Characteristic X-Rays are generated when excited sample atoms undergo a relaxation process. For that the atoms need to be excited first and this can be done with high energy electromagnetic radiation (in XRF) or accelerated particles such as electrons (in SEM). The primary beam kicks out an inner shell electron and a vacant spot is left behind. As this state is unstable, a higher shell electron will soon move into this vacant spot and during this process energy is emitted in the form of X-Rays. This emitted radiation has a specific energy which depends on the binding energies of the two electrons that participated in this process. If this emitted ( characteristic ) x-ray radiation is detected then the composition of the material can be measured.
X-ray tubes are devices that produce x-ray radiation, which is useful in various applications. For example in medicine this high energy electromagnetic radiation is used for imaging your body. In airports x-rays are used to scan your luggage for prohibited items. X-rays can also be used for materials characterization in techniques such as x-ray fluorescence spectroscopy or photoelectron spectroscopy.
An x-ray tube consists of an anode and a cathode within a casing that can hold vacuum. The cathode is heated to high temperatures, where it starts emitting electrons – this process is known as thermionic emission. A high voltage applied between the cathode and the anode accelerates the emitted electrons towards the anode. When these high energy electrons interact with the anode some of the energy is converted into x-ray radiation and some into heat. Thats why water cooling is needed to prevent the overheating of the anode.The emitted x-ray radiation consists of two components – bremsstrahlung and characteristic x-rays. In the case of bremsstrahlung the electromagnetic radiation is emitted from the negative electron when its trajectory is changed by a positively charged atoms nucleus. This radiation has a very broad energy range. Its energy and intensity depends on the voltage between the anode and the cathode, on the cathode filaments heating current and on the atomic number of the anode material. Characteristic x-rays however have a very specific energy, which strongly depends on the anode material. This radiation is generated when the accelerated electrons excite the anode atoms by kicking out inner shell electrons. In the relaxation process a higher shell electron moves to the vacant spot and the excess energy is emitted in the form of x-rays. The energy of these characteristic x-rays depend on the binding energy of the electron that was kicked out and the binding energy of the electron that occupied the vacant spot. The generated x-rays leave the tube through a beryllium window. Beryllium is used as a window material because it doesnt absorb much of the x-rays as it has a low atomic number. Be sure to follow us in youtube for more awesome videos in the future!