CHARACTERISTICS OF INTERACTIONS

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• CHARACTERISTICS OF INTERACTIONS

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• In a radiation interaction, the radiation and the
  material with which it interacts may be
  considered as a single system.
• When the system is compared before and after the
  interaction, certain quantities will be found to
  be invariant.
• Invariant quantities are exactly the same before
  and after the interaction.
• Invariant quantities are said to be conserved in
  the interaction.

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• One quantity that is always conserved in an
  interaction is the total energy of the system,
  with the understanding that mass is a form of
  energy.
• Other quantities that are conserved include
  momentum and electric charge.

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• Some quantities are not always conserved during
  an interaction.
• For example. the number of particles may not be
  conserved because particles may be
• fragmented,
• fused,
• created (energy converted to mass), or
• destroyed (mass converted to energy).

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• Interactions may be classified as either
• elastic or
• inelastic.

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• An interaction is elastic if the sum of the
  kinetic energies of the interacting entities is
  conserved during the interaction.
• If some energy is used to free an electron or
  nucleon from a bound state, kinetic energy is not
  conserved and the interaction is inelastic.
• Total energy is conserved in all interactions,
  but kinetic energy is conserved only in
  interactions designated as elastic.

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ENTC 4390

• DIRECTLY IONIZING RADIATION

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• When an electron is ejected from an atom, the
  atom is left in an ionized state.
• Hydrogen is the element with the smallest atomic
  number and requires the least energy (binding
  energy of 13.6 eV) to eject its K-shell electron.

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• Radiation of energy less than 13.6 eV is termed
  nonionizing radiation because it cannot eject
  this most easily removed electron.
• Radiation with energy above 13.6 eV is referred
  to as ionizing radiation.

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• If electrons are not ejected from atoms but
  merely raised to higher energy levels (outer
  shells), the process is termed excitation, and
  the atom is said to be excited.
• Charged particles such as electrons, protons, and
  atomic nuclei are directly ionizing radiations
  because they can eject electrons from atoms
  through charged-particle interactions.

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• Neutrons and photons (x and g rays) can set
  charged particles into motion, but they do not
  produce significant ionization directly because
  they are uncharged.
• These radiations are said to he indirectly
  ionizing.

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• Energy transferred to an electron in excess of
  its binding energy appears as kinetic energy of
  the ejected electron.
• An ejected electron and the residual positive ion
  constitute an ion pair.
• An average energy of 33.85 eV termed the
  W-quantity or W, is expended by charged particles
  per ion pair produced in air.
• The average energy required to remove an electron
  from nitrogen or oxygen (the most common atoms in
  air) is much less than 33.85 eV.

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• The W-quantity includes not only the electrons
  binding energy but also the average kinetic
  energy of the ejected electron and the average
  energy lost as incident particles excite atoms,
  interact with nuclei, and increase the rate of
  vibration of nearby molecules.
• On the average. 2.2 atoms are excited per ion
  pair produced in air.

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ENTC 4390MEDICAL IMAGING

• Interactions of Radiation

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Particle Interactions

• Particles of ionizing radiation include charged
  particles, such as
• alpha particles
• protons
• electrons,
• beta particles
• positrons, and
• neutrons.

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• The behavior of heavy charged particles is
  different from that of lighter charged particles
  such as electrons and positrons.

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Excitation, Ionization, and Radiative Losses

• Energetic charged particles all interact with
  matter by electrical forces and lose kinetic
  energy via
• excitation,
• ionization, and
• radioactive losses.

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• Excitation and ionization occur when charged
  particles lose energy by interacting with orbital
  electrons.
• Excitation is the transfer of some of the
  incident particles energy to electrons in the
  absorbing material, promoting them to electron
  orbits farther from the nucleus.
• Higher energy levels.

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• In excitation, the energy transferred to an
  electron does not exceed its binding energy.
• The electron doesnt leave the atom.
• Following excitation, the electron will return to
  a lower energy level,
• With the emission of the excitation energy in the
  form of
• Electromagnetic radiation or
• Auger electrons.

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Atomic Emissions

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Characteristic X-Rays

• Electron transitions between atomic shells
  results in the emission of radiation in the
  visible, ultraviolet, and x-ray portions of the
  electromagnetic (EM) spectrum.
• Characteristic x-rays are named according to the
  orbital in which the vacancy occurred.

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Electromagnetic radiation
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Electromagnetic Radiation

• Electromagnetic radiation consists of oscillating
  electric and magnetic fields.
• An electromagnetic wave requires no medium for
  propagation,
• That is, it can travel in a vacuum as well as
  through matter.

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• The wavelength of an electromagnetic wave is
  depicted as the distance between adjacent crests
  of the oscillating fields.
• The constant speed of electromagnetic radiation
  in a vacuum is the product of the frequency n and
  the wavelength l of the electromagnetic wave.
• c ln

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• Often it is convenient to assign wavelike
  properties to electromagnetic rays.
• At other times it is useful to regard these
  radiations as discrete bundles of energy termed
  photons or quanta.
• The two interpretations of electromagnetic
  radiation are united by the equation
• E hn
• where E represents the energy of a photon and n
  represents the frequency of the electromagnetic
  wave. The symbol h represents Plancks constant,
  6.62 x 10 -34 J-sec.

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• The frequency n is
• n c/l
• and the photon energy may be written as
• E (hc)/l

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• The energy in keV possessed by a photon of
  wavelength, l, in nanometers is
• E 1.24/l
• Electromagnetic waves ranging in energy from a
  few nano-electron volts up to gigaelectron volts
  make up the electromagnetic spectrum.

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Ultraviolet Light

• Ultraviolet (UV) light is usually characterized
  as nonionizing.
• UV light is used to
• sterilize medical instruments,
• destroy cells,
• produce cosmetic tanning, and
• treat certain dermatologic conditions

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Visible Light

• Visible light is the part of the EM spectrum to
  which the retina is most sensitive.

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Infrared

• Infrared is the energy released as heat by
  materials near room temperature.
• Infrared sensitive devices can record heat
  signatures.
• To date, they have not found a good medical
  imaging application.

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• The radiation resulting from a vacancy in the K
  shell is called a K-characteristic x-ray.
• The radiation resulting from a vacancy in the L
  shell is called a L-characteristic x-ray.

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• If the vacancy in one shell is filled by the
  adjacent shell it is identified by a subscript
  alpha.
• L ? K transition Ka and
• M ? L transition La

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• If the electron vacancy is filled from a
  nonadjacent shell it is identified by a subscript
  beta.
• M ? K transition Kb

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• The energy of the characteristic x-ray is the
  difference between the electron binding energies
  (Eb) of the respective shells.

-2.5 keV
-11 keV
-69.5 keV
-67 keV Kb Characteristic X-Ray
M
L
K
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Auger Electrons

• An electron cascade does not always result in a
  characteristic x-ray.
• A competing process that predominates in low Z
  elements is Auger electron emission.
• The energy released is transferred to an orbital
  electron, typically in the same shell as the
  cascading electron.

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-2.5 keV
-11 keV


-69.5 keV
64.5 keV Auger electron

Vacant

M
L
K
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• If the transferred energy exceeds the binding
  energy of the electron, ionization occurs,
  whereby the electron is ejected from the atom.
• The result of ionization is an ion pair
  consisting of the ejected electron and the
  positively charged atom.
• Sometimes the ejected electrons possess enough
  energy to produce further ionizations called
  secondary ionization.

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• The term interaction may be used to describe
• the crash of two automobiles
• (an example in the macroscopic world) or
• the collision of an x ray with an atom
• (an example in the submicroscopic world).

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• Interactions in both macroscopic and microscopic
  scales follow fundamental principles of physics
  such as
• (a) the conservation of energy and
• (b) the conservation of momentum.

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• Neutrons are uncharged particles.
• Neutrons do not interact with electrons and do
  not directly cause excitation and ionization.
• They do, however, interact with atomic nuclei,
  sometimes liberating charged particles or nuclear
  fragments.
• Neutrons may also be captured by atomic nuclei.

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• In some cases the neutron is reemitted.
• In other cases the neutron is retained,
  converting the atom to a difference nuclide.
• In this case, the binding energy my be emitted
  via spontaneous gamma-ray emission

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Neutron
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• Pair production only occurs when the energies of
  x-ray or gamma ray exceed 1.02 MeV.
• An x-ray or gamma ray interacts with the electric
  field of the nucleus of an atom.

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0.51 MeV
annihilation

• The photons energy is transformed into an
  electron-positron pair.

b
0.51 MeV
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• When the positron (a form of antimatter) comes to
  rest, it interacts with a negatively charged
  electron, resulting in the formation of two
  oppositely directed 0.511 MeV annihilation
  photons.
• Positrons are important in imaging of positron
  emitting radiopharmaceuticals in which the
  resultant annihilation photon pairs emitted from
  the patient are detected by positron emission
  tomography (PET) scanners.

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Ionizing Radiation
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