kgm^-3

• Acts between all nucleons

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• Short range, effective over just a few femtometres

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Between two nucleons

• Attractive to a separation of about 3 fm

• Repulsive below a separation of about 0.5 fm

• Antimatter counterpart of a particle

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• Opposite charge to the particle

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• Same rest mass as the particle

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Electron - positron
Proton - antiproton
Neutron - antineutron
Neutrino - antineutrio

• Strong nuclear
  • Experienced by nucleons
  • Relative strength = 1
  • Range = 10^-15 m
• Weak nuclear
  • Responsible for beta-decay
  • Relative strength = 10^-3
  • Range = infinite
• Electromagnetic
  • Experienced by static and moving charged particles
  • Relative strength = 10^-6
  • Range = 10^-18 m
• Gravitational
  • Experienced by all particles with mass
  • Relative strength = 10^-40
  • Range = infinite

• Has no internal structure

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• Cannot be split into smaller particles

• Particles/ antiparticles affected by the strong nuclear force

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• Protons/ neutrons/ mesons

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• Contain quarks

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• If charged, experience the electromagnetic force

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• Decay by the weak nuclear force

• Particles/ antiparticles not affected by the strong nuclear force

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• Electrons/ neutrinos/ muons

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• If charged, experience the electromagnetic force

• Up
  • u
  • + 2/3 e

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• Down
  • d
  • - 1/3 e

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• Strange
  • s
  • - 1/3 e

• Anti-up
  • u
  • - 2/3 e

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• Anti-down
  • d
  • + 1/3 e

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• Anti-strange
  • s
  • + 1/3 e

• uud

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• + 1 e

• udd

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• 0 e

Any hadron made with a combination of three quarks

Any hadron made with a combination of a quark and an anti-quark

A neutron in an unstable nucleus decays into a proton, electron and electron antineutrino

¹_on → ¹₁p + ⁰_-1e + v_e

d → u + ⁰_-1e + v_e

A proton decays into a neutron, positron and electron neutrino

¹₁p → ¹_on + ⁰₁e + v_e

u → d + ⁰₁e + v_e