modern ch.10-11: nuclear physics physical systems, 6.march.2003 ejz
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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
Structure and properties of the nucleus
• Compare ATOMS (r~1Å ) vs NUCLEI (r~10-4 Å)
Coulomb force ~ 1/ Strong force~1E ~ ev: visible – X E ~ 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)
Binding energy per nucleon
Binding energies and nuclear forces
• Binding energy = mass defect = (Minitial – Mfinal) c2 = mc2 =Q
• Strong force: Yukawa potential
• High-Z nuclei need extra n, relax by fission
• Low-Z nuclei need fewer n, relax by fusion
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
Detectors
Geiger counter Scintillation detector more sensitive
Semiconductor detectors can measure particle energy.
Fission
n+235U fission
chain reaction, if critical mass
FusionCompress protons together, opposing Coulomb repulsion with
gravity (Sun) or magnetic pressure (tokamak)
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 1/2 < 3 days) Bio calcium-like bones
Fault loss of plasma Fault meltdown
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
X rays and CAT scans
PET scans detect beta-decay from injected or inhaled 11C6, 13N7, 15O8, 18F9
NMR = Nuclear Magnetic Resonance
• H makes spin-flip transitions
in RF field.
• Frequency of transitions = E/h
depends on external B
MRI = Magnetic Resonance Imaging
• Impose B0 which varies in space,
constant in time.
• Measure absorption of B1
which varies in time (RF)
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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