Photon Interactions with Matter Flashcards
What are the four main processes by which photons interact with matter?
- Coherent scattering = Rayleigh scattering = Thompson scattering 2. Photoelectric effect 3. Compton scattering 4. Pair production
There are two energy regimes one is lower-energy and the other is high-energy, what is the cut-off in number in KeV?
100 KeV
At the lowest energies what type of interaction is the main?
Coherent scattering
What is unique about Coherent scattering?
No energy is gained nor lost, thus is NOT relevant for medical applications
What are the two types of Coherent scattering?
Thompson and Rayleigh scattering
Define Thompson scattering
A type of coherent scattering in which the photon scatters from a free electron
Define Rayleigh scattering
A type of coherent scattering in which the photon scatters from an electron bound in an atom.
Give an example of Rayleigh scattering from molecules in the atmosphere?
The reason the sky is blue.
What is photoelectric effect?
a photon interacting with an electron in an atom
What are the 4 types of particles that may be produced from photoelectric interactions?
- Photoelectrons 2. Auger electrons 3. Characteristic X-rays 4. Ionized atoms
In what energy range is the photoelectric effect the most dominant type?
below ~ 100 KeV (diagnostic energy range)
What is mass attenuation coefficient?
This quantity is related to the probability of interaction of photon (think of the graph of mass absorption coefficient vs. energy)
What is the unit of mass attenuation?
cm^2/g
In the lower energy range below 100 KeV, how does the mass absorption coefficient behave?
In lower energy range, the mass absorption coefficient decreases as energy increases (the scaling is ~ 1/E^3)
At lower energy range, rank the order of mass absorption coefficients of lead, bone, muscle in order from the highest to the lowest
Lead > Bone > Muscle
Why is the mass absorption coefficient of lead higher than the one of muscle?
Because of the higher number atomic number of lead Z = 82 vs. muscle (the mass absorption coefficient scales are approximately Z^3)
In what energy and Z (atomic number) are desired for photoelectric effect?
lower energy and higher Z ( ~ Z^3/E^3)
At what energy range is Compton scattering becomes dominant?
when the energy of photon starts to exceed ~ 100 KeV
Which one is more important for Compton scattering? energy of the incident photon or number of electrons per unit gram
number of electrons per unit gram in the material as more electrons results in more scattering events
Which particular element make a material more electron rich?
Materials with higher hydrogen atom content have more electrons per unit gram.
Which one has fewer electrons? High Z element or low Z element
High Z element, b/c they have fewer protons since these elements tend to be more neutron-rich and therefore fewer electrons
Which materials tend to have higher probability for Compton interaction?
materials with more electrons per gram in low-Z elements, hydrogen-rich materials such as plastics, water, or tissues compared to high-Z materials such as bone, or metals.
What determines the scattering angle in Compton scattering?
Energy of the incident photon
At low energy such as 1 KeV the scattering is usually
symmetric
At higher energy the scattering becomes more
asymmetric
At high energy such as 10 MeV, the Compton scattered in what direction?
More forward direction rather than the backward direction.
Which interaction of photons and matter dominant in the highest energy such as higher than 1 MeV?
Pair Production
Describe pair production
quantum mechanical interaction of the photon in the field of the nucleus. The photon is converted to a pair of particles: one electron (e-) and one positron (e+).
What two factors contribute to higher probability of pair production?
- Energy above 1.02 MeV 2. Larger for higher Z nuclei
Draw the mass attenuation coefficient vs Energy graph showing both water and titanium for photoelectric interaction, Compton, and Pair production.
What is photonuclear reaction or photodisintegration?
it’s a high-energy photon (> 10 MeV) interacting with a nucleus to eject one of the neutrons.
This can happen in high-Z material in and around the head of a linear accelerator. This creates an extra neutron compsonent which may need to be considered in shielding fo rhigh energy linear acceleratros
For pair production to occur, what is the minimum incoming photon energy from a high energy photon?
The final photon matter interaction that we consider is pair production which happens at the highest energies. This is a quantum mechanical interaction of the photon in the field of the nucleus.
The photon is converted to a pair of particles: one electron and one positron.
Because of conservation of eneergy, the total energy of the pair that is creatd is equal to the energy before. This means that the energy of the incident photon must be at least twice the rest masss energy of the electron.
