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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