X-ray tomography

Ullrike Worm, summer semester 2013

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Radiographic testing using X-rays (X-ray tomography, radiography): An imaging method for radiographing objects using X-rays to depict an internal structure. Above all, radiographic testing is used to identify defects in the material and any inhomogeneities. Depending on the wave length, different thickness and their changes can also be examined.

History

The X-ray was discovered by Wilhelm Conrad Röntgen (1845 - 1923) in 1985. He accidentally discovered what he termed X-rays during an experiment with a cold cathode ray tube. He ascertained that the spark discharge in the tub lit a crystal in a dark room and thus that X-rays caused the fluorescence of certain materials. The same radiation is capable of blackening photographic films. Above all, X-rays were used in the field of medicine to radiograph the human body. In addition, it was known that they could be used to observe at metallic objects. The health risk of radiation on the human body had not yet been considered at this time. Nowadays, in addition to the field of medicine, X-rays are used mainly for carrying out non-destructive testing as well as for inspecting luggage at airports.

X-ray radiation

The X-rays consist of electromagnetic waves that propagate rectilinearly in the wavelength range from 0.1 to 10 nanometres. They are formed by changing the state of electrons. Depending on the way they are formed, a distinction is made between bremsstrahlung and k-shell emission.

In bremsstrahlung, the electrons are braked, which is why its also sometimes called braking radiation. The X-ray tube consists of a cathode enclosed in a metal housing and an anode aligned to it obliquely. By heating up the cathodes, free electrons are created which accelerate in the direction of the anode owing to the application of high voltage. This brakes the electrons and the radiation becomes free. This principle is depicted in fig. 1. Only up to 1% are X-rays, the rest is converted into heat. The majority is directed to an exit window owing to the the obliquely aligned anode, the remainder has to be shielded by the metal housing from going outwards. The anode is often made of wolfram as it has a very high melting point and conducts heat well. Nevertheless, the anode is usually cooled using air or water.

The energy of X-rays is given in KeV (kilo electron volts). It depends on kinetic energy which is directly proportional to the high voltage applied The continuous spectrum is created by electrons being braked at different strengths. If the kinetic energy is completely converted into a single photon, what is termed threshold energy is achieved. In addition to the tube voltage, the intensity of the X-rays depends on the tube current; this correlation is represented in diagram 1 and diagram 2. Here, tube current and tube voltage are plotted in AU (arbitrary units).

X-rays of higher intensity are gained with K-shell emission, where a free electron transfers so much energy to an electron in the inner shell, that it can leave its energy state. The gap thus created is closed by an electron of a higher energy state whereby energy being released is given off in the form of X-ray photons.

Properties of X-rays:

• ionising radiation
• activate fluorescence in certain materials
• blacken photographic films
• can be characterised through their frequency, wave length and speed
• diffraction is possible at boundary layers of different materials
• not visible or perceptible

Attenuation of rays

When X-rays reach a material, they are attenuated. This is the fundamental way radiographic testing works as the difference between various materials can only be recognised owing to attenuation of the rays. Attenuation depends on three factors in particular:

1. the material being radiographed, in particular its density and chemical composition
2. the thickness of the material
3. the radiation energy, determined by the preset voltage of the X-ray equipment

Absorption and diffusion always contribute to attenuation. In the case of scattered radiation, the original direction of the rays is always changed and energy diminishes. Those rays that maintain their original direction exactly are termed primary rays. The following mechanisms, which are also portrayed in table 1, can contribute to attenuation of rays.

Photoabsorption

The quantum of light (photon) has enough energy to eject an electron from the inner shell. The photon is destroyed and a photoelectron is formed that converts the excess photon energy into kinetic energy.

Classic diffusion

The photon is simply diverted by the electrons of atomic electron shell. This takes place elastically without transferring energy so that there is only a change of direction.

 Compton scattering

The photon strikes a weakly bound electron from a outer shell and transmits part of its energy to it. This is how a Compton electron is formed. The photon flies on with the remainder of the energy, whereby the wavelengths become longer due to the loss of energy.

Pair formation

If the energy of the photon is greater than 1.02 MeV (mega-electron volt), it can be converted into an electron-positron pair in the electrical field of the atom. In doing so, the photon disappears as its total energy is needed for generating the electrons and for kinetic energy. However, the effect is negligible as the energy in most X-ray machines is below this value.

Half-value layer

The half-value layer or half-value thickness is a way to measure the shielding effect of a material. It indicates the thickness of a material that is needed to half the dose rate of the radiation. This depends on the relevant radiation energy. Several values are indicated in table 2.

Table 2: Half-value layer

Radiation protection

X-rays can not only harm the human body acutely by causing immediate burns following a high dose of radiation, but also lead to long-term health conditions and even hereditary defects. The radiation causes a change in the molecules and damages the properties and behaviour of cells. This is why radiation protection regulations are to be complied with. The inverse square law has a positive effect on radiation protection. At a single distance, the dose per unit area is set to 1. When the distance is doubled, the dose per unit area is only a quarter and when the distance is tripled, it is only a ninth. Thus, even small changes in distance result in a large difference in dose of radiation. The basic principle of radiation protection is to only work with the strength needed and to not stay longer than necessary in the hazardous area. Moreover, the largest possible distances are to be observed and sufficient radiation shielding is to be used. Preferred materials for this are lead, reinforced concrete and lime-sand brick.

X-ray machines

There are machines in which the X-ray tube is accommodated together with generators in a housing or separate machines with several separate housings. In addition, a cooling unit is necessary. As described under the heading 'X-ray radiation', the X-rays are generated in an X-ray tube and directed to the test object via an outlet window. Shielding is often installed between the X-ray tube and the test object to limit the radiation to a specific area. The individual components can be fixed on to what is termed an optical bench. The X-rays penetrate through the damaged areas more easily than the material and blacken the film, whereas intact areas mainly shield the X-rays. This is also shown in fig. 2. The X-ray image is created from these differences in radiation. Radiation detection is carried out by using a film or flat panel detector. Either the film is developed or the detector signal is converted into a digital image.

X-ray tomography

In comparison with basic X-rays, where all depth levels in an object are shown with the same definition on an image, only one depth level is shown in focus in X-ray tomography (tomography - Ancient Greek: tome "cutting, grafëin "recording") and all other depth levels appear blurred. This principle is represented in fig. 3. The X-ray tubes and the film cassette rotate in opposite directions so that only the intended axis's centre of rotation between tube and cassette is sharply in focus, here marked with a triangle. Everything else is shown either not in focus or blurred. Normally, several images over a range of depths are taken to ensure good results regarding the object. A further development of this process is X-ray computed tomography.

Imaging technique and image quality

Digital image: The images are recorded digitally using special phosphor screens or flat panels with microelectronic sensors. The following effect is used in phosphor screens where certain materials are excited by the X-rays and release light, what is termed fluorescence. The signals have to converted into visible light or electric signals so that a digital image can be produced. In comparison, there are also scintillators which release visible light or UV-radiation that has been excited by the X-rays.

Film images: Film images which are complicated to use are still widely used. The X-ray image is formed by causing microscopically small silver bromide grains found on the film to decompose during the radiation process, which then subsequently appear black owing to a chemical process. During the development of the film, the unchanged bromide grains are used to fix the image. The image quality is crucial for the evaluation of defects and depends on the contrast as well as geometric and internal distortion.

Contrast: The contrast describes the maximum change in optical density between object or defect and its surroundings. It increases by reducing scattered radiation, e.g. by using shielding as well as as side and rear covers. The material and the thickness of the test object also have an influence on the contrast.

Geometrical distortion: Owing to the rectilinear propagation of electromagnetic waves, the laws of optics apply. This means that in addition to the desired deepest shadow a partial shadow is also created that is referred to as geometric distortion. The focus is smaller when there is a large distance between the radiation source and object, when there is a small distance between the object and film or when radiation source has a small diameter.

Internal distortion: Internal distortion depends on the grain of the film and is determined by the size distribution of the silver bromide grains. The more finely grained the material, the higher the image resolution of the defect. In radiation detectors, the internal distortion depends on the detector material used. In addition, ionisation processes and the radiation energy have an influence on internal distortion.

All the effects mentioned influence image quality. This is determined using an image quality indicator (fig. 4) as per the German DIN-EN 462 standard. It consists of wires of various thicknesses that are combined in a plastic housing. Depending of the material to be tested, there are a variety of different indicators. The image quality is determined according to the thinnest recognisable wire. Identification of the relevant indicator is indicated in the field over the wires through the use lead letters. It states the image quality value of the coarsest wire, the material type and the standardisation.

Application

This process is used to test workpieces for defects and inhomogeneities. In contrast to materials without defects, these can be recognised particularly well as they let through a lot of radiation. Moreover, installation and construction details that are not superficially visible can be recognised and different thicknesses can be made visible. Similarly, material cracks are usually easily to detect. In fig. 5, an example of a full protection X-ray machine is shown for such an application. Even crystals can be identified on the basis of their unique crystalline structure. This is done by determining the crystal lattice by irradiating over a certain angle and the Bragg's law applies here. The goniometer used to set the angle of incidence can be removed, thus enabling normal radiographing of smaller objects to be carried out. The X-ray image is made visible for the observer by means of X-ray fluorescence through the use of a scintilllator. Additional shielding can also be installed on the optical bench. Moreover, this process can be generally used to test and investigate historical stone sculptures and rocks.

Literature

• Askeland, D.R.: Materialwissenschaften. Spektrum Akademischer Verlag. Heidelberg, 1996.
• DIN EN 462: Bildgüte von Durchstrahlungsaufnahmen. Beuth Verlag GmbH. Berlin, 1994.
• Hoxter, E. A.; Schenz, A.: Röntgenaufnahmetechnik: Grundlagen und Anwendungen. Siemens-Aktiengesellschaft, München. Berlin, 1991.
• Introduction to Radiographic Testing. NDT Education Resource Center, Iowa State University.
• Lossau, N.: Röntgen: Eine Entdeckung verändert unser Leben. VGS Köln, 1995.
• Stark, F.; Grosse, C. U.: Die Durchstrahlungsprüfung, Grundlagen der Zerstörungsfreien Prüfung. Skript Lehrstuhl für Zerstörungsfreie Prüfung from Technische Universität München (pp. 82-92). München, 2011.
• Stegmann, D.: Zerstörungsfreie Prüfverfahren: Radiografie und Radioskopie. Teubner, Stuttgart, 1995.