1. X-rays : Their Production Their Interaction with Matter
and
Their Interaction with Matter

2. The Electromagnetic Spectrum
1 electron Volt = 1.6 x Joules
E = hn

3. Forms of Electromagnetic Waves
Wave form
Wavelength
Frequency (Hz)
Photon energy
(eV)
Gamma
< 0.001nm
3 x 1020
1.2 MeV
X-ray
0.001nm to 0.9nm
3 x 1020 to
3 x1017
1.2 x MeV to
1.2keV
Ultraviolet
0.9nm to 350nm
3 x1017 to
8.6x1014
1.2x1keV to
3.6eV
Visible
350nm to 750nm
8.6x1014 to 4x1014
3.6eV to 1.6eV

4. The production of x-rays
X-rays are the result of highly energetic processes.
X-rays used in radiography are the result of accelerating electrons
to velocities of a hundred keV before allowing them to collide with
a heavy metal, tungsten target.

5. X-ray Spectrum
1: Continuum
2: Characteristic x-rays

6. X-ray Spectrum
Continuous spectrum
Braking Radiation
a fast moving electron comes very close to the nucleus.
strong Coulomb attraction means the electron is accelerated.
radiation produced due to the braking of the electron by the
nucleus is called “Bremsstrahlung”

7. The production of characteristic X-raysK L M
high velocity electron
ejected K band
electron
hole in K shell K L M
electron from L shell
falls into K shell
K X-ray
High energy electron knocks out inner K electron from atom

8. Energy Levels in Targets
Shell
Occupancy
1 N
18 M
8 L
2 K
Shell
Occupancy
2 P
12 O
32 N
18 M
8 L
2 K
keV
0.01
0.07
0.9
8.98
keV
0.02
0.06
0.50
2.5
10.2
69.5
K ALPHA 59.3 keV
Copper Z=29
Tungsten Z=74

9. The Interaction of X-rays with Matter
Coherent scattering
Coherent scattering results when the incident X-ray interacts with an atom and is scattered in a new direction without loss of energy. It is of minor importance in absorption processes in the 20 keV to 100keV range of energies.
Photoelectric effect
In the photoelectric effect the X-ray ejects electrons from the inner shells of the atoms producing photoelectrons, positive ions and characteristic X-ray emission.
Compton effect
In the Compton Effect the X-ray photon strikes an outer shell electron ejecting it from its orbit. The scattered X-ray moves off in a different direction with slightly lower energy.

10. The Interaction of X-rays with Matter

11. The Interaction of X-rays with Matter
Heggie et al 2001

12. The Interaction of X-rays with Matter
Coherent scattering
Interaction between photon and bound electrons.
Photon changes direction but without energy change
Also known as Rayleigh Scattering
Dominates at low energies and large Z

13. The Interaction of X-rays with Matter
2. Photoelectric effect
For photon energies > electron binding energies
energy of photon totally absorbed by atom
( single atom event)
electron ejected from atom
characteristic x-ray produced in subsequent cascade
positive ion remains
Probability related to:
electron energy match to photon energy
Attenuation coefficient m/r proportional to:
Ephoton3 Z3

14. The Interaction of X-rays with Matter
Compton Scattering
For photon energies >> electron binding energies
outer electrons act independently of nucleus
collisonal process between photon and electron
energy/ momentujm transfer from photon to electron
produces a:
positive ion
free electron
photon with different direction and lower energy
Diagnostic Imaging 10 keV to 150 keV
Outer electrons in High Z materials appear free
All electrons in soft tissue appear free

15. The Interaction of X-rays with Matter
Compton Scattering K L M

16. Radiography
The X-rays are
1: scattered from the beam by Compton scattering
2: absorbed from the beam by the photoelectric effect.
3: scattered X-rays and the primary X-rays then fall on the X-ray film or X-ray intensifier

17. Scattered x-rays produce a fog background
Primary x-rays produce the image

19. Attenuation and Half Value Layers
attenuated primary beam No Io Dx
scattered beam
Loss from beam
N = No exp(-m x)
Half Value Layer
When is N = No/2?
exp(-m x) = ½
HVL = 0.693/ m
-DN = mNo Dx

20. Attenuation and Half Value Layers
Mass attenuation coefficient m
Depends on energy of photons
100 10 1
60 keV photons
Transmitted intensity %
60 kVp +2.5mm Al
Beam hardening occurs since m30kev> m60kev> m100kev
Spectrum changes as low energy photons are
preferentially absorbed