Radiation and the Atom

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Radiation and the Atom
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Electromagnetic Radiation

• Visible light, radio waves, x-rays
• No mass unaffected by electrical or magnetic
  fields constant speed in a given medium
• Travels in straight lines trajectory can be
  altered by interaction with matter
• Absorption removal of the radiation
• Scattering change in trajectory

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Electromagnetic Spectrum
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EM Radiation in Imaging

• Gamma rays originate within nuclei of
  radioactive atoms used to image the distribution
  of radiopharmaceuticals
• X-rays produced outside the nucleus used in
  radiography and computed tomography
• Visible light produced in detecting x- and
  gamma rays used for observation and
  interpretation of images
• Radiofrequency EM in the FM region used as the
  transmission and reception signal for MRI

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EM Wave Characteristics

• Waves characterized by amplitude, wavelength (?),
  frequency (?), and period
• Speed (c), wavelength, and frequency related by
• Wavelengths typically measured in nanometers
  (10-9 m) frequency expressed in hertz (Hz) (1 Hz
  1 cycle/sec 1 sec-1)

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Characterization of Waves
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Electric and Magnetic Field Components of
Electromagnetic Radiation
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EM Particle Characteristics

• When interacting with matter, EM radiation can
  exhibit particle-like behaviour
• Particle-like bundles of energy called photons
  energy is given by
• where h 6.62 x 10-34 J-sec
• Energies of photons commonly expressed in
  electron volts (eV)

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Ionizing vs. Nonionizing Radiation

• EM radiation of higher frequency than
  near-ultraviolet region of spectrum carries
  enough energy per photon to remove bound
  electrons from atomic shells, producing ionized
  atoms and molecules
• Radiation in this region is called ionizing
  radiation
• Visible light, infrared, radio and TV broadcasts
  is called nonionizing radiation

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Electromagnetic Spectrum
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Particulate Radiation

• Protons found in nuclei of all atoms single
  positive charge
• Electrons exist in atomic orbits emitted by
  nuclei of some radioactive atoms (referred to as
  beta-minus particles (?-), negatrons, or simply
  beta particles)
• Positrons positively charged electrons (?)
  emitted from some nuclei during radioactive decay
• Neutrons uncharged nuclear particle released
  by nuclear fission and used for radionuclide
  production

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Mass Energy Equivalence

• Einsteins theory of relativity states that mass
  and energy are interchangeable
• where E represents the energy equivalent to mass
  m at rest and c is the speed of light in a vacuum

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Fundamental Properties of Particulate Radiation
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Structure of the Atom

• Smallest division of an element in which the
  chemical identity of the element is maintained
• Composed of extremely dense positively charged
  nucleus containing protons and neutrons and an
  extranuclear cloud of light negatively charged
  electrons
• Electrically neutral in its nonionized state

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Bohr Model of the Atom

• Electrons orbit around a dense positively charged
  nucleus at fixed distances
• Each electron occupies a discrete energy state in
  a given electron shell
• Shells assigned the letters K, L, M, N, , with K
  denoting the innermost shell also assigned
  quantum numbers 1, 2, 3, 4, , with 1 designating
  the K shell
• Each shell can contain a maximum of (2n2)
  electrons, where n is the quantum number of the
  shell

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Binding Energy

• Energy required to remove an electron completely
  from an atom
• By convention, binding energies are negative with
  increasing magnitude for electrons in shells
  closer to the nucleus
• Binding energy of electrons in a particular orbit
  increases with the number of protons in the
  nucleus (i.e., atomic number, Z)

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Energy-Level Diagrams for Hydrogen and Tungsten
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Radiation from Electron Transitions

• When an electron is removed from its shell, a
  vacancy is created in that shell
• Usually filled by an electron from an outer
  shell, leaving a vacancy in the outer shell that
  may in turn be filled by an electron transition
  from a more distant shell
• Series of transitions called electron cascade
• Energy released by each transition equals
  difference in binding energy between original and
  final shells of the electron

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Characteristic X-Rays

• Electron transitions between atomic shells
  results in emission of radiation in the visible,
  ultraviolet, and x-ray portions of the EM
  spectrum
• Energy is characteristic of each atom, since the
  electron binding energies depend on Z
• Emissions from transitions exceeding 100 eV are
  called characteristic or fluorescent x-rays

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Characteristic X-Rays (cont.)

• Named according to the orbital in which the
  vacancy occurred
• Radiation resulting from vacancy in K shell is
  called a K-characteristic x-ray
• If vacancy in one shell is filled by adjacent
  shell it is identified by a subscript alpha
• If vacancy is filled from a nonadjacent shell,
  the subscript beta is used

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De-excitation of a Tungsten Atom
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Auger Electrons and Fluorescent Yield

• Competing process that predominates in low Z
  elements is Auger electron emission
• Energy released is transferred to an orbital
  electron, typically in the same shell as the
  cascading electron
• Probability that electron transition will result
  in emission of characteristic x-ray is called
  fluorescent yield (?)
• K-shell fluorescent yield is essentially zero for
  elements Z lt 10, and approaches 80 for Z gt 60

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De-excitation of a Tungsten Atom
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Atomic Nucleus

• Composed of protons and neutrons (collectively,
  nucleons)
• Number of protons is atomic number (Z) total
  number of protons and neutrons (N) is the mass
  number (A)
• Notation specifying an atom with chemical symbol
  X is

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Nuclear Energy Levels

• Nucleus has energy levels that are analogous to
  orbital electron shells often much higher in
  energy
• Lowest energy state is called the ground state
• Nuclei with energy in excess of the ground state
  are said to be in an excited state
• Excited states that exist longer than 10-12
  seconds are called metastable or isomeric states
  denoted by the letter m after the mass number of
  the atom (e.g., Tc-99m)

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Nuclear Families
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Nuclear Stability

• Only certain combinations of neutrons and protons
  in the nucleus are stable
• A higher neutron-to-proton ratio is required in
  heavy elements to offset the coulombic repulsive
  forces between protons by providing increased
  separation of protons
• Nuclei with odd number of neutrons and odd number
  of protons tend to be unstable

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Nuclide Line of Stability
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Radioactivity

• Combinations of neutrons and protons that are not
  stable do exist over time they permute to nuclei
  that are stable
• They achieve stability by conversion of a neutron
  to a proton or vice versa
• These events are accompanied by emission of
  energy
• Emissions include particulate and electromagnetic
  radiations
• Nuclei that decay to more stable nuclei are said
  to be radioactive process itself called
  radioactive decay

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Gamma Rays

• Radioactive decay often results in formation of
  daughter nucleus in an excited state
• EM radiation emitted from nucleus as excited
  state decays to lower (more stable) energy state
  is called a gamma ray
• When this process takes place in a metastable
  isomer (e.g., Tc-99m), it is called isomeric
  transition

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Internal Conversion Electrons

• Nuclear de-excitation does not always result in
  emission of a gamma ray
• Alternative form is internal conversion, in which
  the de-excitation energy is completely
  transferred to an orbital (typically K, L, or M)
  electron
• Conversion electron is ejected from the atom,
  with kinetic energy equal to that of the gamma
  ray less the electron binding energy