The cathode and anode are housed inside a glass "tube" and contained within a vacuum. If you spin the anode, the electrons are spread out over the full 360, increasing the target area by a factor of 300 Lets assume the electrons cover an arc of 1.2 degrees on the anode. Think of it this way - have you ever put a pen on a spinning top (or wheel)? The tip of the pen is very small but you create one big line. This allows creates a larger target for the electrons. In modern x-ray tubes, the normal anode spins (very fast - around 3,500 rpm). The anode is a relatively flat (circular) disc that acts as the target for the stream of electrons that are emitted from the cathode. As a whole, the anode assembly is important as the source of the x-rays, as the primary conductor of heat out of the tube and as an integral part of the high voltage circuit. The anode assembly consists of the anode, the stator and the rotor. The anode assembly is the positive end of the x-ray tube. Essentially, the force that causes the electrons to repel each other is overpowered by the repulsive force of the focusing cup and the electrons tend to converge rather than diverge.ĭiscussion of the associated wiring is relatively questionable, except when discussing (a) focal spot size selection or (b) grid-biased (or grid-controlled or grid-pulsed) tubes. As this is counterproductive in x-ray tubes, the focusing cup is a negatively charged housing that "encourages" the electrons to stay together. Since negative repels negative, the electrons that have been emitted have a tendency to diverge. The filament emits electrons, all of which have a negative charge. The filament current is not quite the same as the mA that the radiographer controls, but they are related.įocusing CupThe focusing cup (made of Molybdenum) is a shallow depression in containing the filament. The amount of electrons that "break free" of the filament (or are emitted) is directly proportional to the amount of electrons flowing inside the filament (i.e., the current). When an electric current is passed through the filament, it heats up to such an extent that some of the electrons have enough (thermal vibrational) energy to break free of the attractive (electrostatic) forces holding them inside the filament. Most radiographers, when talking about the cathode, are actually talking about the filament.įilamentThe filament is actually a very tightly wound coil of wire. It is important to note that in radiography, the terms cathode and filament are generally interchangeable. It consists of the filament (or filaments), the focusing cup and all the associated wiring. The cathode is more correctly called the cathode assembly. The purpose of the cathode is to produce the thermionic cloud, conduct the high voltage current, and focus the emitted electrons (1). The cathode is the negative side of the x-ray tube. In any electric circuit, current flows from the negative side of a circuit to the positive side, and it is no different when an x-ray tube is involved. The negative end of the x-ray tube is called the cathode, and the positive end is called the anode. The x-ray tube consists of a cathode and an anode within a vacuum. The x-ray tube is essentially a massive valve, or switch, completing an electric circuit.
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