Presentation on theme: "X-rays : Their Production Their Interaction with Matter"— Presentation transcript:
1 X-rays : Their Production Their Interaction with Matter andTheir Interaction with Matter
2 The Electromagnetic Spectrum 1 electron Volt = 1.6 x JoulesE = hn
3 Forms of Electromagnetic Waves Wave formWavelengthFrequency (Hz)Photon energy(eV)Gamma< 0.001nm3 x 10201.2 MeVX-ray0.001nm to 0.9nm3 x 1020 to3 x10171.2 x MeV to1.2keVUltraviolet0.9nm to 350nm3 x1017 to8.6x10141.2x1keV to3.6eVVisible350nm to 750nm8.6x1014 to 4x10143.6eV 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 electronsto velocities of a hundred keV before allowing them to collide witha heavy metal, tungsten target.
6 X-ray SpectrumContinuous spectrumBraking Radiationa 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 thenucleus is called “Bremsstrahlung”
7 The production of characteristic X-rays KLMhigh velocity electronejected K bandelectronhole in K shellKLMelectron from L shellfalls into K shellK X-rayHigh energy electron knocks out inner K electron from atom
9 The Interaction of X-rays with Matter Coherent scatteringCoherent 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 effectIn 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 effectIn 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.
11 The Interaction of X-rays with Matter Heggie et al 2001
12 The Interaction of X-rays with Matter Coherent scatteringInteraction between photon and bound electrons.Photon changes direction but without energy changeAlso known as Rayleigh ScatteringDominates at low energies and large Z
13 The Interaction of X-rays with Matter 2. Photoelectric effectFor photon energies > electron binding energiesenergy of photon totally absorbed by atom( single atom event)electron ejected from atomcharacteristic x-ray produced in subsequent cascadepositive ion remainsProbability related to:electron energy match to photon energyAttenuation coefficient m/r proportional to:Ephoton3Z3
14 The Interaction of X-rays with Matter Compton ScatteringFor photon energies >> electron binding energiesouter electrons act independently of nucleuscollisonal process between photon and electronenergy/ momentujm transfer from photon to electronproduces a:positive ionfree electronphoton with different direction and lower energyDiagnostic Imaging 10 keV to 150 keVOuter electrons in High Z materials appear freeAll electrons in soft tissue appear free
15 The Interaction of X-rays with Matter Compton ScatteringKLM
16 RadiographyThe X-rays are1: scattered from the beam by Compton scattering2: 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 beamNoIoDxscattered beamLoss from beamN = No exp(-m x)Half Value LayerWhen is N = No/2?exp(-m x) = ½HVL = 0.693/ m-DN = mNo Dx
20 Attenuation and Half Value Layers Mass attenuation coefficient mDepends on energy of photons10010160 keV photonsTransmitted intensity %60 kVp +2.5mm AlBeam hardening occurs since m30kev> m60kev> m100kevSpectrum changes as low energy photons arepreferentially absorbed
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