3
IONIZING RADIATION
3.1 PHOTON INTERACTIONS
What are the five major types of photon interactions with matter?
Question 2
What is Rayleigh scattering?
Question 3
What is the photoelectric effect?
Question 4
What is Compton scattering?
The five major types of photon interactions with matter in order of energy threshold are (1) Rayleigh scattering, also known as coherent scattering, classical scattering, or elastic scattering; (2) photoelectric effect, also known as photoelectric absorption; (3) Compton scattering, also known as incoherent scattering, or Compton effect; (4) pair production; and (5) photodisintegration or photonuclear.
Answer 2
In Rayleigh scattering, the incident photon interacts with the atom as a whole, scattering off in a different direction without losing energy.
Answer 3
In the photoelectric effect, the entire energy of the incident photon is transferred to an orbital electron, which is then ejected from the atom. The energy of the ejected electron (photoelectron) is the energy of the incident photon minus the binding energy. Characteristic X-ray or Auger electron emission will subsequently occur, filling the vacancy of the photoelectron.
Answer 4
Compton scattering is a collision between a photon and a loosely bound outer shell orbital electron of an atom, resulting in a scattered photon of lower energy and an electron. This is the dominant interaction of therapeutic X-ray beams with tissue. Because the incident photon energy greatly exceeds the binding energy of the outer shell electron, the Compton interaction looks like a collision between the photon and a free electron (termed nearly free).
Question 6
What is photodisintegration?
Question 7
With one incoming photon, what are the outgoing particles produced during Rayleigh scattering, photoelectric effect, Compton scattering, and pair production, respectively?
Question 8
Does ionization occur in Rayleigh scattering?
In pair production, the photon interacts strongly with the electromagnetic field of an atomic nucleus and gives up all of its energy to create an electron and a positron.
Answer 6
In photodisintegration, an incident photon with high energy (>7 to 10 MeV) collides with the nucleus of an atom and all of the photon’s energy gets absorbed. The nucleus then emits a neutron. This effect causes extra shielding requirements for high-energy beams.
Answer 7
With one incoming photon, the outgoing particles produced by the four photon interactions are listed as follows:
Rayleigh scattering: one photon
Photoelectric effect: one electron
Compton scattering: one electron and one photon
Pair production: one electron and one positron
Answer 8
Ionization does not occur in Rayleigh scattering because electrons are not ejected.
Question 10
What is the dependence of Compton scattering on atomic number (Z), energy (E), and electron density (Ne)?
Question 11
What is the dependence of pair production on atomic number (Z) and energy (E)?
Question 12
Is Compton scattering most likely to occur with inner or outer shell electrons?
The photoelectric effect is proportional to Z3/E3, where Z is the atomic number of the target atom, and E is the energy of the incident photon.
Answer 10
The probability of Compton scattering is independent of Z and decreases with E. Compton scattering depends on the electron density Ne of the material.
Answer 11
The probability of pair production changes with Z2 and log(E).
Answer 12
Compton scattering is most likely to occur with outer shell electrons.
Question 14
Which interaction is the main cause for the absorption in water of photons with energy 10 to 100 keV?
Question 15
Which interaction is the main cause for the absorption in water of photons with energy 100 keV to 10 MeV?
Question 16
When is the photoelectric effect most probable with respect to the energy of the photon and the electron binding energy?
The minimum energy of an incident photon for the pair-production process is 1.02 MeV. This energy corresponds to the mass of the electron and positron produced (0.511 MeV each). Additional energy above the 1.02 MeV threshold is carried away as kinetic energy by the pair.
Answer 14
Photoelectric effect is the main cause for the absorption in water of photons with energy 10 to 100 keV. This is the range of diagnostic X-rays. The dependence of the absorption on atomic number Z3 provides the excellent contrast of diagnostic X-ray images.
Answer 15
Compton scattering is the main cause for the absorption in water of photons with energy of 100 keV–10 MeV. This is the range of therapeutic X-rays.
Answer 16
The photoelectric effect is most probable when the energy of the photon is slightly higher than the electron binding energy. In the absorption spectrum, the shell binding energies appear as sharp peaks known as “absorption edges,” labeled by the corresponding shell, for example, K-edge, for absorption from the inner most shell.
Question 18
At what energy are the probabilities of Compton scattering and photoelectric effect about the same for soft tissue?
Related posts:
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
