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

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Presentation on theme: "X-rays : Their Production and Their Interaction with Matter."— Presentation transcript:

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

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

3 Wave formWavelengthFrequency (Hz) Photon energy (eV) Gamma< 0.001nm3 x MeV X-ray0.001nm to 0.9nm 3 x to 3 x x MeV to 1.2keV Ultraviolet0.9nm to 350nm 3 x10 17 to 8.6x x1keV to 3.6eV Visible350nm to 750nm 8.6x10 14 to 4x eV to 1.6eV Forms of Electromagnetic Waves

4 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. The production of x-rays

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

6 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

7 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

8 Energy Levels in Targets keV Copper Z=29Tungsten Z=74 keV Shell Occupancy 1 N 18 M 8 L 2 K Shell Occupancy 2 P 12 O 32 N 18 M 8 L 2 K K ALPHA 59.3 keV

9 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

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11 Heggie et al 2001

12 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

13 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: E photon 3 Z 3 For photon energies > electron binding energies

14 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 keV Outer electrons in High Z materials appear free All electrons in soft tissue appear free

15 The Interaction of X-rays with Matter 3.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

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19 Attenuation and Half Value Layers attenuated primary beam scattered beam NoIoNoIo -  N =  N o  x xx Loss from beam N = N o exp(-  x) Half Value Layer When is N = N o /2? exp(-  x) = ½ HVL = 0.693/ 

20 Attenuation and Half Value Layers Mass attenuation coefficient  Depends on energy of photons Transmitted intensity % keV photons 60 kVp +2.5mm Al Beam hardening occurs since   kev   kev   kev Spectrum changes as low energy photons are preferentially absorbed


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