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## PowerPoint Slideshow about ' Nuclear Physics' - vienna

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### Nuclear Physics

20th 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
- 1913: Millikan measured electron charge

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

- 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

Isotopes

- 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

- 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

- 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 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 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

- 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

- 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

- 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 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

- 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 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

- 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 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

- 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 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

- 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 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 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

- 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

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