Fission Neutrons Neutrons are like neutral protons Mass

Fission

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

Neutron Energies ] Neutrons in nuclear reactions have distinct ranges of energy. • Slow or thermal neutrons with energies under 1 e. V • Fast neutrons with energies from 100 ke. V to 10 Me. V • Relativistic neutrons with energies over 1 Ge. V Useful Fact ] What is the kinetic energy of a thermal neutron? ] It must be about k. T. • At 20 °C, k. T = 1/40 e. V ] Better is (3/2)k. T • 3 degrees of freedom • K = 0. 038 e. V

Mass Defect ] Heavy nuclei have higher rest masses than two lighter nuclei. 100 Mo • • • 132 Sn 235 U Binding energy U-235: -7. 2 Me. V/nucleon Daughters: -8. 5 Me. V/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.

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 238 U absorbs fast neutrons ] Neutrons sustain the reaction. • Moderators slow down fast neutrons • Absorbers capture neutrons ] Typical fission: • Releases 208 Me. V

Nuclear Bomb ] 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 next