Photon Interactions

Subgroup I: Radiation Physics and Detection

photon interactions with matter

1. photoelectric absorption

2. compton scatter

3. rayleigh scatter

4. pair production

What determines how a photon will interact with matter?

photon energy and absorber characteristics (electronic density, atomic number, and physical density)

photoelectric interactions

• a photon interacts with an atom, usually an inner shell electron

• all of the photon energy is transferred to the atom

• the photon’s energy exceeds the binding energy of the electron in the atom’s shells

• an orbital electron is ejected from the atom

During photoelectric interaction, the ejected electron is referred to as __

photoelectron

Energy of a photoelectron is equal to

the energy of the incident photon minus the binding energy of the ejected electron

photoelectrons tend to come from __ electron shells

K and L

If an inner shell vacancy is produced,

an electron will drop to fill the vacancy and release a characteristic x-ray or auger electron

absorption edges

there is a strong increase in the probability of photoelectric interaction at the absorption edge because the incident photon now exceeds the binding energy of the electron shell and the photon can now interact with more electrons

At lower energies in low atomic number materials (water), the probability of photoelectric absorption is __

>96%

At higher energies in low atomic number materials (water), the probability of photoelectric absorption is __

~0%

At lower energies in high atomic number materials (lead), the probability of photoelectric absorption is __

>99%

At higher energies in high atomic number materials (lead), the probability of photoelectric absorption is __

~50%

Compton scatter

• a photon interacts with an atom, usually an outer shell electron

• the photon is redirected with a decrease in energy

  • the photon becomes a scattered photon

• during the process, an electron is also ejected from the atom

• the energy of the photon is divided between the scattered photon and the ejected electron

In Compton scatter, the ejected electron is termed __

Compton electron

In Compton scattering, when does maximum energy to the photon occur?

at 180 degree scattering angle

In Compton scatter, a greater scattering angle will ___ energy to the incident photon and ___ energy to the electron.

decrease, increase

In Compton scatter, the higher the incident photon energy, ___ energy given to the electon.

the more

In Compton scatter, why can we ignore the binding energy of the ejected electron?

It’s quite small because it’s from an outer shell.

Compton scatter equation

hv’ = hv / ( 1 + (hv / 511) * (1 - cos θ)

Probability of Compton scatter is dependent on

• electron density of the absorber

• energy of the incident photon

Probability of Compton scatter is INdependent from

atomic number (Z) of the absorber

an increase in electron density results in ___ of Compton scatter

an increase

an increase in the energy of the incident photons results in ___ of Compton scatter

a decrease

Rayleigh scatter

• a photon interacts with the ‘whole atom’

• the electrons in the atom oscillate and emit a photon

• energy of the emitted photon is equal to the incident photon energy

• the emitted photon is on a different trajectory than the incident photon

• ionization does NOT occur

If a Rayleigh scatter photon is detected, it will

degrade image quality

Rayleigh scatter is also known as

coherent scattering

The probability of Rayleigh scatter ___ as energy of the photon increases.

decreases

Is Rayleigh scatter significant to Nuclear Medicine?

no

pair production

• the energy of a photon is converted to mass in the form of an electron and a positron

• the conversion takes place in the electric field of the atom

• the rest mass of an electron (negatron and positron) is 0.511 keV

• since electrons are created during this interaction, the incident photon must possess at least 1.022 MeV

Is pair production significant to Nuclear Medicine?

no

Is photoelectric absorption significant in Nuclear Medicine?

yes, it dominates as the first interaction in lead

Is Compton scatter significant to Nuclear Medicine?

yes, it dominates as the first interaction in tissue