modern ch.10-11: nuclear physics physical systems, 6.march.2003 ejz
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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