Xray Production

X-Ray Production

X-rays are produced by two main mechanisms and come in two varieties.

Bremsstrahlung xrays

Bremsstrahlung is a German word meaning “braking radiation” which describes the process of Xray generation. The high speed electron impacts on the target and at the atomic level approaches the nucleus. There is no actual collision between electron and nucleus because the electron interacts with the Coulombic nuclear forces and its vector quantities of direction and velocity are changed. Since kinetic energy derives from velocity ($K.E. = {1 \over 2}mv^2$). The change in energy is radiated as electromagnetic radiation. The amount of energy means a short wavelength within the Xray band.

As the electron is not destroyed, it can undergo multiple interactions, and even initial interactions will vary from minor to major energy changes depending on the actual angle and proximity of attack, and the point of 'impact' on the nucleus. As a result, bremsstrahlung radiation will have continuous spectrum where the maximum energy relates to the entire KE of the electron but will be infrequent. The energy spectrum without filtration is a straight line that matches the formula $I_{E} = kZ(E_{m} - E)$ where $I{E}$ = intensity of photons of energy E, k is a constant, Z is the atomic number of the target, and $E_{m}$ is the maximum photon energy which is numerically equal to the applied kilovolts peak (kVp).

The average energy of a bremsstrahlung-derived beam is approximately 1/3 of the maximum energy (or kVp).

The direction of bremsstrahlung Xrays is decidedly horticultural (I know, read on!). Where the energy (kVp) of the incident electron beam is around 100 keV, bremsstrahlung production has a spatial orientation described as 'isotropic', that is equally in all directions - like an apple! However as the energy of the electron beam increases the direction becomes spatially asymmetrical or 'anisotropic', projecting forward into a pear shape at 400keV, a cucumber shape at 4MV and a carrot at 20MV! Now do you understand the horticultural reference?

The probability of bremsstrahlung production is proportional to the Z2 of the target material, and the efficiency of production is proportional to the target material Z and the tube voltage.

Characteristic xrays

Electrons are the same whether orbiting in shells around the nucleus, or produced inside an xray generator. When ever their velocity or position is changed, there is a loss of energy that takes a radiative form (xrays). When electrons traveling at the target have their direction changed, a spectrum of xrays results as shown in the diagram. However the electrons circulating in the atoms can also change.

While most of the electrons have their path changed and little else, some will collide with electrons. Sufficient energy in such collisions can result in the ejection of an orbiting electron. 'Sufficient energy' means enough to overcome the bonding energy of the orbiting electron. The impacting electron will move off with reduced energy, and the ejected electron will move off in a different direction and speed with the remaining energy, there is an empty position in one of the shells. The remaining orbiting electrons will 'pack down' to fill the hole, and when changing orbits will lose energy and emit this as radiation. The orbiting levels are fixed as a physical property fixing the elemental identity of an atom, and so the energy emission will be characteristic of that atom. The energy will be mono-energetic and so appear as a spike rather than a continuous spectrum. Electrons ejected come from the K, L or M orbits. The other corollary of this type of interaction is that the atom becomes an ion (it has lost an ejected electron!).

All atoms will produce characteristic radiation but not all are visible in the xray portion of the electromagnetic spectrum. Elements with higher atomic numbers have their K, L, M or N shells of sufficient energy to be called 'xrays'. The discrete characteristic radiation energies are equal to the difference in the energy level of the outer and inner orbital electrons.

The xray energy is proportional to the atom's Z. Where the incident electrons have energies less than the electron binding energy, there will be no characteristic radiation emitted. As the electron energy increases past the threshold level, the maximum level of characteristic radiation reaches 20% of total production, and then starts to fall to 10% in the 50-100 keV range and 3% in the 200 keV range. In the megavoltage range, characteristic radiation is negligible.

Xray Energy Spectra in real life

The spectrum produced by an xray generator is heterogeneous with characteristic x-rays superimposed on the bremsstrahlung distribution. In addition there is inherent filtration in tube (window, target) that primarily attenuates the low energy photons in the spectrum. The filtration preferentially removes more of the lower energy than the high energy photons, and the resulting transmitted beam will have a higher average photon energy, therefore being of higher quality and more penetrating.

page revision: 10, last edited: 27 Dec 2007 12:35