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Modern Ch.10-11: Nuclear physics Physical Systems, 6.March.2003 EJZ

Modern Ch.10-11: Nuclear physics Physical Systems, 6.March.2003 EJZ. Structure and properties of the nucleus Binding energies and nuclear forces Radioactivity Detectors Fission Fusion other applications of nuclear physics Figures from Giancoli and Kaufmann+Freedman.

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Modern Ch.10-11: Nuclear physics Physical Systems, 6.March.2003 EJZ

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  1. Modern Ch.10-11: Nuclear physicsPhysical Systems, 6.March.2003 EJZ • Structure and properties of the nucleus • Binding energies and nuclear forces • Radioactivity • Detectors • Fission • Fusion • other applications of nuclear physics • Figures from Giancoli and Kaufmann+Freedman

  2. Structure and properties of the nucleus • Compare ATOMS (r~1Å ) vs NUCLEI (r~10-4Å) • Coulomb force ~ 1/a Strong force~1DE ~ ev: visible – X DE ~ Mev: gamma rays • Magic numbers: nucleons fill in shells related to spins • Liquid-drop model: • Binding energy ~ +c1A – c2A2/3 – c3 Z2 /A1/3 –c4 (N-Z)2/A • ~ # of bonds – surface area – coulomb repulsion – proton excess • (fusion) (fission)

  3. Binding energy per nucleon

  4. Binding energies and nuclear forces • Binding energy = mass defect = (Minitial – Mfinal) c2 = Dmc2 =Q • Strong force: Yukawa potential • High-Z nuclei need extra n, relax by fission • Low-Z nuclei need fewer n, relax by fusion

  5. Radioactivity • Alpha decay (MeV): nucleus emit 4He2 nucleus, transmutes to Z-2. • Ex: smoke detector: 241Am95 ionizes N2, O2; smoke blocks current. • Beta decay: (MeV): neutrons become protons: n  p + e- + neutrino. • Ex: carbon dating: 14C6 14N7 + e- + neutrino (Prob.11.6 p.410) • Gamma decay: Excited nucleus relaxes by emitting photon

  6. Detectors Geiger counter Scintillation detector more sensitive Semiconductor detectors can measure particle energy.

  7. Fission n+235U  fission  chain reaction, if critical mass

  8. Fusion Compress protons together, opposing Coulomb repulsion with gravity (Sun) or magnetic pressure (tokamak)

  9. Fusion vs Fission Experimental stage Reactors operational Hydrogen  Helium Uranium  Z~ 30-60 3H Radioactive 90Sr38 n  irradiated metal waste toxic sludge and gas 12 years Half life 30 years beta (Q =15 MeV) Decay beta (Q = 86 MeV) Water (biological t1/2 < 3 days) Bio calcium-like  bones Fault  loss of plasma Fault  meltdown

  10. other applications of nuclear physics • Medical treatment: radiation therapy • Medical diagnoses • X-rays: harder tissue (tumors, bones) in 2D • tomography: slices  3D image • CAT scans: computerized axial tomography • PET scans: Positron emission tomography: detect beta-decay from injected or inhaled 11C6, 13N7, 15O8, 18F9 • MRI (NMR): detect H spin-flip in water-rich tissue

  11. X rays and CAT scans

  12. PET scans detect beta-decay from injected or inhaled 11C6, 13N7, 15O8, 18F9

  13. NMR = Nuclear Magnetic Resonance • H makes spin-flip transitions • in RF field. • Frequency of transitions = DE/h • depends on external B

  14. MRI = Magnetic Resonance Imaging • Impose B0 which varies in space, • constant in time. • Measure absorption of B1 • which varies in time (RF)

  15. Modern physics summary • Structure and properties of the nucleus • Binding energies and nuclear forces • Radioactivity • Nuclear reactions • Fission • Fusion • other applications of nuclear physics

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