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Nuclear Reactions

Nuclear Reactions. Fission and Fusion. A brief history…. 1919: Ernest Rutherford experimented with bombarding nitrogen gas molecules with alpha particles emitted from bismuth-214 Discovery: faster moving particles were produced, and these could travel farther than the alpha particles!

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Nuclear Reactions

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  1. Nuclear Reactions Fission and Fusion

  2. A brief history… • 1919: Ernest Rutherford experimented with bombarding nitrogen gas molecules with alpha particles emitted from bismuth-214 • Discovery: faster moving particles were produced, and these could travel farther than the alpha particles! • “New” particles also deflected in a magnetic field like a positive particle

  3. A brief history… • Conclusion: The faster moving particles were protons • Artificial Transmutation: • The change of one element to another through the bombardment of a nucleus • More experiments to determine exact nature of the particles and how they were “created” done with a cloud chamber…

  4. Cloud Chambers • Invented ~1911 by a Scottish Atmospheric Physicist (C.T.R. Wilson) to experiment with rain clouds. • Enclosed environment made to be supersaturated (originally with water vapor, now commonly ethanol) • Ions introduced to this environment would attract water molecules (which are polar), forming clouds… • Earned a share in the 1927 Nobel Prize in Physics for the invention…

  5. Cloud Chambers • Why would this be useful for Rutherford? • Water vapor condenses around ions • An alpha particle is ionizing radiation, thus leave a LOT of ions in its path • Water vapor would condense around these ions, leaving a vapor trail showing where an alpha particle had been… • Video

  6. Rutherford’s Theories… • If proton was simply “chipped off” the Nitrogen nucleus by the alpha particle, there should be 4 visible tracks in the cloud chamber: • The original alpha particle BEFORE collision • The alpha particle AFTER the collision • The “chipped off” proton • The Nitrogen nucleus, now charged, as it recoiled after the collision

  7. Rutherford’s Theories… • If alpha particle was absorbed, and that caused the proton to be pushed out, then there should be 3 visible tracks: • The alpha particle before collision • The proton emitted after the collision • The path of the recoiling Nitrogen nucleus • This theory was supported in 1925

  8. Balancing Nuclear Equations: • Note: Deuteron = Hydrogen-2 atom, a.k.a Deuterium • Example problem: • A sample of Oxygen-16 is bombarded with neutrons. If one of the resulting products is a deuteron, what is the resulting nucleus?

  9. Unified Mass Unit (u) • A unit adopted by scientists that is more appropriate for masses along the order of magnitude of atomic masses 1 u = 1.66 x 10-27 kg Mass of an electron (me) = 0.000549 u Mass of a proton (mp) = 1.007277 u Mass of a neutron (mn) = 1.008665 u Mass of 1 H atom (mH) = 1.007825 u

  10. Mass-energy equivalence • Einstein hypothesized a relationship between mass and energy in 1905 • Many years later, data from nuclear reactions showed that his hypothesis was indeed true c = 3.00 x 108 m·s-1 m = mass (kg) E = Energy (J)

  11. Mass-energy equivalence • Used to calculate the Rest Energy of a mass • Used to calculate the amount of energy released in nuclear reactions For Example: Calculate the amount of energy released when 1.00 kg of fuel is used up in a nuclear reactor…

  12. The unified mass unit is defined as • the mass of one neutral atom of Carbon-12 • 1/12 of the mass of one neutral atom of Carbon-12 • 1/6 of the mass of one neutral atom of Carbon-12 • The mass of the nucleus of Carbon-12

  13. Binding Energy • All atomic nuclei have a total mass that is lower than the sum of the masses of each individual particle • For example: The EXPECTED mass of an atom of Helium would be the sum of the mass of 2 neutrons, 2 protons, and 2 electrons: 2(0.000549 u) + 2(1.007277 u) + 2(1.008665 u) = 4.032982 u The MEASURED mass of an atom of helium has been found to be 4.002602 u a difference of 0.03038 u This difference is known as the Mass Defect of the atom

  14. Binding Energy • …a measure of the energy needed to keep a nucleus together • Binding Energyis the energy equivalent of themass defect E = mc2 E = (1.66 x 10-27 kg)(3.00 x 108 m·s-1)2 E = 1.49 x 10-10 J = 931 MeV (Since 1 eV = 1.6 x 10-19 J)

  15. What is the energy equivalent of 1 u? • 319 MeV • 931 eV • 319 keV • 931 MeV

  16. Binding Energy Example: • Calculate the binding energy of Oxygen-16. The measured mass of Oxygen-16 is 15.994915 u 8 electrons+8 protons+8 neutrons 8me + 8mp + 8mn = mexpected = 8(0.000549 u) + 8(1.007277 u) + 8(1.008665 u) = 0.004392 u + 8.058216 u + 8.069320 u = 16.131928 u

  17. Binding Energy Example: • Calculate the binding energy of Oxygen-16. The measured mass of Oxygen-16 is 15.994915 u mdefect = mexpected – mmeasured = 16.131928 u – 15.994915 u = 0.137013 u Eb = mdefect · (931 MeV·u-1) Eb = (0.137013)(931) = 128 MeV

  18. How many Joules of energy is 128 MeV? • 8.00 x 1020 J • 8.00 x 1026 J • 2.05 x 10-17 J • 2.05 x 10-11 J

  19. Nuclear Reactions • Fission: A reaction that involves the splitting of a large, unstable nucleus into 2 or more smaller, more stable nuclei

  20. Which nucleus is most likely to be part of a fission reaction? • Carbon-14 • Deuterium • Plutonium • Potassium-40

  21. Nuclear Reactions • Fusion: A reaction that joins two very light nuclei to form a heavier nucleus • Picture source: www.atomicarchive.com

  22. Nuclear Reactions and Binding Energy • Nuclei with higher amounts of binding energy per nucleon are more stable than those with lower amounts of binding energy per nucleon. • Fission and fusion processes each release large amounts of energy as the nuclei join or split to form more stable products. • To predict how much energy can result from a nuclear reaction, we use a binding energy curve…

  23. Binding Energy Curve

  24. Binding Energy Curve • Example: Use the binding energy curve to predict the amount of energy released when Uranium-235 undergoes fission to produce two Palladium-117 fragments. • Eb for 235U = 7.6 MeV/nucleon • Eb for 117Pd = 8.4 MeV/nucleon The difference between these values, multiplied by the total number of nucleons, is equal to the amount of energy released in the reaction: (0.8 MeV/nucleon) x (235 Nucleons) = 188 MeV

  25. Nuclear Fission • Only takes place in certain very heavy elements, such as Uranium-235 • Fissile Uranium-235 is used in nuclear reactions: • Nucleus bombarded with a neutron to begin a chain reaction…

  26. Fission Reactions • Self-sustaining (chain) reactions: when enough neutrons are produced to naturally enable the reaction to continue until all fissile material is gone • Examples: Nuclear Reactors in Power Plants; Bombs dropped on Hiroshima and Nagasaki in WWII • Critical Mass: The amount of fissile material required to sustain a fission reaction

  27. Figure from Physics for Scientists and Engineers (6th ed.) by Serway and Jewett (Thomson Brooks/Cole, 2004).

  28. Nuclear Fusion Reactions • Conditions required for fusion reactions: • Very high temperatures(because nuclei need very high kinetic energies) • Very densely packed (to ensure that enough collisions will occur), therefore: • Very high pressures • Problems with creating fusion on Earth: • Containment is a huge problem • At temps required, atoms would ionize and technically would become a plasma

  29. Nuclear Fusion Reactions • Proton-Proton Cycle = the fusion reaction that is the source of energy in young/cool stars such as the sun: • The first two reactions in the cycle must occur twice • Total energy released = 24.7 MeV

  30. Fusion Example • Calculate the energy released when a proton and a deuteron undergo fusion to produce helium-3.

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