PARTICLE PHYSICS

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• Created by: CPev3
• Created on: 07-04-21 11:33

Rutherford's alpha-scattering experiment

Narrow beam of alpha particles, all of the same Ek, 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

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Experiment analysis

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 > 90o

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

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Alpha particle - gold nucleus collision

• Rebounds back
• Scattering angle = 180o
• 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
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Upper limit for the radius of a gold nucleus

At d, the alpha particle momentarily stops

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

1.2 x 10-12 = Qq / 4πεod

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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↑ Ek = ↓ d

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Isotopes

• 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
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Atomic mass units

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

= 1.661 x 10-27 kg

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R = roA1/3

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

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↑ mass and ↓ volume

∴ ↑ density of 1017 kgm-3

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Strong nuclear force

• 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
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Antiparticle

• 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

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Fundamental forces

• 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
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Fundamental particle

• Has no internal structure

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• Cannot be split into smaller particles
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• 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
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Leptons

• 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
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• Up
• u
• + 2/3 e

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• Down
• d
• - 1/3 e

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• Strange
• s
• - 1/3 e
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• 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
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• uud

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• + 1 e
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• udd

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• 0 e
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Meson

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

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Beta-minus (ß-) decay

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

1on → 11p + 0-1e + ve

d → u + 0-1e + ve

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Beta-plus (ß+) decay

A proton decays into a neutron, positron and electron neutrino

11p → 1on + 01e + ve

u → d + 01e + ve

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