Photon Interactions with Matter

Question 1

What are the four main processes by which photons interact with matter?

Answer 1

1. Coherent scattering = Rayleigh scattering = Thompson scattering 2. Photoelectric effect 3. Compton scattering 4. Pair production

Question 2

There are two energy regimes one is lower-energy and the other is high-energy, what is the cut-off in number in KeV?

Answer 2

100 KeV

Question 3

At the lowest energies what type of interaction is the main?

Answer 3

Coherent scattering

Question 4

What is unique about Coherent scattering?

Answer 4

No energy is gained nor lost, thus is NOT relevant for medical applications

Question 5

What are the two types of Coherent scattering?

Answer 5

Thompson and Rayleigh scattering

Question 6

Define Thompson scattering

Answer 6

A type of coherent scattering in which the photon scatters from a free electron

Question 7

Define Rayleigh scattering

Answer 7

A type of coherent scattering in which the photon scatters from an electron bound in an atom.

Question 8

Give an example of Rayleigh scattering from molecules in the atmosphere?

Answer 8

The reason the sky is blue.

Question 9

What is photoelectric effect?

Answer 9

a photon interacting with an electron in an atom

Question 10

What are the 4 types of particles that may be produced from photoelectric interactions?

Answer 10

1. Photoelectrons 2. Auger electrons 3. Characteristic X-rays 4. Ionized atoms

Question 11

In what energy range is the photoelectric effect the most dominant type?

Answer 11

below ~ 100 KeV (diagnostic energy range)

Question 12

What is mass attenuation coefficient?

Answer 12

This quantity is related to the probability of interaction of photon (think of the graph of mass absorption coefficient vs. energy)

Question 13

What is the unit of mass attenuation?

Answer 13

cm^2/g

Question 14

In the lower energy range below 100 KeV, how does the mass absorption coefficient behave?

Answer 14

In lower energy range, the mass absorption coefficient decreases as energy increases (the scaling is ~ 1/E^3)

Question 15

At lower energy range, rank the order of mass absorption coefficients of lead, bone, muscle in order from the highest to the lowest

Answer 15

Lead > Bone > Muscle

Question 16

Why is the mass absorption coefficient of lead higher than the one of muscle?

Answer 16

Because of the higher number atomic number of lead Z = 82 vs. muscle (the mass absorption coefficient scales are approximately Z^3)

Question 17

In what energy and Z (atomic number) are desired for photoelectric effect?

Answer 17

lower energy and higher Z ( ~ Z^3/E^3)

Question 18

At what energy range is Compton scattering becomes dominant?

Answer 18

when the energy of photon starts to exceed ~ 100 KeV

Question 19

Which one is more important for Compton scattering? energy of the incident photon or number of electrons per unit gram

Answer 19

number of electrons per unit gram in the material as more electrons results in more scattering events

Question 20

Which particular element make a material more electron rich?

Answer 20

Materials with higher hydrogen atom content have more electrons per unit gram.

Question 21

Which one has fewer electrons? High Z element or low Z element

Answer 21

High Z element, b/c they have fewer protons since these elements tend to be more neutron-rich and therefore fewer electrons

Question 22

Which materials tend to have higher probability for Compton interaction?

Answer 22

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.

Question 23

What determines the scattering angle in Compton scattering?

Answer 23

Energy of the incident photon

Question 24

At low energy such as 1 KeV the scattering is usually

Answer 24

symmetric

Question 25

At higher energy the scattering becomes more

Answer 25

asymmetric

Question 26

At high energy such as 10 MeV, the Compton scattered in what direction?

Answer 26

More forward direction rather than the backward direction.

Question 27

Which interaction of photons and matter dominant in the highest energy such as higher than 1 MeV?

Answer 27

Pair Production

Question 28

Describe pair production

Answer 28

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+).

Question 29

What two factors contribute to higher probability of pair production?

Answer 29

1. Energy above 1.02 MeV 2. Larger for higher Z nuclei

Question 30

Draw the mass attenuation coefficient vs Energy graph showing both water and titanium for photoelectric interaction, Compton, and Pair production.

Answer 30

Question 31

What is photonuclear reaction or photodisintegration?

Answer 31

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

Question 32

For pair production to occur, what is the minimum incoming photon energy from a high energy photon?

Answer 32

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.