02 -interaction_of_radiation_with_matter_i
DESCRIPTION
RADIATION &MATTERTRANSCRIPT
Interaction of Radiation with Matter I
Particle interactions
• Energetic charged particles interact with matter by electrical forces and lose kinetic energy via:– Excitation– Ionization– Radiative losses
• ~ 70% of charged particle energy deposition leads to nonionizing excitation
Specific Ionization
• Number of primary and secondary ion pairs produced per unit length of charged particle’s path is called specific ionization– Expressed in ion pairs (IP)/mm
• Increases with electrical charge of particle
• Decreases with incident particle velocity
Specific ionization for 7.69 MeV alpha particle from polonium 214
Charged Particle Tracks
• Electrons follow tortuous paths in matter as the result of multiple scattering events– Ionization track is sparse and nonuniform
• Larger mass of heavy charged particle results in dense and usually linear ionization track
• Path length is actual distance particle travels; range is actual depth of penetration in matter
Path lengths vs. ranges
Linear Energy Transfer
• Amount of energy deposited per unit path length is called the linear energy transfer (LET)
• Expressed in units of eV/cm• LET of a charged particle is proportional to the
square of the charge and inversely proportional to its kinetic energy
• High LET radiations (alpha particles, protons, etc.) are more damaging to tissue than low LET radiations (electrons, gamma and x-rays)
Bremsstrahlung
Bremsstrahlung
• Probability of bremsstrahlung production per atom is proportional to the square of Z of the absorber
• Energy emission via bremsstrahlung varies inversely with the square of the mass of the incident particle– Protons and alpha particles produce less than
one-millionth the amount of bremsstrahlung radiation as electrons of the same energy
Bremsstrahlung
• Ratio of electron energy loss by bremsstrahlung production to that lost by excitation and ionization = EZ/820– E = kinetic energy of incident electron in MeV
– Z = atomic number of the absorber
• Bremsstrahlung x-ray production accounts for ~1% of energy loss when 100 keV electrons collide with a tungsten (Z = 74) target in an x-ray tube
Neutron interactions
• Neutrons are uncharged particles• They do not interact with electrons
– Do not directly cause excitation or ionization
• They do interact with atomic nuclei, sometimes liberating charged particles or nuclear fragments that can directly cause excitation or ionization
• Neutrons may also be captured by atomic nuclei– Retention of the neutron converts the atom to a
different nuclide (stable or radioactive)
Neutron interaction
X- and Gamma-Ray Interactions
• Rayleigh scattering
• Compton scattering
• Photoelectric absorption
• Pair production
Rayleigh Scattering
• Incident photon interacts with and excites the total atom as opposed to individual electrons
• Occurs mainly with very low energy diagnostic x-rays, as used in mammography (15 to 30 keV)
• Less than 5% of interactions in soft tissue above 70 keV; at most only 12% at ~30 keV
Rayleigh Scattering
Compton Scattering
• Predominant interaction in the diagnostic energy range with soft tissue
• Most likely to occur between photons and outer (“valence”) shell electrons
• Electron ejected from the atom; photon scattered with reduction in energy
• Binding energy comparatively small and can be ignored
Compton Scattering
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Compton scatter probabilities
• As incident photon energy increases, scattered photons and electrons are scattered more toward the forward direction
• These photons are much more likely to be detected by the image receptor, reducing image contrast
• Probability of interaction increases as incident photon energy increases; probability also depends on electron density– Number of electrons/gram fairly constant in tissue;
probability of Compton scatter/unit mass independent of Z
Relative Compton scatter probabilities
Compton Scattering
• Laws of conservation of energy and momentum place limits on both scattering angle and energy transfer
• Maximal energy transfer to the Compton electron occurs with a 180-degree photon backscatter
• Scattering angle for ejected electron cannot exceed 90 degrees
• Energy of the scattered electron is usually absorbed near the scattering site
Compton Scattering
• Incident photon energy must be substantially greater than the electron’s binding energy before a Compton interaction is likely to take place
• Probability of a Compton interaction increases with increasing incident photon energy
• Probability also depends on electron density (number of electrons/g density)– With exception of hydrogen, total number of electrons/g
fairly constant in tissue– Probability of Compton scatter per unit mass nearly
independent of Z
Photoelectric absorption
• All of the incident photon energy is transferred to an electron, which is ejected from the atom
• Kinetic energy of ejected photoelectron (Ec) is equal to incident photon energy (E0) minus the binding energy of the orbital electron (Eb)
Ec = Eo - Eb
Photoelectric absorption (I-131)
Photoelectric absorption
• Incident photon energy must be greater than or equal to the binding energy of the ejected photon
• Atom is ionized, with an inner shell vacancy• Electron cascade from outer to inner shells
– Characteristic x-rays or Auger electrons
• Probability of characteristic x-ray emission decreases as Z decreases– Does not occur frequently for diagnostic energy photon
interactions in soft tissue
Photoelectric absorption (I-131)
Photoelectric absorption
• Probability of photoelectric absorption per unit mass is approximately proportional to
• No additional nonprimary photons to degrade the image
• Energy dependence explains, in part, why image contrast decreases with higher x-ray energies
33 / EZ
Photoelectric absorption
• Although probability of photoelectric effect decreases with increasing photon energy, there is an exception
• Graph of probability of photoelectric effect, as a function of photon energy, exhibits sharp discontinuities called absorption edges
• Photon energy corresponding to an absorption edge is the binding energy of electrons in a particular shell or subshell
Photoelectric mass attenuation coefficients
Photoelectric absorption
• At photon energies below 50 keV, photoelectric effect plays an important role in imaging soft tissue
• Process can be used to amplify differences in attenuation between tissues with slightly different atomic numbers, improving image contrast
• Photoelectric process predominates when lower energy photons interact with high Z materials (screen phosphors, radiographic constrast agents, bone)
Percentage of Compton and photoelectric contributions
Pair production
• Can only occur when the energy of the photon exceeds 1.02 MeV
• Photon interacts with electric field of the nucleus; energy transformed into an electron-positron pair
• Of no consequence in diagnostic x-ray imaging because of high energies required
Pair Production