x-rays : their production and their interaction with matter
TRANSCRIPT
X-rays : Their Productionand
Their Interaction with Matter
10
700 nm600 nm500 nm400 nm
blue yellow red far red
10-15 10-9 10-6
10 -3 10 3 -12 1
Gamma rays Xrays
ultraviolet infrared microwaves radiowaves
visible light
m m m m m m m
1 electron Volt = 1.6 x 10-19 Joules
E = h
The Electromagnetic Spectrum
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 to3 x1017
1.2 x MeV to
1.2keV
Ultraviolet 0.9nm to 350nm
3 x1017 to8.6x1014
1.2x1keV to3.6eV
Visible 350nm to 750nm
8.6x1014 to 4x1014
3.6eV to 1.6eV
Forms of Electromagnetic Waves
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.
electron source
100 kV -ve+ve
tungsten target
anode cathode
X-rays
The production of x-rays
1: Continuum2: Characteristic x-rays
20 40 60 80 100 keV
Characteristic X-raysContinuum emission
high voltage
low voltage
X-ray Spectrum
+ve nucleus
-ve electron
X-rays
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”
X-ray Spectrum
K
L
M
high velocity electron
ejected K band electron
hole in K shell
High energy electron knocks out inner K electron from atom
K
L
M
electron from L shell
falls into K shell
K X-ray
The production of characteristic X-rays
Energy Levels in Targets
keV0.01
0.07
0.9
8.98
Copper Z=29 Tungsten Z=74
keV0.02
0.06
0.50
2.5
10.2
69.5
ShellOccupancy
1 N
18 M
8 L
2 K
ShellOccupancy
2 P12 O
32 N
18 M
8 L
2 K
K ALPHA 59.3 keV
1. 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.
2. 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.
3. 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.
The Interaction of X-rays with Matter
100%
20 60 100 20 60 100 20 60 100keV keV keV
50%
0%
Photoelectric Compton Coherent
Water Bone/Tissue Sodium iodide
The Interaction of X-rays with Matter
The Interaction of X-rays with Matter
Heggie et al 2001
The Interaction of X-rays with Matter
1. 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
The Interaction of X-rays with Matter
2. Photoelectric effect
• 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 proportional to:• Ephoton
3
• Z3
For photon energies > electron binding energies
The Interaction of X-rays with Matter
3. 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 keVOuter electrons in High Z materials appear freeAll electrons in soft tissue appear free
The Interaction of X-rays with Matter
3. Compton Scattering
K
L
M
2/cos11 mcE
EE
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
•Scattered x-rays produce a fog background•Primary x-rays produce the image
Attenuation and Half Value Layers
attenuated primary beam
scattered beam
No
Io
-N = No x
x
Loss from beam
N = No exp(-x)
Half Value Layer When is N = No/2? exp(-x) = ½
HVL = 0.693/
Attenuation and Half Value Layers
Mass attenuation coefficient
• Depends on energy of photons
100
10
1
Tra
nsm
itte
d in
tens
ity
%
0 4 8 12 16 20
60 keV photons
60 kVp +2.5mm Al
Beam hardening occurs since kevkevkev
Spectrum changes as low energy photons are preferentially absorbed