The principle of x-ray tube operation

When the filament is fed with the heating current, it emits electrons. They are attracted and accelerated in the direction of the anode (~ 20 mm). Since for total reflection condition alignment a good focusing is required, fine focus tube are preferred, for which the bombarded area is ~ 0.25 mm x 10 mm. Fine-focus X-ray tubes are available with several target (anode) materials: Au, W, Ag, Mo, Cu, Co, Fe, Cr, among others.

A proper flux for TXRF excitation can be obtained from x-rays tubes operating at maximal power in the range of 2 – 3 kW. Besides x-ray emission, some energy is released in the form of heat at the anode material, and therefore the x-ray tube must be cooled to avoid its damage. Typically water cooling systems are used for this purpose, with a water consumption of about 4L/min at a pressure of 3-5 bar and a temperature of 20-25^oC. The tubes’ life can span some 3000-6000 working hours. To ensure a long live operation warm-up conditioning should take between 15 to 45 min, depending on the period of interruption, and the cooling-down time should last for at least 10 min. The nominal value for maximum power of operation should never be exceeded.

The X-rays are emitted in all directions but, out of the tube they emerge only through a side window. The diameter of the window is ~ 6mm and its distance to the bombarded area is ~ 30mm. This means that the beam leaving the tube will be a cone with an aperture of ~ 12^o. By tilting the tube by 6^o, the beam axis can run horizontally. From the outside the focus of the tube is “observed” under 6^o and “looks” like a line of 0.25 mm x 10 mm.