X-Ray Production

X-Ray Tube Diagram (Source: Xstrahl.com)
X-Ray Tube Diagram (Source: Xstrahl.com)

When a high electrical potential (~103 Volts) is applied across an X-Ray tube, the produced electrons at the filament side (cathode) will be attracted and accelerated toward the anode, where they will strike a fixed tungsten target. The moving electrons are then suddenly decelerated, where the energy transferred is then converted into either x–rays or heat.

There are two different mechanisms by which X-Rays are produced. One gives rise to Bremsstrahlung X-Rays through radiative interactions; a process by which the electron, while passing near an atomic nucleus, will decelerate and lose energy in a form of X-Ray or Bremmstrahlung radiation. The other process is characteristic X-Ray, resulting from the interaction of the high-speed electron with a bound orbital electron in an atom. This mechanism may knock the electron from one of its orbital shells.

If the incoming electron has enough energy to overcome the binding threshold energy of the orbiting electron, the orbital electron will move off from the atom, resulting in an empty shell and thus leaving the atom in an excited state. The atom will then return to a steady state by emitting characteristic photons. However, if the accelerating electrons do not transfer sufficient energy to this outer-shell electron to ionise it, the outer-shell electron is simply raised to an excited state and immediately drops back to its normal energy state with the emission of infrared radiation. This process is responsible for the heat generated in the anodes of the X-Ray tubes.

The proportion of Bremsstrahlung characteristic X-Rays produced and heat will vary as a function of the target material and the tube voltage. Typically, at 60 kVp only 0.5% of the electron energy is converted to X-Rays.

Attenuation Processes

X-Rays can be used for therapeutic purposes as a result of the process by which they affect the tissues through which they pass. When X-Rays pass through tissue the interaction between photons and the medium results in energy transfer. This energy transfer may set electrons in motion, where ionisation and excitation may occur. The gradual loss of the photon energy is referred to as attenuation.

The probability of photon attenuation by the absorbing medium depends on the photon energy, as well as the density and atomic number (z) of the absorber; this can be classified into five major types of interaction. One of these is the interaction between the photons and the nucleus of an atom; the photodisintegration or photonuclear interaction. This process is only important at high photon energy, typically energy greater than 10 MeV. The other four dominant processes are coherent scattering (Rayleigh), photoelectric effect, incoherent scattering (Compton effect), and pair production.

Attenuation Process Diagram (Source: Xstralh.com)
Attenuation Process Diagram (Source: Xstralh.com)

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