KJ3055-Chapter 8 (X-ray spectrometry)

The x-ray tube

Fig. 1. Schematics of an x-ray tube.

In an x-ray tube (Fig. 1) electrons emitted by the glowing cathode K are accelerated towards the target anode A due to the high tube voltage U_a (10 to 100 kV). As the electron beam impacts on the target, the high kinetic energy of the electrons is partially converted into x-ray photons. Much of electron energy is released as heat and the target should be cooled by a water stream (W). U_a is the heating voltage applied to the cathode.

The x-ray spectrum produced the tube consists of bremsstrahlung (braking radiation, continuum spectrum) and characteristic radiation (Fig. 2). Continuum component arises from the radiation emitted when electrons brakes when colliding with target. Characteristic radiation is caused by electron transitions in target atoms as a result of excitation by the electron beam. It is characteristic to the chemical element in the target (hence its name) and appears only if the electron energy overcomes the threshold value required to remove an inner shell electron. Often, radiation intensity is plotted as a function of photon energy (Fig. 3).

Fig. 2. Spectrum of a rhodium target tube operated at 60 kV, showing continuous component and characteristic K lines.

Fig. 3. X-Ray spectrum produced by an Ag target. Notice the use of photon energy as independent variable ().

The design of a contemporary x-ray tube is shown in Fig. 4 which displays a side-window tube. End-window tubes are also available.

Fig. 4. The principle of the side-window tube. Source

F.G. Banica, 09-03-24