The intensity of beam if then given by the number of photons passing through a given area per unit time. As the intensity is also proportional to the square of the electric field it follows that magnitude of the field is quantised (Dendy & Heaton, 1999).
Thus A beam of x-ray photons is heterogenous and presents both fields: electrical and magnetic. Because X-ray beam is not originated from a point source it's divergent by its nature. In this way the magnification of the image could be achieved by the increase of the focus distance (direct dependence).
Because of the existence of two different types of photon interaction within the x-ray tube there is important what peak voltage is used. The energy of electrons depends on the voltage between the anode and cathode. Higher peak voltage produce photons with higher energy (Aichinger et al., 2003). Thus measurement of the practical peak voltage is used for the quality control of X-ray units (Ramrez-Jimnez et al., 2004).
There are some types of X-ray photons dependently on their origin and type of interaction with target atom. If the projectile electron interacts with an inner-shell electron of the target atom rather than an outer-shell electron than characteristic x-radiation can be produced (Christensen et al., 1979;). Contrarily to the characteristic interaction Bremsstrahlung (braking) x-radiation occur when the projectile electrons lose their kinetic energy in the interaction with the nucleus of a target atom (Aichinger et al., 2003).
Actually, X-ray tube is very ineffective device - only small part of energy is transformed in X-ray beams while the rest just produce heat (Aichinger et al., 2003). Only small part of the anode surface is involved in x-ray production. This area is called as the "focal spot". There is known that smaller focal spots is more useful for imaging purposes because they generate less blurring and provide better visibility of image details. Thus X-ray tubes with small focal spots are useful for decrease of radiation loading (Aichinger et al., 2003).
X rays interact with irradiated substances in form of photoelectric effect, dispersion and forming of pairs "electron-positron" (Aichinger et al., 2003) They have high penetrating capacity, its property is described by formula: I=I0e-rt, where I0 - is intensity of the beam of X-rays, - attenuation coefficient (Aichinger et al., 2003).
The depth of penetrating depends on the half-value thickness, i.e. the thickness of an absorbing substance that reduces the intensity of x-ray beam to half its value (i.e. I0/2).
The half-value thickness depends on the attenuating properties of the substance itself and the penetrating power of the radiation incident upon it. This property is used for such radiographic tests as roentgen-densitometry when density of tested object is compared with etalon, e.g. aluminium equivalent (Wells & Ryan, 2000).
Another practical application of the half-value thickness knowledge is shielding/filtration. Protective equipment is an example of added filtration. Filtration allows increasing the average energy of X-ray beams and reduce amount of low-energy photons. It's helpful for radiation dose control. Minimum adequate filtration of the x-ray beam is achievable at the half value thickness which is equal or more than 2.5 mm of aluminium (Bushberg et al., 1994).
Both Bremsstrahlung and characteristic radiation are emitted in the anode