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