Fission
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.
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 = eV
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
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
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
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