X-ray Tube Theory
As discussed earlier, it was quite by accident that X-rays were discovered. When electrons hit matter, approximately 99% of the energy is changed into heat energy. However, about 1% are changed into photons or X-radiation. An X-ray tube is not needed to create X-rays. In the past the use of Cathode Ray Tubes (CRT) for your family television set produced a small amount of radiation. If one were to put a dosimeter or a meter that measures radiation in front of a CRT the reading would show the presents of X-rays. In fact, when all of our desktop computers used CRTs, radiation workers would have to be careful not to set their personal dosimetry device close to the CRT.
The X-ray Tube is designed to produce as much X-rays as possible at the lowest Wattage. With that in mind, whenever charged particles (electrons or ions) of sufficient energy hit a material, X-rays are produced. Electrons are released by a hot cathode (K). Collision with the anode, a metal target (A). The kinetic energy of the electrons at the target is converted to 99% heat and 1% X-ray. For most X-ray tubes, generating X-Rays requires two parts. 1) Electron emitter (Cathode) K, which uses a Tungsten wire filament that is heated up using current from a voltage applied across it. The heated filament electrons are leaving the atomic bond and attracted to the Anode target. 2) Target (Anode) A, made of a dense material and the X-Rays are generated here, due to high speed collisions between electrons and target. Electron acceleration is achieved between cathode (-) and anode (+) by applying a high voltage at Ua. The resultant acceleration of free electrons is controlled by the voltage or electromotive force, the higher the Voltage (kV) the higher the speed of the electrons. A vacuum is needed to avoid collisions with air molecules.
Filament Current and High Voltage
There are two types of current in an X-ray tube. The first type is the filament current. The filament current is applied to the Uh terminals depicted on the figure above right. As you increase the filament current it heats up and more atoms are accelerated to the anode of the tube. The result is that the total current through the X-ray tube or commonly called tube current increases. Increase in current (I) changes intensity of the X-Ray by increasing the number of X-ray photons emitted and the heat generated. However, the frequency or the wavelength does not change. When Tube voltage is increased at Ue the intensity and the frequency or wavelength increases thereby increasing the energy of the X-ray photons. Remember that the intensity depends on the voltage and current. The more energized the X-ray, the better the penetration but also the more scatter as we will talk about next.
A case in point would be a mammographic X-ray tube that typically runs below 30 KeV. The target is made from Be and the scatter is limited to the photoelectric absorption. Small changes in target material and energy can therefore significantly affect photoelectric absorption. The ideal or "perfect" x-ray spectrum for mammography would be made up of photons all having the same energy (mono-energetic) and with the ability to adjust the energy for different breast conditions. The filters are also beryllium and adjusted to optimize the contrast of the image by reducing scatter. The same optimization of the characterization of the X-ray beam should occur with industrial applications. The diondo filter changer provides the operator with a remarkably diverse set of 14 filters that will optimize the effects of scatter through many different materials. In NDT CT or Industrial CT, it is important to preprogram the correct beam characteristics for the ever changing materials.
K-Characteristic Radiation and Bremsstrahlung
There are two types of X-rays generated or produced at the target of the X-ray Tube, 1) Characteristic X-rays and 2) Bremsstrahlung. “When a fast-moving electron collides with a K-shell electron, the electron in the K-shell is ejected (provided the energy of the incident electron is greater than the binding energy of K-shell electron) leaving behind a 'hole'. An outer shell electron fills this hole (from the L-shell, M-shell, etc. ) with an emission of a single x-ray photon, called characteristic radiation, with an energy level equivalent to the energy level difference between the outer and inner shell electron involved in the transition.”
As opposed to the continuous spectrum of bremsstrahlung radiation, characteristic radiation is represented by a line spectrum. As each element has a specific arrangement of electrons at discrete energy level, then it can be appreciated that the radiation produced from such interactions is 'characteristic' of the element involved.
For example, in a tungsten target electron transitions from the L-shell to the K-shell produce x-rays photons of 57.98 and 59.32 keV. The two energy levels are as a result of the Pauli exclusion principle which states that no two particles of half-integer spin (such as electrons) in an atom can occupy exactly the same energy state at the same time; therefore the K-shell represents two different energy states, the L-shell eight states and so on.
When an electron falls (cascades) from the L-shell to the K-shell, the x-ray emitted is called a K-alpha x-ray. Similarly, when an electron falls from the M-shell to the K-shell, the x-ray emitted is called a K-beta x-ray 1. However, it is possible to have M-L transitions and so on but their likelihood is so low they can be safely ignored.
Each element differs in nuclear binding energies, and characteristic radiation depends on the binding energy of particular element.
X-rays are produced by high-energy electrons bombarding a target, especially targets that have a high proton number (Z). When bombarding electrons penetrate into the target, some electrons travel close to the nucleus due to the attraction of its positive charge and are subsequently influenced by its electric field. The course of these electrons would be deflected, and a portion or all of their kinetic energy would be lost. The principle of the conservation of energy states that in producing the X-ray photon, the electron has lost some of its kinetic energy (KE):
final KE of electron = initial KE of electron - energy of X-ray photon
The 'lost' energy is emitted as X-ray photons, specifically bremsstrahlung radiation (bremsstrahlung is German for 'braking radiation'). Bremsstrahlung can have any energy ranging from zero to the maximum KE of the bombarding electrons (i.e., 0 to Emax), depending on how much the electrons are influenced by the electric field, therefore forming a continuous spectrum. The 'peak' of the spectrum typically occurs at approximately one-third of Emax so for a bremsstrahlung spectra with an Emax value of say 120 keV, the peak of the spectrum would be at approximately 40 keV.
The intensity of bremsstrahlung radiation is proportional to the square of the atomic number of the target (Z), the number of unit charges of the bombarding particle (z) and inversely with the mass of the bombarding particle (m): Z² z / m. It follows that light particles such as electrons and positrons bombarding targets of high atomic number are more efficient producers of bremsstrahlung radiation than heavier particles such as alpha particles or neutrons (which can also cause X-rays to be produced through bremsstrahlung, though it's much more unlikely than with electrons).
Focal Spot Size
The X-ray tube filament is the source of the electrons that are accelerated to the Anode. The filament is typically a tungsten coil. The Cathode of the tube contains one or two of these coils so that a selection can be made between different focal spot sizes. The size of the coil along with how it is mechanically situated in the cathode cup will determine size of the electron beam that strikes the Anode target and emits X-rays. The size of the electron beam hitting the anode is your focal spot size. In some cases a grid cup is used to create an electromagnetic ring around the electron beam to adjust the focal spot size.
How Does Focal Spot Size and Focal Distance Affect Image Quality?
The focal spot or the tube filament look similar to the filament in an incandescent light bulb. We will demonstrate the affects that size of the focal spot will have using a light and the resultant shadow.
Note that picture to the right shows a light with a large filament shining on a hand that is far away from it and casting a shadow on a wall that is clear and of similar size to the hand in the way of the light beam.
In the next picture the hand has moved closer to the large light filament and casts a shadow on the wall that is magnified but blurry. The blurred edges are called penumbra and are caused by a large light source. Magnification or Mag Factor (M) can be determined by dividing the source or focus to image or detector (the shadow) distance by the focal spot to object (the hand) distance. M=FID/FOD. Penumbra or geometric un-sharpness (U) can be determined by U=(M-1)*F where F is the focal spot size.
The third picture to the right shows a very small focal spot and the resultant image or shadow cast on the wall is magnified and sharp. For demonstration purposes this shows how the focal spot will influence your imaging results.
X-ray Large Focal Spot (Left) vs. Small Focal Spot (Right)
With a large focal spot and high magnification the resolution decreases due to the physics of penumbras (half-shadows), the image becomes blurry. For high resolution and high magnification scans the focal spot should be as small as possible For micro-focus tubes the rule of thumb is Power [W] = Resolution [µm] . For high magnification the focal spot determines the spatial resolution and for low magnification the detector determines the spatial resolution.