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Nuclear Physics

Nuclear Physics. AP Physics B Chapter 30 Notes. Nuclear Notation.

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Nuclear Physics

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  1. Nuclear Physics AP Physics B Chapter 30 Notes

  2. Nuclear Notation Standard notation for a given nuclide is to list the chemical symbol (uranium here) with a subscript (Z) used to show the number of protons (atomic number) and a superscript (A) to show the number of nucleons (protons plus neutrons). The neutron number is N = A – Z mp≅mn= 1.67 x 10-27

  3. Isotopes Nuclei with the same Z (they are the same element) but with different N are isotopes. Masses of atoms are measured with reference to the carbon-12 atom which has an assigned mass of 12u (unified atomic mass unit).

  4. Binding Energy and Nuclear Forces The total mass of a stable nucleus is always less than the sum of the masses of its separate constituents—what happens to the mass? It goes into binding energy—found by Einstein’s famous E=mc2—the strong nuclear force. This is also known as the mass defect

  5. Mass Defect--Example Compare the mass of He to the total mass of its components (use Appendix B).

  6. Radioactive Decay Many unstable isotopes occur in nature, and it was discovered that they spontaneously produced three types of “rays” that were classified by their ability to penetrate: α, β and γ.

  7. Radioactive Decay Alpha and beta rays are bent in opposite directions by B, gamma are not deflected at all

  8. Alpha Decay Alpha decay releases .An example would be radium-226 decaying to radon-222.

  9. Alpha Decay In general, alpha decay can be written as: With the parent nucleus of N and the daughter nucleus of N’ Alpha particles tend to be emitted because they are very tightly bound (stable)—energy is released.

  10. Alpha Decay--Application Most smoke detector contain which emits alpha particles. The radiation continually ionizes the air between two charged plates—this allows a small current to flow across the plates. When smoke enters, the radiation is absorbed by the smoke particles, breaking the circuit.

  11. Beta Decay β- decay is the release of an electron (negative beta decay) and a neutrino. The electron released is not an orbital electron—it comes from the nucleus. No nucleons are lost, but the charge on the daughter nucleus is +1e greater than on the parent. The neutrino is particle discovered by Fermi and was critical to the laws of conservation.

  12. Beta Decay β+ decay is the release of a positron (positive beta decay) and a neutrino. The positron has the same mass as an e but a charge of +1e. Again, no nucleons are lost, but the charge on the daughter nucleus is -1e greater than on the parent ( a proton is converted to a positron and a neutrino).

  13. Gamma Decay Gamma rays are photons having very high energy. In gamma decay, the parent nucleus is in an excited state, and when it jumps down to a lower state it emits a photon called a γ ray. Gamma rays are very penetrating, and can be used to sterilize food or medical devices, and to screen large containers.

  14. Decay Energy Calculations To determine the amount of energy released in alpha and beta decay problems, use E = mc2 and determine the mass before the decay and the mass after. Use the useful conversion 1u = 931.5 Mev/c2. Examples: 232U alpha decay to 228Th 14C beta decay to 14N

  15. Radioactive Decay Summary The three types of radioactive decay are summarized at left (don’t worry about electron capture). Notice that in all three types of decay that there is a conservation of nucleons.

  16. Radioactive Decay Summary A lithium nucleus, while at rest, decays into a helium nucleus of rest mass 6.6483 x 10‑27 kilogram and a proton of rest mass 1.6726 x 10‑27 kilogram, as shown by the following reaction. In this reaction, momentum and total energy are conserved. After the decay, the proton moves with a nonrelativistic speed of 1.95 x 107 m/s. a. Determine the kinetic energy of the proton. b. Determine the speed of the helium nucleus. c. Determine the kinetic energy of the helium nucleus. d. Determine the mass that is transformed into kinetic energy in this decay. e. Determine the rest mass of the lithium nucleus.

  17. Nuclear Fission A nuclear reaction takes place when a nucleus is struck by another nucleus or particle. If the original nucleus is transformed into another, this is called transmutation. An example:

  18. Nuclear Fission Energy and momentum are conserved in nuclear reactions Neutrons are very effective in nuclear reactions, as they nave no charge and therefore are not repelled by the nucleus.

  19. Nuclear Fission After absorbing a neutron, a uranium-235 nucleus will split into two roughly equal parts. One way to visualize this is to view the nucleus as a kind of liquid drop.

  20. Nuclear Fission A lot of energy is released in the fission process as well as several neutrons. These neutrons can be used to induce fission in other nuclei, causing a chain reaction.

  21. Nuclear Fission The reaction for this process results in two fission fragments: The U-236 compound nucleus exists for less than 10-12 s So a typical reaction looks like this: Other reactions may also occur

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