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Ch. 5: Interactions of Ionizing Radiation
N/A
26
Physics
07/18/2011

Term
 What types of particles are directly ionizing?
Definition
 Charged particles, such as electrons, protons, and α-particles, provided they have sufficient kinetic energy.
Term
 What types of particles are indirectly ionizing?
Definition
 Uncharged particles, such as neutrons and photons, provided they have sufficient kinetic energy.
Term
 An ejected electron that receives sufficient energy from ionizing radiation to produce a secondary ionization track of its own is called a ______.
Definition
 delta (δ) ray
Term
 What are the three major processes by which ionizing photons produce high-speed electrons in medium?
Definition
 (1) Photoelectric effect(2) Compton scattering (3) Pair Production
Term
 A. What is the fluence of photons?   B. What is its symbol?   C. Give the formula for photon fluence.
Definition
 A. The fluence of photons is the number of photons entering per unit cross-sectional area.   B. Photon fluence, Φ   C. Φ = dN/da,   where dN = number of photons, da = cross-sectional area
Term
 A. What is fluence rate?  What is another name for fluence rate?   B. What is its symbol?   C. Give the formula for fluence rate.
Definition
 A. Fluence rate is the photon fluence per unit time, also called flux density.   B. Fluence rate, φ   C. φ = dΦ/dt
Term
 A. What is energy fluence?   B. What is its symbol?   C. Give the formula for energy fluence.   D. How can this formula be simplified for a monoenergetic beam?
Definition
 A. Energy fluence is the ratio of energy entering (here, in the form of photons) per unit cross-sectional area.   B. Energy fluence, Ψ   C. Ψ = dEfl / da,   where dEfl = the sum of all photons that enter a cross-sectional area   D. In the case of a monoenergetic photon beam: Ψ = dN*hν / da
Term
 A. What is energy fluence rate?  What is another name for it?   B. What is its symbol?   C. Give the formula for energy fluence rate.
Definition
 A. Energy fluence rate is the measure of energy fluence per unit time.  It is also known as energy flux density.   B. Energy fluence rate, ψ   C. ψ = Ψ/dt
Term
 What formula describes the attenuation of photons in matter?
Definition
 dN is proportional to Ndx,   where dN is the number of photons absorbed, N is the number of incident photons, and dx is the thickness of absorbing material   dN = -μNdx,   where μ is a proportionality constant called the attenuation coefficient.
Term
 Give the formula for photon beam attenuation in terms of intensity.
Definition
 dI = -μIdx   dI/I = -μdx   Solving the differential equation,   I(x) = I0e-μx   Note: This formula only applies to monoenergetic beams.
Term
 When discussing photon beam attenuation, if absorber thickness is given as a length, what is 'μ' called?
Definition
 The linear attenuation coefficient
Term
 A. What is the half-value layer (HVL)?   B. Give the formula relating HVL to attenuation coefficient.
Definition
 A. The half-value layer, HVL, is the thickness of absorber required to attenuate the intensity of a photon beam to half its initial value.  It is analagous to half-life.   B. HVL = 0.693/μ
Term
 For a heterogeneous photon beam, place the following in order from least to greatest:   HVL1HVL2HVL3
Definition
 HVL1 < HVL2 < HVL3   This is due to the hardening of the beam by filtration.
Term
 A. What is the mass attenuation coefficient?   B. What is its symbol?   C. What are its units?   D. What units are its corresponding 'thickness' expressed in?   E. Why is it used?
Definition
 A. The mass attenuation coefficient relates the absorption of a photon beam to the thickness of material independent of its density.   B. Mass attenuation coefficient, μ/ρ   C. μ/ρ = cm-1 / (g/cm3) = cm2/g   D. ρx is given in g/cm2   E. The mass attenuation coefficient is more fundamental than the linear attenuation coefficient because it depends not on the density of the absorbing material but only on its atomic composition.
Term
 A. One coefficient used in describing photon beam attenuation is the electronic attenuation coefficient.  What is its symbol?    B. What are its units?   C. What units are its corresponding 'thickness' expressed in?
Definition
 A. Electronic attenuation coefficient, eμ    B. eμ = μ/ρ * 1/N0 cm2/electron,   where N0 is the number of electrons per gram:   N0 = NA * Z / AW   C. electrons / cm2
Term
 A. One coefficient used in describing photon beam attenuation is the atomic attenuation coefficient.  What is its symbol?    B. What are its units?   C. What units are its corresponding 'thickness' expressed in?
Definition
 A. Atomic attenuation coefficient, aμ    B. aμ = μ/ρ * Z/N0 = μ/ρ * AW/NA cm2/atom,   where N0 is the number of electrons per gram:   N0 = NA * Z / AW   C. atoms / cm2
Term
 A. What is the energy transfer coefficient?   B. What is its symbol?   C. Give the formula that describes energy transfer coefficient.
Definition
 A. The energy transfer coefficient is the fraction of photon energy transferred into kinetic energy of charged particles per unit thickness of absorber.   B. Energy transfer coefficient, μtr   C. μtr = (Etr/hν)μ,   where Etr is the average energy transferred per interaction
Term
 A. What is the energy absorption coefficient?   B. What is its symbol?   C. Give the formula that describes energy absorption coefficient.
Definition
 A. The energy absorption coefficient is the fraction of photon energy transferred into kinetic energy of locally-interacting (i.e., ionization and excitation) charged particles per unit thickness of absorber.  Energy absorption coefficient ignores interactions in which produced charged particles suffer bremsstrahlung interactions and do not deposit their energy locally.   B. Energy absorption coefficient, μen   C. μen = μtr(1-g),   where g is the fraction of energy of secondary particles lost to bremsstrahlung interactions
Term
 Compton scattering, coherent scattering, the photoelectric effect, and pair production each have their own linear attenuation coefficients.  Give the symbol for each.
Definition
 Coherent scattering: σcoh   Compton scattering: σc   Photoelectric effect: τ   Pair production: Π
Term
 A. Describe the process of coherent scattering.   B. What are two other names for it?   C. Describe the probability of coherent scattering as it relates to photon beam energy and atomic number of the absorbing material.
Definition
 A. In coherent scattering, a passing electromagnetic wave is absorbed by an electron which then begins to oscillate (is excited).  The oscillating electron re-emits the energy as an electromagnetic wave of the same frequency, but at a slightly different angle.  No energy is absorbed in the medium.   B. (1) Rayleigh scattering(2) Classical scattering   C. Coherent scattering increases with Z, decreases with energy.
Term
 A. Describe the process of the photoelectric effect.   B. Describe the kinetic energy of the ejected electron.   C. What two types of emissions can occur following a photoelectric interaction?   D. Describe the probability of photoelectric effect as it relates to photon beam energy and atomic number of the absorbing material.
Definition
 A. In a photoelectric interaction, the entire energy of an incident photon is absorbed by an atom, ejecting an orbital electron.   B. K.E. = hv - EB   C.(1) Characteristic x-rays(2) Auger electrons, which are monoenergetic electrons produced by the internal absorption of characteristic x-rays   D. Photoelectric effect is proportional to Z3/E3
Term
 What is an absorption edge?
Definition
 An absorption edge is a spike in the curve of mass photoelectric attenuation coefficient (τ/ρ) vs. photon energy that corresponds to the binding energy of a specific orbital electron.  When the energy of a photon is exactly equal to that of an orbital electron, resonance occurs and the probability of P.E. becomes extremely high.
Term
 Describe the process of the Compton effect.
Definition
 The Compton process generally occurs when the energy of the incident photon beam is much greater than the binding energy of the orbital electrons involved.  Thus, the photon is interacting with essentially a "free" electron.  The electron receives a certain energy from the incident photon (hv0) and is emitted at angle θ.  The scattered photon (hv') is deflected at an angle φ.
Term
 Describe the probability of the Compton effect as it relates to photon beam energy and atomic number of the absorbing material.
Definition
 As the energy of an incident photon beam increases past the K-shell binding energy of the absorbing material, the photoelectric effect decreases drastically and the Compton effect becomes more important.  Its probability, however, also decreases with photon energy.   Because the electrons are essentially "free" (relatively low binding energy compared to incident photon energy), the probability of Compton scattering is independent of Z.   Compton scattering is proportional to electron density, ρe, which is a measure of the number of electrons per volume.   To find electron density, simply multiply density by electrons per gram.   g/cm3 * electrons/g = electrons/cm3
Term
 A. Describe the process of pair production.   B. What is the threshold energy of the incident photon beam required for pair production to occur.  Why?   C. What is the reverse of this process called?
Definition
 A. In pair production, an incident photon interacts with the electromagnetic field of an atomic nucleus, giving up all of its energy in the creation of an electron and positron.  The electron and positron tend to be emitted in the forward direction.    B. The threshold energy for pair production to occur is ~1.022 MeV.  This is because the rest mass energy of the electron (and positron) is 0.511 MeV.   C. The reverse of pair production is annihilation radiation, where a positron and electron combine to form two photons, each with energy = 0.511 MeV, travelling in opposite directions.
Term
 Describe the probability of pair production as it relates to photon beam energy and atomic number of the absorbing material.
Definition
 Probability of pair production increases with energy beyond the 1.022 MeV threshold.  Probability also increases with atomic number (~Z2 per atom, Z per electron, Z per gram)
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