Resident Physics LecturesResident Physics Lectures
• Christensen, Chapter 2C
Production of X-Rays
George DavidAssociate ProfessorDepartment of RadiologyMedical College of Georgia
The Atomic NucleusThe Atomic Nucleus
• Protons+ Charges
# protons = atomic # (Z)
• Neutrons No charge Mass about the same as proton
Atomic Weight(mass)= # protons + # neutrons
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kVp = kilovolts peakkVp = kilovolts peak
• peak kilovoltage applied across x-ray tube
• voltage applied across x-ray tube pulses and varies single phase three phase
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Single Phase
Three Phase
kVp
kVp
keV = kilo-electron voltkeV = kilo-electron volt
• energy of an electron
• Kinetic energy
• Higher energy electron moves faster
• Electrons can be manipulated by electric fields Accelerated Steered
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Orbital ElectronsOrbital Electrons• Electrons
- chargesvery small mass compared with protons /
neutrons
• Electrons reside only at certain energy levels or ShellsShells
Designations start at K shellK shell closest to nucleusL shell next closestShells proceed up from K, L, M, N, etc.Except for K shell, all shells contain sub-shells
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K
L
Binding EnergyBinding 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
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K
L
Electron Shells (cont.)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
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K
L
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The Shell GameThe Shell Game• Electrons can move from shell to
shell
• to move to higher energy shell requires energy input equal to difference between shells
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K
L
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Requiresenergyinput!
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The Shell Game (cont.)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
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K
L
Energyreleased
X-Ray Production(cont.)X-Ray Production(cont.)
• X-Rays are produced in the x-ray tube by two distinct processes Characteristic Characteristic
radiationradiation BremsstrahlungBremsstrahlung
Characteristic RadiationCharacteristic 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
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K
L
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Characteristic RadiationCharacteristic 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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K
L
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Characteristic RadiationCharacteristic 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
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K
L
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Characteristic Radiation (cont.)Characteristic Radiation (cont.)
• threshold energy required for incident electron (from cathode) to eject orbital electron = electron’s binding energy
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K
L
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BremsstrahlungBremsstrahlung• 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
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K
L
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Bremsstrahlung (cont.)Bremsstrahlung (cont.)• Bremsstrahlung means braking braking
radiationradiation
• Moving electrons have many Bremsstrahlung reactions
» small amount of energy lost with each
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K
L
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Bremsstrahlung (cont.)Bremsstrahlung (cont.)• Energy lost by moving electron is
random & depends on distance from nucleus charge (Z) of nucleus
• Bremsstrahlung Energy Spectrum0 - peak kilovoltage (kVp) applied to x-ray tube most x-ray photons low energy lowest energy photons don’t escape tube
» easily filtered by tube enclosures or added filtration
Beam IntensityBeam 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
Intensity & Target MaterialIntensity & 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
Intensity & TechniqueIntensity & Technique
• beam intensity proportional to mA
• beam Intensity ~ proportional to kVp2
+ filamentvoltagesource
highvoltagesource