6.4 Nuclear and Particle Physics

module 6

What did alpha scattering show?

Atom is mostly empty space.

Nucleus is small, dense, positive.

Most mass is concentrated in nucleus.

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Alpha scattering: most particles pass straight through.

Shows atom is mostly empty space.

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Alpha scattering: some particles deflect slightly.

Shows nucleus is positively charged.

Alpha particles are repelled.

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Alpha scattering: very few bounce back.

Shows nucleus is tiny and dense.

Mass and positive charge concentrated in small volume.

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Why was gold foil thin in alpha scattering?

To reduce multiple scattering.

Alpha particles should interact with one nucleus at most.

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Why was alpha scattering done in vacuum?

Air would absorb or scatter alpha particles.

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Simple nuclear model of atom.

Nucleus contains protons and neutrons.

Electrons surround nucleus.

Nucleus is much smaller than atom.

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Define proton number.

Number of protons in nucleus.

Symbol:

Z

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Define nucleon number.

Total number of protons and neutrons.

Symbol:

A

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Define isotope.

Atoms of same element with same protons but different neutrons.

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How do you find neutrons from A and Z?

neutrons = A - Z

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Relative size of atom and nucleus.

Atom radius about 10^-10 m.

Nucleus radius about 10^-15 m.

Nucleus is about 100000 times smaller in radius.

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Define strong nuclear force.

Force that holds nucleons together in nucleus.

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Strong nuclear force range.

Very short range.

Attractive up to about 3 fm.

Repulsive below about 0.5 fm.

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Why is strong nuclear force needed?

Protons repel electrically.

Strong nuclear force overcomes repulsion at nuclear distances.

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Nuclear radius equation.

R = r0 A^(1/3)

R = nuclear radius

A = nucleon number

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What does R = r0 A^(1/3) show?

Nuclear radius depends on nucleon number.

Larger A gives larger nucleus.

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How do you show nuclear density is roughly constant?

R is proportional to A^(1/3)

Volume is proportional to R^3

So volume is proportional to A

Mass is proportional to A

Density is roughly constant.

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How do you calculate nuclear density?

density = mass / volume

mass about A x nucleon mass

volume = 4/3 pi R^3

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Define hadron.

Particle made from quarks.

Hadrons experience strong nuclear force.

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Types of hadrons.

Baryons:

3 quarks

Mesons:

quark-antiquark pair

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