x ray production Flashcards
ionization interactions
Consider an incident electron moving with kinetic energy through a material. The incident electron comes close to an electron in the electron cloud surrounding an atom in the material. There will be a force of repulsion between the two electrons. This will cause a change of direction for the incident electron and also, if the force is strong enough, it may lead to the atomic electron being ejected from its orbital shell. This will leave the atom with an overall positive charge of +1 and it is said that the atom has been ionised.
excitation interactions
If the force of interaction is not sufficient to ionise the atom, the atomic electron may be raised to a higher energy shell. The atom is now said to be in an excited state, which will quickly revert back to its normal state giving up some energy in the form of heat. As we shall see this excitation process occurs a lot in the target of an x-ray tube resulting in a lot of heat being produced, in fact 99% of the energy that the electrons from the filament have is converted to heat and only 1% to x-rays !!!
electrons through a material
The electrons traveling through the target material can undergo very small and very large angular deflections as they interact. They can therefore lose correspondingly small or large amounts of their energy. The results of this is a very irregular path through the target material.
bremmstrahlung radiation
Once again consider an electron which has been accelerated from the filament, moving through the target of the tube. It encounters an atom of the target material but this time penetrates the electron cloud of the atom and interacts with the nucleus of the tungsten atom. The forces cause the electron to change direction which results in it slowing down. If it has slowed down it means it has lost some of its kinetic energy which is emitted in the form of an x-ray. This type of x-ray is called Bremsstrahlung radiation (Bremsstrahlung means braking or slowing). The energy lost by the electron in this type of interaction is called a radiation loss.
The energy of the Bremsstrahlung x-ray is equal to the amount of kinetic energy lost by the incident electron. The amount of energy lost by the incident electron depends on how “close” it comes to the nucleus and hence how much it is deflected by. If the incident electron is only slightly deflected it losses only a little energy and hence the emitted x-ray is low energy. At the other extreme, if the electron has a “head on” collision with the nucleus and is completely stopped, it will lose all its kinetic energy and the emitted Bremsstrahlung x-ray will have an energy equal to that of the electron.
characteristic radiation
In order to understand how the spikes or peaks in the spectrum are formed you need to recall what is meant by the binding energy of electron shells.
The second kind of interaction that an electron can have in the target material was that of ionisation of the atom. Let us assume that the incoming electron has ejected a K-shell electron from its orbit. This will leave a “vacancy” in the K-shell which leaves the atom in an unstable state. This is rectified by an electron from another shell filling this vacancy. Lets say that the vacancy is in the K shell and it is filled by an electron in the L shell. The result of this transition is the emission of an x-ray, the energy of which is equal to the difference of the binding energies of the K and L shell. The vacancy could be filled by an electron in the M shell in which case the energy of the emitted x-ray is equal to the difference of the binding energies of the K shell and the M shell.
These x-rays are called characteristic x-rays of type K alpha or K beta as shown in the animation above. The reason for this is that every element has a unique binding energy because of its unique atomic number, Z. So if the energy of the emitted x-ray is dependent on the binding energies of the electron shells it means they are unique or characteristic of that element. For a tungsten atom the K alpha and K beta characteristic energies are 59 keV and 67 keV respectively.
the general shape of an x-ray
You can see that the x-axis of the graph is the energy of the x-rays in keV, and the y-axis is the number of x-rays produced at any given energy (which gives us the intensity of the x-ray beam). So we can see that there are no x-rays of very low energy and only start at approximately 15 keV. We get a maximum number at around 35 keV then a gradual reduction to a peak energy of 90 keV. There are also two spikes that appear on the graph at energies of 59 keV and 67 keV. This means that we have a lot of x-rays being produced with these energies. We need to be able to explain how the shape of this energy spectrum comes about
beam hardening
Any filter placed in the beam will preferentially absorb the low energy photons. The diagram below shows a series of spectra starting with no added filtration and adding various thicknes of aluminium in 0.5 mm stages up to 2.5 mm. With no filtration the average energy is approximately 27 keV. With 0.5 mm aluminium the lower energy photons are absorbed so the average energy is increased to approximately 41 keV. The final medical spectra with 2.5 mm aluminium the average energy is about 47 keV.
As the lower energy x-rays are absorbed, the average beam energy increases and the beam is said to be harder.
factors that affect the quality and intensity of radiation produced
The term quality describes the penetrating power of the x-ray beam
The intensity of an x-ray beam is more a measure of the amount of radiation produced in a given time
Generating potential (kVp). this affects both quality and intensity. The higher the tube voltage the more penetrating the beam is, hence the quality will change (more later). If the kVp is small, we have seen that no characteristic lines are produced in the spectrum hence the intensity is affected. If we increase the kVp then the intensity increases in proportion to kVp squared i.e.
Intensity proportional to (kVp)2
2. Tube current (mA). Changing the tube current only changes the intensity of the beam, it does not affect the quality. The intensity of the beam is directly proportional to the tube current i.e.
Intensity proportional to mA
- Tube filtration. As we have seen, placing a filter in the beam affects the shape of the spectrum. Adding filtration will change the quality and the intensity of the beam. The beam will become more penetrating because of beam hardening and the intensity will be reduced.
- Target material. The atomic number of the target material affects the intensity of the beam. The higher the atomic number, the higher the intensity.
Intensity proportional to Z
- Type of x-ray tube rectification. Affects the quality and quantity of the x-ray spectrum. Basically the closer we are to having a constant potential, the higher the quantity and quality of the radiation produced at the target.
- The distance from the source. The beam from an x-ray tube is a diverging beam, hence the further from the tube you get the less the intensity is. The reduction in intensity is described by the inverse square law.
