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

The Electromagnetic Spectrum 1 electron Volt = 1.6 x 10-19 Joules E = hn

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

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.  

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

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”

The production of characteristic X-rays K 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

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

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.

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 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 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

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

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

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 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

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 0 4 8 12 16 20 Beam hardening occurs since m30kev> m60kev> m100kev Spectrum changes as low energy photons are preferentially absorbed