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Chapter 29

Chapter 29. Nuclear Physics. Nuclear Physics Sections 1–4. Milestones in Nuclear Physics. 1896 – the birth of nuclear physics Becquerel discovered radioactivity in uranium compounds 1911 Rutherford, Geiger and Marsden performed scattering experiments

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Chapter 29

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  1. Chapter 29 Nuclear Physics

  2. Nuclear Physics Sections 1–4

  3. Milestones in Nuclear Physics • 1896 – the birth of nuclear physics • Becquerel discovered radioactivity in uranium compounds • 1911 Rutherford, Geiger and Marsden performed scattering experiments • Established the point mass nature of the nucleus • Nuclear force was a new type of force • 1919 Rutherford and coworkers first observed nuclear reactions • Naturally occurring alpha particles bombarded nitrogen nuclei to produce oxygen

  4. History • Becquerel – discovered radioactivity (1896) (from radiare = emit rays) • Curie – discovery of polonium and radium (1898) • Rutherford – nuclear model • classified ,, radiation,  particle = 4He nucleus (nuclear transmutation) • used  scattering to discover the nuclear model • postulated ‘neutrons’ A=Z+N (1920); bound p+ e- state? • Mosley – studied nucleus via X-ray spectra • correlated (Z = charge of nucleus) with periodic table • extra particles in nucleus: A = Z + ? • Chadwick – discovered neutron (1932) • Pauli – postulated neutral particle from -decay (1930) • Fermi – theory or weak decay (1933) ‘neutrino’ • Fission – Hahn, Strassmann, (&Meitner!) (1938) • first reactor (chain reaction), Fermi (1942) • Bohr, Wheeler – liquid drop model • Mayer, Jensen – shell model (1949) • Hofstadter – electron scattering (1953-) • measured the charge density of various nuclei • discovered structure in the proton (not point-like particle)

  5. Some Properties of Nuclei • All nuclei are composed of protons and neutrons • Exception is ordinary hydrogen with just a proton • Atomic number, Z • Number of protons in the nucleus • Neutron number, N • Number of neutrons in the nucleus • Mass number, A • Number of nucleons in the nucleus: A = Z + N • Nucleon is a generic term used to refer to either a proton or a neutron in the nucleus • The mass number is not the same as the mass

  6. Symbolism • Symbol: • X is the chemical symbol of the element • Example: • Mass number: A = 27 nucleons • Atomic number: Z = 13 protons • Neutron number: N = 27 – 13 = 14 neutrons • The Z may be omitted since the element can be used to determine Z

  7. More Properties • The nuclei of all atoms of a particular element must contain the same number of protons • They may contain varying numbers of neutrons • Isotopes of an element have the same Z but differing N and A values • Example: • See Appendix B – An Abbreviated Table of Isotopes Radioactive Stable Stable Radioactive

  8. Charge and Mass • The proton has a single positive charge, +e • e = 1.60217733 x 10-19 C • The electron has a single negative charge, –e • The neutron has no charge – difficult to detect • unified mass unit: u • 1 u = 1.660559 x 10-27 kg • Based on definition that mass of one atom of 12C is exactly 12 u • E = m c2 1 u = 931.494 MeV/c2

  9. Summary of Charges & Masses

  10. Binding Energy • The total energy of the bound system (the nucleus) is less than the combined energy of the separated nucleons • This difference in energy is called the binding energy of the nucleus • It can be thought of as the amount of energy you need to add to the nucleus to break it apart into separated protons and neutrons

  11. Binding Energy per Nucleon • Except for light nuclei, the binding energy is ~ 8 MeV per nucleon • The curve peaks in the vicinity of A = 60 • Nuclei with mass numbers greater than or less than 60 are not as strongly bound as those near the middle of the periodic table • The curve is slowly varying at A > 40

  12. Size and Density of Nuclei • Since the time of Rutherford, many other experiments have concluded: • Most nuclei are approximately spherical • Average radius is • ro = 1.2 x 10-15 m or 1.2 fm • The volume of the nucleus (assumed to be spherical) is directly proportional to the total number of nucleons • This suggests that all nuclei havenearly the same density • Nucleons combine to form a nucleus as though they were tightly packed spheres

  13. Z A Z=N = A/2 A=100 Atomic element Nuclear isotope N

  14. ++ decay - decay (isobar) + decay (isobar)  electron capture (isobar) p decay (isotone) n decay (isotope)  decay (isomers) electron conversion (EC) spontaneous fission (SF) double beta decay (2) neutrino-less double beta decay (0) beta-delayed n,p, decay Nuclear decay modes: ISOTONES ISOBARS ISOMERS ISOTOPES Z N

  15. Chart of Nuclides – decay mode magic numbers stable nuclide - decay , electron capture decay p decay n decay spontaneous fission http://www.nndc.bnl.gov/chart

  16. Chart of Nuclides – island of stability magic numbers http://en.wikipedia.org/wiki/Island_of_stability

  17. Alpha Decay • When a nucleus emits an alpha particle it loses two protons and two neutrons • N decreases by 2 • Z decreases by 2 • A decreases by 4 • Symbolically • X is called the parent nucleus • Y is called the daughter nucleus

  18. Alpha Decay – Example • Decay of 226 Ra • Half life for this decay is 1600 years • Excess mass is converted into kinetic energy • Momentum of the two particles is equal and opposite Active Figure: Alpha Decay of Radium-226

  19. Beta Decay • Beta– decay: a neutron is transformed into a proton, and an electron and antineutrino are emitted • A stays the same, Z  Z+1 • Beta+ decay: a proton is transformed into a neutron, and a positron and neutrino are emitted • A stays the same, Z  Z–1 • Symbolically • Energy must be conserved •  is the symbol for the neutrino (carries away excess KE) •  is the symbol for the antineutrino (carries away excess KE)

  20. Beta Decay – Example • Radioactive Carbon-14 decay • Used to date organic samples

  21. Gamma Decay • Gamma rays are given off when an excited nucleus “falls” to a lower energy state • Similar to the process of electron “jumps” to lower energy states and giving off photons • The photons are called gamma rays, very high energy relative to light • The excited nuclear states result from “jumps” made by a proton or neutron • The excited nuclear states may be the result of violent collision or more likely of an alpha or beta emission

  22. Gamma Decay – Example • Example of a decay sequence • The first decay is a beta decay emission • The second step is a gamma decay emission • C* indicates the Carbon nucleus is in an excited state • Gamma emission doesn’t change either A or Z

  23. Medical Applications of Radiation • Tracing • Radioactive particles can be used to trace chemicals participating in various reactions • Example, 131I to test thyroid action • Sterilization • Radiation has been used to sterilize medical equipment • Used to destroy bacteria, worms and insects in food • Bone, cartilage, and skin used in graphs is often irradiated before grafting to reduce the chances of infection

  24. Medical Applications of Radiation • CAT scans • Computed Axial Tomography • Produces pictures with greater clarity and detail than traditional x-rays

  25. Medical Applications of Radiation • MRI scans • Magnetic Resonance Imaging • When a nucleus having a magnetic moment is placed in an external magnetic field, its moment precesses about the magnetic field with a frequency that is proportional to the field • Transitions between energy states can be detected electronically to produce cross-sectional images

  26. Medical Applications of Radiation • 3D-CRT Treatment • Three-dimensional conformal radiation therapy uses sophisticated computers, CT scans and/or MRI scans to create detailed 3-D representations of a tumor and surrounding organs • Radiation beams are then shaped exactly to treat the size and shape of the tumor – nearby normal tissue receives less radiation exposure

  27. 3D-CRT Treatment Planning

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