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A modern dental X-ray tube. The heated cathode is on the left. Centre is the anode which is made from tungsten and embedded in the copper sleeve. William Coolidge explains medical imaging and X-rays. An X-ray tube is a vacuum tube that converts electrical input power into X-rays. [1]
In early and basic X-ray equipment, the applied voltage varies cyclically, with one, two, or more pulses per mains AC power cycle. One standard way to measure pulsating DC is its peak amplitude, hence kVp. Most modern X-ray generators apply a constant potential across the X-ray tube; in such systems, the kVp and the steady-state kV are identical.
Another way of producing X-rays are particle accelerators. They produce X-rays from vectorial changes in their direction through magnetic fields. Whenever a moving charge changes direction, it has to give off radiation with the corresponding energy. In X-ray tubes, this directional change is the electron hitting the metal target (anode).
X-ray generators produce X-rays by applying a high voltage between the cathode and the anode of an X-ray tube and in heating the tube filament to start the electron emission. The electrons are then accelerated in the resulting electric potential and collide with the anode, which is usually made of Tungsten .
Megavoltage X-rays are produced by linear accelerators ("linacs") operating at voltages in excess of 1000 kV (1 MV) range, and therefore have an energy in the MeV range. The voltage in this case refers to the voltage used to accelerate electrons in the linear accelerator and indicates the maximum possible energy of the photons which are subsequently produced. [1]
The energy and penetrating ability of the X-rays produced by an X-ray tube increases with the voltage on the tube. External beam radiotherapy began around the turn of the 20th century with ordinary diagnostic X-ray tubes, which used voltages below 150 kV. [6]
An X-ray spectrograph consists of a high voltage power supply (50 kV or 100 kV), a broad band X-ray tube, usually with a tungsten anode and a beryllium window, a specimen holder, an analyzing crystal, a goniometer, and an X-ray detector device. These are arranged as shown in Fig. 1.
The kinetic energy of the electrons is converted into heat and radiation (X-ray photons) due to these interactions. Most of the energy carried by the electrons is converted to heat (99%). Only 1% is converted into radiation or X-rays. In order to assist with the dissipation of such high heat, a larger focal spot or focal track is needed.