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Chapter 29

Chapter 29. Nuclear Physics. Ernest Rutherford. 1871 – 1937 Discovery that atoms could be broken apart Studied radioactivity Nobel prize in 1908. Some Properties of Nuclei. All nuclei are composed of protons and neutrons Exception is ordinary hydrogen with just a proton

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Chapter 29

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  1. Chapter 29 Nuclear Physics

  2. Ernest Rutherford • 1871 – 1937 • Discovery that atoms could be broken apart • Studied radioactivity • Nobel prize in 1908

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

  4. Symbolism • Symbol: • X is the chemical symbol of the element • Example: • Mass number is 27 • Atomic number is 13 • Contains 13 protons • Contains 14 (27 – 13) neutrons • The Z may be omitted since the element can be used to determine Z

  5. 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:

  6. Charge • The proton has a single positive charge, +e • The electron has a single negative charge, -e • The neutron has no charge • Makes it difficult to detect • e = 1.602 177 33 x 10-19 C

  7. Mass • It is convenient to use unified mass units, u, to express masses • 1 u = 1.660 559 x 10-27 kg • Based on definition that the mass of one atom of C-12 is exactly 12 u

  8. Density of Nuclei • 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

  9. Maria Goeppert-Mayer • 1906 – 1972 • Best known for her development of shell model of the nucleus • Shared Nobel Prize in 1963

  10. Nuclear Stability • There are very large repulsive electrostatic forces between protons • These forces should cause the nucleus to fly apart • The nuclei are stable because of the presence of another, short-range force, called the nuclear force • This is an attractive force that acts between all nuclear particles • The nuclear attractive force is stronger than the Coulomb repulsive force at the short ranges within the nucleus

  11. Nuclear Stability, cont • Light nuclei are most stable if N = Z • Heavy nuclei are most stable when N > Z • As the number of protons increase, the Coulomb force increases and so more nucleons are needed to keep the nucleus stable • No nuclei are stable when Z > 83

  12. Marie Curie • 1867 – 1934 • Discovered new radioactive elements • Shared Nobel Prize in physics in 1903 • Nobel Prize in Chemistry in 1911

  13. Radioactivity • Radioactivity is the spontaneous emission of radiation • Experiments suggested that radioactivity was the result of the decay, or disintegration, of unstable nuclei

  14. Radioactivity – Types • Three types of radiation can be emitted • Alpha particles • The particles are 4He nuclei • Beta particles • The particles are either electrons or positrons • A positron is the antiparticle of the electron • It is similar to the electron except its charge is +e • Gamma rays • The “rays” are high energy photons

  15. Distinguishing Types of Radiation • A radioactive beam is directed into a region with a magnetic field • The gamma particles carry no charge and they are not deflected • The alpha particles are deflected upward • The beta particles are deflected downward • A positron would be deflected upward

  16. Penetrating Ability of Particles • Alpha particles • Barely penetrate a piece of paper • Beta particles • Can penetrate a few mm of aluminum • Gamma rays • Can penetrate several cm of lead

  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

  19. Decay – General Rules • When one element changes into another element, the process is called spontaneous decay or transmutation • The sum of the mass numbers, A, must be the same on both sides of the equation • The sum of the atomic numbers, Z, must be the same on both sides of the equation • Conservation of mass-energy and conservation of momentum must hold

  20. Beta Decay • During beta decay, the daughter nucleus has the same number of nucleons as the parent, but the atomic number is changed by one • Symbolically

  21. Beta Decay, cont • The emission of the electron is from the nucleus • The nucleus contains protons and neutrons • The process occurs when a neutron is transformed into a proton and an electron • Energy must be conserved

  22. Beta Decay – Electron Energy • The energy released in the decay process should almost all go to kinetic energy of the electron (KEmax) • Experiments showed that few electrons had this amount of kinetic energy

  23. Neutrino • To account for this “missing” energy, in 1930 Pauli proposed the existence of another particle • Enrico Fermi later named this particle the neutrino • Properties of the neutrino • Zero electrical charge • Mass much smaller than the electron, recent experiments indicate definitely some mass • Spin of ½ • Very weak interaction with matter

  24. Beta Decay – Completed • Symbolically •  is the symbol for the neutrino • is the symbol for the antineutrino • To summarize, in beta decay, the following pairs of particles are emitted • An electron and an antineutrino • A positron and a neutrino

  25. 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

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

  27. Enrico Fermi • 1901 – 1954 • Produced transuranic elements • Other contributions • Theory of beta decay • Free-electron theory of metals • World’s first fission reactor (1942) • Nobel Prize in 1938

  28. Uses of Radioactivity • Carbon Dating • Beta decay of 14C is used to date organic samples • The ratio of 14C to 12C is used • Smoke detectors • Ionization-type smoke detectors use a radioactive source to ionize the air in a chamber • A voltage and current are maintained • When smoke enters the chamber, the current is decreased and the alarm sounds

  29. More Uses of Radioactivity • Radon detection • Radon is an inert, gaseous element associated with the decay of radium • It is present in uranium mines and in certain types of rocks, bricks, etc that may be used in home building • May also come from the ground itself

  30. Natural Radioactivity • Classification of nuclei • Unstable nuclei found in nature • Give rise to natural radioactivity • Nuclei produced in the laboratory through nuclear reactions • Exhibit artificial radioactivity • Three series of natural radioactivity exist • Uranium • Actinium • Thorium • See table 29.2

  31. Decay Series of 232Th • Series starts with 232Th • Processes through a series of alpha and beta decays • Ends with a stable isotope of lead, 208Pb

  32. Nuclear Reactions • Structure of nuclei can be changed by bombarding them with energetic particles • The changes are called nuclear reactions • As with nuclear decays, the atomic numbers and mass numbers must balance on both sides of the equation

  33. Nuclear Reactions – Example • Alpha particle colliding with nitrogen: • Balancing the equation allows for the identification of X • So the reaction is

  34. Radiation Damage in Matter • Radiation absorbed by matter can cause damage • The degree and type of damage depend on many factors • Type and energy of the radiation • Properties of the absorbing matter • Radiation damage in biological organisms is primarily due to ionization effects in cells • Ionization disrupts the normal functioning of the cell

  35. Types of Damage • Somatic damage is radiation damage to any cells except reproductive ones • Can lead to cancer at high radiation levels • Can seriously alter the characteristics of specific organisms • Genetic damage affects only reproductive cells • Can lead to defective offspring

  36. Units of Radiation Exposure • Roentgen [R] • That amount of ionizing radiation that will produce 2.08 x 109 ion pairs in 1 cm3 of air under standard conditions • That amount of radiation that deposits 8.76 x 10-3 J of energy into 1 kg of air • Rad (Radiation Absorbed Dose) • That amount of radiation that deposits 10-2 J of energy into 1 kg of absorbing material

  37. Radiation Levels • Natural sources – rocks and soil, cosmic rays • Background radiation • About 0.13 rem/yr • Upper limit suggested by US government • 0.50 rem/yr • Excludes background and medical exposures • Occupational • 5 rem/yr for whole-body radiation • Certain body parts can withstand higher levels • Ingestion or inhalation is most dangerous

  38. Applications of Radiation • 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

  39. Applications of Radiation, cont • Tracing • Radioactive particles can be used to trace chemicals participating in various reactions • Example, 131I to test thyroid action

  40. Applications of Radiation, final • MRI • Magnetic Resonance Imaging • When a nucleus having a magnetic moment is placed in an external magnetic field, its moment processes about the magnetic field with a frequency that is proportional to the field • Transitions between energy states can be detected electronically

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