1. 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

2. Structure and properties of the nucleus Compare ATOMS (r~1Å )vsNUCLEI (r~10 -4 Å) Coulomb force ~ 1/  Strong force~1  E ~ ev: visible – X  E ~ Mev: gamma rays Magic numbers: nucleons fill in shells related to spins Liquid-drop model: Binding energy ~ +c 1 A – c 2 A 2/3 – c 3 Z 2 /A 1/3 –c 4 (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 = (M initial – M final ) c 2 =  mc 2 =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 4 He 2 nucleus, transmutes to Z-2. Ex: smoke detector: 241 Am 95 ionizes N 2, O 2 ; smoke blocks current. Beta decay: (MeV): neutrons become protons: n  p + e - + neutrino. Ex: carbon dating: 14 C 6  14 N 7 + e - + neutrino (Prob.11.6 p.410) Gamma decay: Excited nucleus relaxes by emitting photon

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

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

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

9. Fusion vs Fission Experimental stageReactors operational Hydrogen  HeliumUranium  Z~ 30-60 3 H Radioactive 90 Sr 38 n  irradiated metal waste toxic sludge and gas 12 yearsHalf life 30 years beta (Q =15 MeV)Decaybeta (Q = 86 MeV) Water (biological  1/2 < 3 days) Bio calcium-like  bones Fault  loss of plasmaFault  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 11 C 6, 13 N 7, 15 O 8, 18 F 9 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 11 C 6, 13 N 7, 15 O 8, 18 F 9

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

14. MRI = Magnetic Resonance Imaging Impose B 0 which varies in space, constant in time. Measure absorption of B 1 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