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Nuclear Physics. Paul J. Thomas Department of Physics and Astronomy UW - Eau Claire. Early Atomic Ideas. Greek atomos = “cannot be cut”. Early atomistic theories of Democritus and Leucippus. Dalton: elements combine in constant ratios of mass. Brownian motion. The Structure of the Atom.
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Nuclear Physics Paul J. Thomas Department of Physics and Astronomy UW - Eau Claire
Early Atomic Ideas • Greek atomos = “cannot be cut”. • Early atomistic theories of Democritus and Leucippus. • Dalton: elements combine in constant ratios of mass. • Brownian motion
The Structure of the Atom • Static electricity implies that atoms contain separate charges. • “Plum pudding” model of Thomson. • Rutherford’s experiment.
Particles Inside Atoms • Inside the nucleus: • Protons, charge +1, mass 1. • Neutrons, charge 0, mass 1. • Surrounding the nucleus: • Electrons, charge -1, mass 1/2000.
Atomic and Mass Number • The type of element is determined by the number of protons. This is the atomic number (Z). • The number of protons + neutrons is the mass number (A). • For light elements, there are roughly as many protons and neutrons. Heavier elements have more neutrons than protons.
Isotopes • Atoms of the same element, with different numbers of neutrons in the nucleus. • They are chemically identical, and thus cannot be separated by any chemical process.
Isotopes • Have the same chemical behavior. • Can be very different in nuclear behavior (e.g. radioactivity). • Example: 12C is stable, but 14C is radioactive, with a half-life of 5730 y. • Half-life: time required for half of the initial sample to decay.
Half-lives 26Al 1.6 × 106 y 3H 12.33 y 14C 5,730 y 90Sr 28.1 y 235U 7.04 × 108 y 239Pu 24,000 y • After N half-lives, ½N of original remains.
Marie and Pierre Curie • Deduced that the radiation emitted by uranium and thorium was identical and must be a property of the interior of the atom. • Discovered the radioactive elements Polonium and Radium. • First known victims of radiation poisoning. • Awarded the 1903 Nobel Prize in Physics; Marie won the 1911 Nobel Prize in Chemistry.
Radioactive Decay • Alpha Decay 212Bi 208Tl + 4He 238U 234Th + 4He • Beta Decay 14C 14N + e- + 12N 12C + e+ + • Gamma Decay 4He*4He +
Neutrinos • Probably most common fundamental particle. • Neutral. • Small mass (recently discovered): 0.14 millionth of electron mass. • Extremely nonreactive with other particles. • Three types: electron neutrino ne, muon neutrino nm and tau neutrino nt.
How are Neutrinos Made? • Nuclear reactions, particularly at very high energies. • During the Big Bang. • In the centers of stars. • In supernovas. • When cosmic rays hit the Earth’s atmosphere. • Radioactive beta decay.
Neutrinos are nonreactive! • 100 billion solar neutrinos pass through each square inch of our bodies every second, day or night! • Big bang models predict there should be 50 billion neutrinos per electron. • Neutrino mass has to be very small, or the universe would have already collapsed.
Super-Kamiokande • In Kamioka, Gifu-ken, Honshu, Japan. • 12.5 million gallons of pure water in a stainless steel lined cavity, with 13,000 photomultiplier tubes. • Data taken during 1996-98 indicates neutrinos undergo “oscillations”. • This means that they possess mass: (0.14 millionth of an electron mass).
Forces inside the Atom • Typical size of a nucleus: 10-15 m. Strong nuclear force holds protons and neutrons together. • Typical size of an atom: 10-10 m. Electromagnetic force holds protons and electrons together.
Particle Accelerator Fermilab Accelerator (Proton Synchrotron), Batavia, Illinois
Particle Accelerator Main accelerator ring, Fermilab
Hundreds of New Particles • “The muon, who ordered that?” I.I. Rabi • “If I wanted to remember the names of all of these particles, I would have been a biologist.” Leon Lederman • “As the number of particles increases, all that increases is our ignorance.” Martinus Veltman
Leptons and Hadrons • Leptons • e, , , e, , , • no internal structure • conserved • do not interact via the strong force • Hadrons • p, n + hundreds of particles • internal structure (quarks) • baryons are conserved, mesons are not • interact via the strong force
Cloud Chamber • First developed in 1911 by Charles Wilson. • Uses a supercritical vapor that is triggered to condense when a charged particle passes through.
Cosmic Rays • Energetic particles from space. • When particles interact with our atmosphere, they release showers of high energy particles.
Where do Cosmic Rays come from? • Most (lower energy) cosmic rays come from the Sun. • The amount of solar cosmic rays correlates with low numbers of sunspots. • This is because the Sun’s magnetic field partially shields the Earth.
Galactic Cosmic Rays • Other cosmic rays come from outside the solar system. (And perhaps the Galaxy). • Possible origins: • Supernovas • Pulsars • Black holes at centers of active galaxies
Nuclear Energy and Nuclear Weapons Paul J. Thomas Department of Physics and Astronomy UW-Eau Claire
Making Plutonium from Uranium • 238U is 99.3% of naturally occurring uranium. It is not fissionable. • 235U is 0.7% of naturally occurring uranium. It is fissionable. • 239Pu is also fissionable, and can be made from 238U in nuclear reactors.
The Trinity Test: July 16, 1945 Plutonium implosion bomb 18.6 kT TNT equivalent
Hiroshima: August 6, 1945 Uranium gun bomb 15 kT TNT equivalent 150,000 dead by end of 1945
Nuclear Fusion: pp chain Mass of 4 protons + 2 electrons: 6.694 × 10-24 g Mass of 4He nucleus: 6.644 × 10-24 g Difference: 0.050 × 10-24 g Energy released (E = mc2): 4.4 × 10-12 J 6 × 1014 g of H He per second!