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Fission

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Fission

Neutrons are like neutral protons.

Mass is 1% larger

No coulomb force

Neutron lifetime is long, but eventually decays to a proton.

l= 624 s

Neutrons can interact directly with nuclei.

Strong nuclear force

Neutron capture reactions usually involve gamma ray emission.

Neutrons in nuclear reactions have distinct ranges of energy.

Slow or thermal neutrons with energies under 1 eV

Fast neutrons with energies from 100 keV to 10 MeV

Relativistic neutrons with energies over 1 GeV

Useful Fact

What is the kinetic energy of a thermal neutron?

It must be about kT.

At 20 °C, kT = 1/40 eV

Better is (3/2)kT

3 degrees of freedom

K = 0.038 eV

Heavy nuclei have higher rest masses than two lighter nuclei.

Binding energy

U-235: -7.2 MeV/nucleon

Daughters: -8.5 MeV/nucleon

The binding energy difference is released when heavy nuclei split.

Requires quantum tunneling

Ejects excess neutrons

Fission is made more likely after neutron capture.

100Mo

132Sn

235U

The energy released energy from uranium fission can heat water.

The heated water can drive a steam turbine and electric generator.

This is the basis for nuclear power plants.

World Nuclear Association

Nuclear fission of 235U produces multiple neutrons per reaction.

Neutron energy is important to reaction.

235U uses thermal neutrons

238U absorbs fast neutrons

Neutrons sustain the reaction.

Moderators slow down fast neutrons

Absorbers capture neutrons

Typical fission:

Releases 208 MeV

Fission is exothermic.

Excess energy available in reaction

Uranium fission produces neutrons.

Needed for chain reaction

An uncontrolled chain reaction creates a nuclear explosion.

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Hiroshima 1945