1. Resident Physics Lectures (year 1)
Christensen, Chapter 4
Basic Interactions Between X-Rays and Matter
George David
Associate Professor
Medical College of Georgia
Department of Radiology

2. Basic Interactions Pair Production Photoelectric Effect
Compton Scattering

3. Pair Production Process
***
high energy photon interacts with nucleus
photon disappears
electron & positron (positive electron) created - - + ~ ~ + + ~ + - -

4. Pair Production Process
***
energy in excess of 1.02 MeV given to electron/positron pair as kinetic energy.
0.51 MeV is energy equivalent of
Electron
Positron - - + ~ ~ + + ~ + - -

5. Positron Phate Positron undergoes ANNIHILATION REACTION
Two MeV photons created
Photons emerge in exactly opposite directions

6. Pair Production Threshold energy for occurrence:
1.02 MeV
energy equivalent of rest mass of 2 electrons
Threshold is above diagnostic energies
does not occur in diagnostic radiology
Principle of PET scanning

7. Photon Phates Nothing Absorbed Scattered * X Photon passes unmolested
completely removed from beam
ceases to exist
Scattered
change in direction
no useful information carried
source of noise X

8. Primary vs Scatter Primary = Good photon Scatter = Bad photon Focal
Spot
“Good” photon
Patient
“Bad” photon X
Receptor

9. Photoelectric Effect **
photon interacts with bound (inner-shell) electron
electron liberated from atom (ionization)
photon disappears
Photon in
Electron out -

10. PHOTOELECTRIC EFFECT

11. Photoelectric Effect Exiting electron kinetic energy
****
Exiting electron kinetic energy
incident energy - electron’s binding energy
electrons in higher energy shells cascade down to fill energy void of inner shell
characteristic radiation
M to L
Electron out
Photon in -
L to K

12. Photoelectric Interaction Probability
inversely proportional to cube of photon energy
low energy event
proportional to cube of atomic number
more likely with inner (higher) shells
tightly bound electrons 1
P.E. ~
energy3
P.E. ~ Z3

13. Photoelectric Effect Interaction much more likely for
low energy photons
high atomic number elements 1
P.E. ~
energy3
P.E. ~ Z3

14. Photoelectric Effect Photon Energy Threshold binding energy depends on
> binding energy of orbital electron
binding energy depends on
atomic number
higher for increasing atomic number
shell
lower for higher (outer) shells
most likely to occur when photon energy & electron binding energy are nearly the same

15. Photoelectric Threshold
Binding Energies
K: 100
L: 50
M: 20
Photon energy: 15
Which shells are candidates for photoelectric interactions?
Photon in

16. Photoelectric Threshold
Binding Energies
K: 100
L: 50
M: 20
Photon energy: 15 NO NO NO
Which shells are candidates for photoelectric interactions?
Photon in

17. Photoelectric Threshold
Binding Energies
K: 100
L: 50
M: 20
Photon energy: 25
Which shells are candidates for photoelectric interactions?
Photon in

18. Photoelectric Threshold
Binding Energies
K: 100
L: 50
M: 20
Photon energy: 25
YES NO NO
Which shells are candidates for photoelectric interactions?
Photon in

19. Photoelectric Threshold
Binding Energies
K: 100
L: 50
M: 20
Photon energy: 25 1
P.E. ~
energy3 A
Which photon has a greater probability for photoelectric interactions with the m shell?
Photon in B
Photon energy: 22

20. Photoelectric Threshold
Binding Energies
K: 100
L: 50
M: 20
Photon energy: 55
Which shells are candidates for photoelectric interactions?
Photon in

21. Photoelectric Threshold
Binding Energies
K: 100
L: 50
M: 20
Photon energy: 55
YES
YES NO
Which shells are candidates for photoelectric interactions?
Photon in

22. Photoelectric Threshold
Binding Energies
K: 100
L: 50
M: 20
Photon energy: 105
Which shells are candidates for photoelectric interactions?
Photon in

23. Photoelectric Threshold
Binding Energies
K: 100
L: 50
M: 20
Photon energy: 105
YES
YES
YES
Which shells are candidates for photoelectric interactions?

24. Photoelectric Threshold1
P.E. ~
energy3
Photoelectric interactions decrease with increasing photon energy BUT …

25. Photoelectric Threshold**
Photoelectric Threshold
When photon energies just reaches binding energy of next (inner) shell, photoelectric interaction now possible with that shell
shell offers new candidate target electrons
L-shell interactions possible
Interaction
Probability
L-shell binding energy
K-shell interactions possible
K-shell binding energy
Photon Energy

26. Photoelectric Threshold
causes step increases in interaction probability as photon energy exceeds shell binding energies
Photon Energy
Interaction
Probability
L-edge
K-edge

27. Characteristic Radiation**
Occurs any time inner shell electron removed
energy states
orbital electrons seek lowest possible energy state
innermost shells
M to L
L to K

28. Characteristic Radiation**
electrons from higher states fall (cascade) until lowest shells are full
characteristic x-rays released whenever electron falls to lower energy state
M to L
characteristic
x-rays
L to K

29. Photoelectric Effect Why is this important?
photoelectric interactions provide subject contrast
variation in x-ray absorption for various substances
photoelectric effect does not contribute to scatter
photoelectric interactions deposit most beam energy that ends up in tissue
always use highest kVp technique consistent with imaging contrast requirements

30. Compton Scattering Source of virtually all scattered radiation Process
***
Source of virtually all scattered radiation
Process
incident photon (relatively high energy) interacts with free (loosely bound) electron
some energy transferred to recoil electron
electron liberated from atom (ionization)
emerging photon has
less energy than incident
new direction -
Electron out
(recoil electron)
Photon out
Photon in

31. Compton Scattering What is a “free” electron? low binding energy
outer shells for high Z materials
all shells for low Z materials
Electron out
(recoil electron)
Photon in
Photon out -

32. Compton Scattering
Incident photon energy split between electron & emerging photon
Fraction of energy carried by emerging photon depends on
incident photon energy
angle of deflection
similar principle to billiard ball collision
Electron out
(recoil electron)
Photon in
Photon out -

33. Compton Scattering & Angle of Deflection
Photons having small deflections retain most incident incident energy
Photons will scatter many times, losing a little energy each time. - - - -

34. Compton Scattering Probability of Occurrence
independent of atomic number (except for hydrogen)
Proportional to electron density (electrons/gram)
fairly equal for all elements except hydrogen (~ double)

35. Compton Scattering Probability of Occurrence
decreases with increasing photon energy
decrease much less pronounced than for photoelectric effect
Interaction
Probability
Compton
Photoelectric
Photon Energy

36. Photon Interaction Probabilities
100
Photoelectric
Pair Production
Z protons
COMPTON 10
E energy (MeV)