Photon interaction processes Flashcards
What is the energy of the photoelectron ejected during the photoelectric process?
The initial photon energy minus the electron binding energy
What is the energy of a characteristic x-ray emitted?
The difference between two electron shell energies
Briefly, what is an Auger electron?
An outer shell electron emitted when an electron vacancy is filled.
Which materials undergo predominantly characteristic x-ray emission?
High-Z materials
What is the dependency of the photoelectric attenuation coefficient with atomic number and energy?
Proportional to Z^3/E^3
For Compton scattering, state two factors which affect the change in photon energy during scatter.
Incident photon energy; scattering angle.
At diagnostic energies, approximately what percentage of incident energy is retained by the scattered photon?
98%
What are the disadvantages of scattering in the patient?
Increased staff dose Increased patient dose Degraded image (reduced contrast)
Describe the Z and energy dependence of Compton scattering
Depends only on electron density, so high probability for hydrogenous material, independent of Z otherwise
What is the dependency of the elastic scattering attenuation coefficient with atomic number and energy?
Proportional to Z^2/E
Describe 3 ways in which a ‘K-edge’ is used in imaging
A discontinuity occurs in the mass attenuation coefficient of a material as the energy reaches the binding energy of a new electron shell. The sudden rise in absorption at this point can be used in detectors (high absorption desirable, so energies just above the k-edge of the detector are useful) or in contrast media (energies just above k-edge will give best contrast) or filters (can be used to preferentially remove energies above k-edge from the beam).
Why is pair production not observed in diagnostic imaging?
A minimum energy of 1.02 MeV is required (rest mass energy of the electron positron pair)
