The X-ray Tube

The most important and most frequently used X-ray source is the X-ray tube. An X-ray tube is a vacuum “tube” in which there are two electrodes. By applying direct voltage to the electrodes, electrons are emitted from the cathode, which reach the surface of the anode after accelerating in the electric field. In order to ensure that a sufficiently large number of electrons are emitted from the cathode, its temperature has to be increased (thermal emission). The easiest way to achieve this is to use a filament, the number of the electrons emitted from which increases with its temperature. The high-speed electrons that strike the anode interact with the atoms of the material of the anode. The electrons decelerate in the electric field of the atoms, i.e. their velocity decreases, which results in the emission of Brehmsstrahlung. The decelerating electrons lose their kinetic energy in several steps, but it is possible that a single photon carries away their total kinetic energy in the first interaction. This means that the frequency distribution of the resulting Brehmsstrahlung is continuous and it is characteristic of the specific X-ray tube with a given structure and operating parameters (Figure 11.). A so-called short-wave limit (the Duane-Hunt limit, 1915) is observable in the spectra, where the intensity of the beams is zero. The wavelength characteristic of the short-wave limit is closely related to the magnitude of the anode-cathode voltage on the X-ray tube. The photons corresponding to this wavelength are created when the electrons striking the anode lose their total energy in a single interaction. This energy is determined by the anode-cathode voltage on the X-ray tube. The shape of the continuous X-ray spectrum is of great significance in diagnostics, since by changing the voltage on the X-ray tube according to the absorption of the X-rayed object, the quality of imaging can be influenced.
It is also of fundamental importance that the short-wave limit is a direct evidence of the quantum nature of the electromagnetic interaction. Experiments have shown that the inverse of the value of the Duane-Hunt limit is proportional to the magnitude of the applied anode-cathode voltage.

Figure 11. The wavelength distribution of a beam emitted by an X-ray tube in the cases of different anode-cathode voltages

 

The relation between the tube spectrum and the number of electrons is significantly simpler; it is proportional to the cathode-anode current. By increasing the tube current, the shape of the spectrum does not change, only its magnitude increases in direct proportion to the current.