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

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