Ionizing Radiation: Photon Interaction

Question 1

What are the five major types of photon interactions with matter?

Answer 1

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.

Question 2

What is Rayleigh scattering?

Answer 2

In Rayleigh scattering, the incident photon interacts with the atom as a whole, scattering off in a different direction without losing energy.

Question 3

What is the photoelectric effect?

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.

Question 4

What is Compton scattering?

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 5

What is pair production?

Answer 5

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.

Question 6

What is photodisintegration?

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.

Question 7

With one incoming photon, what are the outgoing particles produced during Rayleigh scattering, photoelectric effect, Compton scattering, and pair production, respectively?

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

Question 8

Does ionization occur in Rayleigh scattering?

Answer 8

Ionization does not occur in Rayleigh scattering because electrons are not ejected.

Question 9

What is the dependence of the photoelectric effect on atomic number (Z ) and energy (E)?

Answer 9

The photoelectric effect is proportional to Z^3/E^3, where Z is the atomic number of the target atom, and E is the energy of the incident photon.

Question 10

What is the dependence of Compton scattering on atomic number (Z ), energy (E), and electron density (Ne)?

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.

Question 11

What is the dependence of pair production on atomic number (Z ) and energy (E)?

Answer 11

The probability of pair production changes with Z^2 and log(E).

Question 12

Is Compton scattering most likely to occur with inner or outer shell electrons?

Answer 12

Compton scattering is most likely to occur with outer shell electrons.

Question 13

What is the minimum energy of an incident photon for the pair-production process?

Answer 13

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.

Question 14

Which interaction is the main cause for the absorption in water of photons with energy 10 to 100 keV?

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 Z 3 provides the excellent contrast of diagnostic X-ray images.

Question 15

Which interaction is the main cause for the absorption in water of photons with energy 100 keV to 10 MeV?

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.

Question 16

When is the photoelectric effect most probable with respect to the energy of the photon and the electron binding energy?

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 17

What is KERMA?

Answer 17

KERMA is the Kinetic Energy Released per unit MAss. This represents the energy transferred to the medium by the photon.

Question 18

At what energy are the probabilities of Compton scattering and photoelectric effect about the same for soft tissue?

Answer 18

The probabilities of Compton scattering and photoelectric effect are about the same at ~30 keV for soft tissue.

Question 19

In Compton scattering, as the incident photon energy increases, are the photons and electrons scattered more toward the forward or backward direction?

Answer 19

In Compton scattering, as the incident photon energy increases, both photons and electrons are scattered more toward the forward direction. Additionally, the proportion of energy carried by the scattered electron increases with incident photon energy.

Question 20

In Compton scattering, what are the maximal energies of a photon scattered through 90° and 180°?

Answer 20

In Compton scattering, the maximal energy of the scattered photon is 511 keV at 90° and 255 keV at 180° (ie, backscatter). These are energies of interest for secondary shielding.