1. Fission

2. Neutrons  Neutrons are like neutral protons. Mass is 1% largerMass is 1% larger No coulomb forceNo coulomb force  Neutron lifetime is long, but eventually decays to a proton. = 624 s = 624 s  Neutrons can interact directly with nuclei. Strong nuclear force  Neutron capture reactions usually involve gamma ray emission.

3. Neutron Energies  Neutrons in nuclear reactions have distinct ranges of energy. Slow or thermal neutrons with energies under 1 eVSlow or thermal neutrons with energies under 1 eV Fast neutrons with energies from 100 keV to 10 MeVFast neutrons with energies from 100 keV to 10 MeV Relativistic neutrons with energies over 1 GeVRelativistic 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

4. Mass Defect  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. 132 Sn 100 Mo 235 U

5. Nuclear Reactor  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

6. Chain Reaction  Nuclear fission of 235 U produces multiple neutrons per reaction.  Neutron energy is important to reaction. 235 U uses thermal neutrons 235 U uses thermal neutrons 238 U absorbs fast neutrons 238 U absorbs fast neutrons  Neutrons sustain the reaction. Moderators slow down fast neutronsModerators slow down fast neutrons Absorbers capture neutronsAbsorbers capture neutrons  Typical fission: Releases 208 MeV

7. Nuclear Bomb next  Fission is exothermic. Excess energy available in reaction  Uranium fission produces neutrons. Needed for chain reaction  An uncontrolled chain reaction creates a nuclear explosion. Hiroshima 1945