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Nuclear Physics. 20 th Century Discoveries. Historical Developments. 1895: Roentgen discovered X-rays 1896: Becquerel discovered radioactivity 1897: Thomson discovered electron 1900: Planck “energy is quantized” 1905: Einstein’s theory of relativity 1911: Rutherford discovered the nucleus

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nuclear physics

Nuclear Physics

20th Century Discoveries

historical developments
Historical Developments
  • 1895: Roentgen discovered X-rays
  • 1896: Becquerel discovered radioactivity
  • 1897: Thomson discovered electron
  • 1900: Planck “energy is quantized”
  • 1905: Einstein’s theory of relativity
  • 1911: Rutherford discovered the nucleus
  • 1913: Millikan measured electron charge
historical developments1
Historical Developments
  • 1925: Pauli’s exclusion principle
  • 1927: Heisenberg’s uncertainty principle
  • 1928: Dirac predicts existence of antimatter
  • 1932: Chadwick discovered neutron
  • 1942: Fermi first controlled fission reaction
  • 1964: Gell-Mann proposed quarks
the nucleus
The Nucleus
  • Mass number (A) is number of nucleons (protons + neutrons)
  • Atomic number (Z) is number of protons
  • Neutron number (N) number of neutrons
  • Often, mass number and atomic number are combined with chemical symbol

aluminum, Z = 13, A = 27

  • Atoms of the same element have same atomic number but can have different mass numbers
  • These are called isotopes: atoms of the same element with different number of neutrons
  • Chemical properties are the same but nuclear properties are different
nuclear mass
Nuclear Mass
  • Nuclei are extremely dense, about 2.3 x 1014 g/cm3
  • Nuclear mass usually measured with atomic mass unit (u)
  • Based on mass of carbon-12 atom whose mass is defined as 12 u
  • 1 u = 1.6605402 x 10-27 kg
mass energy
  • Nuclear mass can also be expressed in terms of rest energy by using Einstein’s famous equation E = mc2
  • Mass is often converted to energy in nuclear interactions
  • Substituting values for mass of 1u and converting to eV, we find 1u =931.50 MeV
nuclear stability
Nuclear Stability
  • Since protons have positive charge, they will repel each other with electric force
  • Must be a stronger, attractive force holding them together in nucleus
  • This force usually called the strong force
  • Strong force acts only over extremely small distances
  • All nucleons contribute to strong force
nuclear stability1
Nuclear Stability
  • Neutrons add to strong force without adding to repelling electrical force, so they help stabilize nucleus
  • For Z > 83, repulsive forces can’t be overcome by more neutrons and these nuclei are unstable
binding energy
Binding Energy
  • Binding energy is difference between energy of free, unbound nucleons and nucleons in nucleus
  • Mass of nucleus is less than mass of component parts
  • Difference in mass is mass defect and makes up binding energy (E = mc2)
nuclear decay
Nuclear Decay
  • Unstable nuclei spontaneously break apart and emit radiation in the form of particles, photons, or both
  • Process is called radioactivity
  • Can be induced artificially
  • Parent nucleus decays into daughter nucleus
alpha radiation
Alpha radiation
  • Least penetrating, can be stopped by sheet of paper
  • Decreases atomic number by 2, mass number by 4
  • Is actually a He nucleus, will quickly attract 2 electrons and become helium
beta radiation
Beta radiation
  • Usually a neutron decays into a proton and an electron
  • Missing mass becomes kinetic energy of electron
  • Atomic number increases by 1, neutron number decreases by 1, mass number is the same
beta radiation1
Beta Radiation
  • Inverse beta decay proton emits positron and becomes neutron, decreasing atomic number
  • Betas can be stopped by sheet of aluminum
  • Involves emission of antineutrinos (with e-) or neutrinos (with e+) also
gamma radiation
Gamma radiation
  • Most penetrating, will penetrate several centimeters of lead
  • High energy photon emitted when nucleons move into lower energy state
  • Often occurs as a result of alpha or beta emission
nuclear decay1
Nuclear Decay
  • In many cases decay of parent nucleus produces unstable daughter nucleus
  • Decay process continues until stable daughter nucleus is produced
  • Often involves many steps called a decay series
writing nuclear reactions
Writing Nuclear Reactions
  • Write chemical symbol with mass number and atomic number of parent nucleus
  • On right side of arrow, leave a space for the daughter element and write the symbol for the type of emission occurring
  • alpha: beta: neutron:
writing nuclear reactions1
Writing Nuclear Reactions
  • Mass and charge are conserved quantities so totals on left side of equation must equal totals on right of equation for both the mass numbers and the atomic numbers
  • Calculate atomic number of daughter and look up its symbol on periodic table
  • Calculate mass number of daughter
half life
  • Decay constant for a material indicates rate of decay
  • Half-life is the time for ½ of a sample to decay; after 2 half-lives, ¼ of sample remains; after 3, 1/8 remains
  • Half-lives range from less than a second to billions of years
nuclear fission
Nuclear Fission
  • Heavy nucleus splits into two smaller nuclei
  • Energy is released due to higher binding energy per nucleon (and so less mass) in smaller nuclei
  • Often started by absorption of a neutron by large nucleus making it unstable
  • U-235 and Pu-239 are usual fission fuels for reactors and atomic bombs
nuclear fission1
Nuclear Fission
  • Fission products include two smaller elements, high energy photons, and 2 or 3 more neutrons
  • Neutrons then can be absorbed by other nuclei creating chain reaction
  • Need a minimum amount of fuel for sustained reaction called critical mass
nuclear fusion
Nuclear Fusion
  • Two light nuclei combine to form heavier nucleus
  • Product has higher binding energy (less mass) so energy is released
  • Fusion occurs in stars and hydrogen bombs (thermonuclear)
  • Stars fuse protons (hydrogen) and helium atoms
nuclear fusion1
Nuclear Fusion
  • Fusion fuel on earth usually deuterium (heavy hydrogen)
  • For fusion to occur, electrostatic repulsion forces must be overcome so nuclei can collide
  • Extremely high temperatures and pressures needed
nuclear fusion2
Nuclear Fusion
  • Sustained, cost-effective fusion reaction has not been achieved
  • Would be better then fission because:
    • products are not radioactive
    • fuel is cheap and plentiful
    • no danger from critical mass
quarks and antimatter
Quarks and Antimatter
  • Protons and neutrons are composed of smaller particles called quarks, considered fundamental particles
  • 6 types of quarks exist but only two in common matter: up and down
  • Proton = uud; neutron = udd
  • Each fundamental particle has a corresponding antimatter particle with opposite charge