Week 5 - X-Ray and Electron Interactions in Matter

Question 1

X-Ray Production

Answer 1

• X-ray tube and linear accelerators generate x-rays in a similar way
• Accelerated beam of election directed onto a thin metal target
• Electrons (negatively charged) interact with the positive electric field of the nuclei in the metal target atoms
• Electrons are deflected and lose energy

Energy absorbed by nucleus and emitted as electromagnetic radiation (Bremsstrahlung x-rays)
o Energy absorbed by nucleus is radiated out in the form of bremsstrahlung x-rays
o Continuous distribution –> until reaching maximum photon energy (approx. 150 kV)
o Characteristic Radiation –> due to ionisation which occurs in the target atoms
 Emission of characteristic x-rays

Question 2

Attenuation of an X-Ray Beam

Answer 2

X-rays
o Some images will pass straight through
 Contribute to the image

o Some will be absorbed
 More are absorbed in bone than soft tissue

o Some will be scattered
 Interact, change direction and degrade quality of image

o Interact with atoms (electrons) in the patient

o Reduction in the number off x-rays - attenuation

Question 3

X-Ray Attenuation

Answer 3

• Attenuation of the x-ray beam occurs due to absorption and scattering of the x-rays
• What influences x-rays interactions?
  o X-ray energy
  o Material through which x-rays travel
• Interactions will lead to:
  o Absorption or scatter
   There will be a probability for each type of interaction occurring

Question 4

Linear Attenuation Coefficient

Answer 4

Measure of a materials ability to attenuate an x-ray beam per unit thickness
o Thickness (cm) is a linear quality
o Large attenuation coefficient µ - high probability of attenuation per cm
o Small attenuation coefficient µ - low probability of attenuation per cm

Question 5

Total Linear Coefficient

Answer 5

• Total linear attenuation coefficient is the fraction of x-rays removed from the beam per unit thickness of the irradiated beam
• Include absorption and scattering events that can occur
• What is important is the number of atoms / unit volume
  o If the pressure or temperature changes, density changes and number of atoms / unit volume would change
   Linear attenuation coefficient would change

Question 6

Mass Attenuation Coefficient

Answer 6

Linear attenuation coefficient divided by the density of a material (p)
o Removes problems with changes in the physical environment that affect atoms / unit volume
o Unit of mass attenuation coefficient (µ/p): cm^2 / g

50 keV is better for radiology
 More attenuation between bone and soft tissue

Question 7

Attenuation Processes

Answer 7

• Each process (absorption and scatter) will have their own linear attenuation coefficient and mass attenuation coefficient

Need to understand

• Two main processes important for diagnostic x-ray imaging
• One extra for radiation therapy due to the higher energy x-rays

Photoelectric effect
Compton Scattering
Pair Production (Therapy Only)

Question 8

Photoelectric Effect (absorption process)

Answer 8

• Incident x-ray photon collide with inner election (typically k-shell)
• X-ray is completely absorbed by atom

If x-ray energy is greater than the electron binding energy
o Electron is ejected from the atom (photo-electron)
o Energy of ejected electron is equal to
 Incident x-ray energy – electron binding energy
o Usually followed by characteristic x-ray emission
 To fill vacancy on K-shell

If x-ray energy is less that electron binding energy
o Photoelectric effect cannot occur

Probability of photoelectric effect occurring
o Increases strongly with increasing atomic number Z (of the absorbing material)
o Decreases with increasing x-ray photon energy

Question 9

Compton Scattering

Answer 9

• Incident x-ray photon interacts with an outer electron – loosely bound to the atom
• X-ray energy&raquo_space;> electron biding energy
  o X-ray is scattered –> change of direction –> energy transferred to electron which recoils (freely)
  o Electron energy = energy lost by x-ray
  o Scattered photon most likely escapes target material or interact again (PE or CS)
  o Electron travels small distance (micron) losing energy in multiple interactions over the distance (ionising atoms)

o X-Rays can be scattered through angles 0 – 180
o Small energy transfer from x-ray to electron = small scattering angle
o Small incident x-ray energy = large scattering angle
o Larger incident x-ray energy = smaller scattering angle

Probability of Compton scattering
 Increases with increasing atomic number Z (of the material)
 Decreases with increasing photon energy – but not as much as with PE

Question 10

Pair Production

Answer 10

• Incident x-ray passes close to the nucleus and interacts with the nuclear field
• Absorption of the x-ray
• Energy converted to electron-positron pair (E=mc^2)
• Stationary election and positron both have mass 511 kEV (0.511 MeV)
• Incident x-ray energy needs to be atleast 2 x 0.511 MeV energy
  o Threshold energy for pair production
  o Doesn’t occur at diagnostic energies around 100 keV
• Energy greater than threshold (1.02 MeV) given to electron + positron as kinetic energy
• Positron will then annihilate with an electron
• Two gamma rays produced at 180
• Electron behaves as for CS

Question 11

Probability for Interaction Occurring

Answer 11

Three major interaction
o Photoelectric effect (absorption)
o Compton scattering
o Pair Production

• Probability of each depends on material and energy

Question 12

Attenuation of X-Rays and Gamma Rays

Answer 12

The attenuation of x-rays depends on the thickness and type of absorber

Question 13

Half-Value Thickness

Answer 13

• Thickness of absorber required to reduce the intensity (number of x-rays in the beam) to 50% of the incident beam

Question 14

Tenth-Value Thickness

Answer 14

• Thickness of absorber required to reduce the intensity to 10% of the incident beam

Question 15

Electron Interactions

Answer 15

• Electrons are charged particles with mass and therefore interact differently to photons (x-rays and gamma rays)

Interact with the electric field (coulomb field) of other charged particles
o Protons
o Orbital electrons

Through these collisions the electrons may
o Lose some of their kinetic energy (collision and radiation loss)
o Change direction of motion (scattering)

Question 16

Electron Interactions with Orbital Electrons

Answer 16

Inelastic collisions between the incident election and an orbital electron are coulomb interactions that result in:

o Atomic Ionisation
 Ejection of the orbital electron from the absorber atom

o Atomic Excitation
 Transfer of an atomic orbital electron from one allowed level (shell) to a higher allowed level

Atomic Ionisation and Excitations result in collision energy losses experienced by the incident electron
o Characterised by collision (ionisation) stopping power

Electrons lose energy in 1000s of collisions losing small among of energy in each
o Begin to slow down

Eventually are stopped

Question 17

Electron: Range

Answer 17

Electrons have a definite range 
o	They are stopped after a certain distance 
o	Red high energy 
o	Green intermediate energy 
o	Blue low energy 

• 100 keV (approx. microns)
• MeV (approx. mm)

Electron range depends on: 
o	Incident electron energy 
o	Material 
	Linear Stopping Power 
	Energy lost per unit linear distance (eV/cm)

Question 18

Electron Interactions with the Nucleus

Answer 18

May pass close to the nucleus of an atom and experience a considerable loss of energy
o Due to the large electrostatic force

• Results in the emission of an X ray photon

Energy of the X-ray photons is distributed over a range (max energy = energy of the incident)
o Depending on how close the electron passes to the atomic nucleus

Question 19

Importance of Bremsstrahlung in Diagnostic Imaging and Radiotherapy

Answer 19

• X-rays in a diagnostic x-ray unit are produced by Bremsstrahlung
• X-rays are produced in a linear accelerator by the Bremsstrahlung process

Question 20

X-Rays, Electrons and Dose

Answer 20

• Electrons cause energy to be deposited in a material (e.g., tissue)
• X-Rays interact through PE, CS, or pair production
• Result: election

The electron then travels some small distance losing energy along its track in multiple energy loss collisions (absorbed energy = absorbed dose)
o Ionisation or excitation with orbital electrons

Also, radiative losses
o Bremsstrahlung from the nucleus