Resident%20Physics%20Lectures

1
Resident Physics Lectures

• Christensen, Chapter 2C
• Production of X-Rays

George David Associate Professor Department of
Radiology Medical College of Georgia
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The Atomic Nucleus

• Protons
• Charges
• protons atomic (Z)
• Neutrons
• No charge
• Mass about the same as proton
• Atomic Weight(mass) protons neutrons

3
kVp kilovolts peak

• peak kilovoltage applied across x-ray tube
• voltage applied across x-ray tube pulses and
  varies
• single phase
• three phase

kVp
kVp
4
keV kilo-electron volt

• energy of an electron
• Kinetic energy
• Higher energy electron moves faster
• Electrons can be manipulated by electric fields
• Accelerated
• Steered

5
Orbital Electrons

• Electrons
• - charges
• very small mass compared with protons / neutrons
• Electrons reside only at certain energy levels or
  Shells
• Designations start at K shell
• K shell closest to nucleus
• L shell next closest
• Shells proceed up from K, L, M, N, etc.
• Except for K shell, all shells contain sub-shells

L
K
-






-
-
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Binding Energy

• energy required to remove orbital electron from
  atom
• Negative electrons attracted to positive nucleus
• more binding energy for shells closer to nucleus
• K shell has highest binding force
• higher atomic materials (higher Z) result in
  more binding energy
• more positive charge in nucleus

7
Electron Shells (cont.)

• Electrons can only reside in a shell
• electron has exactly the energy associated with
  its shell
• electrons attempt to reside in lowest available
  energy shell

L
K
-






-
-
-
8
The Shell Game

• Electrons can move from shell to shell
• to move to higher energy shell requires energy
  input equal to difference between shells

L
Requiresenergyinput!
K
-


-




-
-
9
The Shell Game (cont.)

• to move to a lower energy shell requires the
  release of energy equal to the difference
  between shells
• characteristic x-rays

-
-
10
X-Ray Production(cont.)

• X-Rays are produced in the x-ray tube by two
  distinct processes
• Characteristic radiation
• Bremsstrahlung

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Characteristic Radiation

• Occurs whenever electrons drop into lower shell
• Inner shell has lower energy state
• Energy difference between shells emitted as
  characteristic x-ray
• 0-28 of total x-ray beam energy

-
-
12
Characteristic Radiation

• High speed electron from cathode slams into
  target knocking out inner shell orbital electron
• orbital electron removed from atom
• electrons from higher energy shells cascade down
  to fill vacancies
• Characteristic x-rays emitted.

-
-
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Characteristic Radiation

• Consists only of discrete x-ray energies
  corresponding to energy difference between
  electron shells of target
• Specific energies are characteristic of target
  material
• for tungsten 59 keV corresponds to the difference
  in energy between K and L shells

14
Characteristic Radiation (cont.)

• threshold energy required for incident electron
  (from cathode) to eject orbital electron
  electrons binding energy

L
K
-
-
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Bremsstrahlung

• interaction of moving electron with nucleus of
  target atoms
• Positive nucleus causes moving electron to change
  speed / direction
• Kinetic energy lost
• Emitted in form of Bremsstrahlung x-ray

-
16
Bremsstrahlung (cont.)

• Bremsstrahlung means braking radiation
• Moving electrons have many Bremsstrahlung
  reactions
• small amount of energy lost with each

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Bremsstrahlung (cont.)

• Energy lost by moving electron is random
  depends on
• distance from nucleus
• charge (Z) of nucleus
• Bremsstrahlung Energy Spectrum
• 0 - peak kilovoltage (kVp) applied to x-ray tube
• most x-ray photons low energy
• lowest energy photons dont escape tube
• easily filtered by tube enclosures or added
  filtration

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Beam Intensity

• Product of
• photons in beam
• energy per photon
• Units
• Roentgens (R) per unit time
• Measure of ionization rate of air
• Depends on
• kVp
• mA
• target material
• filtration
• waveform

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Intensity Target Material

• higher target atomic results in greater x-ray
  production efficiency
• higher positive charge of nucleus causes more
  Bremsstrahlung
• discrete energies of characteristic radiation
  determined by anode material
• Energy differences between shells
• molybdenum used in mammo
• characteristic radiation of 17 19 keV

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Intensity Technique

• beam intensity proportional to mA
• beam Intensity proportional to kVp2

filament voltage source

high voltage source