About "X-ray Tube"
How much do you know about "X-ray tubes" :
They are vacuum diodes that operate at high voltages. Contains two electrodes: a filament that emits electrons as a cathode, and a target that receives electron bombardment as an anode. Both stages are sealed in a high vacuum glass or ceramic enclosure. A vacuum electronic device that produces X-rays by impinging high-speed electrons on a metal target surface. X - ray tubes can be divided into gas - filled tubes and vacuum tubes according to the way electrons are generated. The inflatable X-ray tube was an early type of X-ray tube. In 1895, W.C. Roentgen discovered X-rays while conducting experiments on the Crooks tube.
The Krooks tube was the first inflatable X-ray tube. After the tube is connected to the high pressure, the gas in the tube ionizes. Under the bombardment of positive ions, electrons escape from the cathode and collide with the target surface after acceleration to produce X-rays. Inflatable X-ray tube has the advantages of low power, short life and difficult control, so it is seldom used later. In 1913, W.D. Coolidge invented the vacuum X-ray tube. The vacuum degree in the tube is not less than 10-4 Pa. The cathode is directly heated spiral tungsten wire and the anode is a metal target embedded in the end face of copper block. The target material and electron beam energy are selected according to the purpose of the tube, and tungsten is usually used as the target material. In some applications, silver, palladium, rhodium, molybdenum, copper, nickel, cobalt, iron, chromium and other materials are used. The operating temperature of the cathode is about 2000K, and the emitted electrons are accelerated by tens of thousands to hundreds of thousands of volts before hitting the target surface. The cathode is surrounded by a metal hood with a slotted front end.
The potential of the metal mask is equal to or lower than that of the cathode, forcing electrons to focus into a narrow area on the target surface, creating a focal spot. X-rays radiate in all directions from the focal spot and are output through Windows in the wall of the tube. Windows are generally made of beryllium, aluminum or light glass with very little X-ray absorption, with beryllium sheets as the best.
How an X-ray tube works
The X-ray tube consists of two electrodes, an anode and a cathode, which are used for the target to be bombarded with electrons and for the filament to emit electrons. Both stages are sealed in a high vacuum glass or ceramic enclosure. The power supply portion of the X-ray tube shall at least consist of a low-voltage power supply for heating the filament and a high-voltage generator for applying a high voltage to the poles.
When enough current passes through the tungsten wire to produce a cloud of electrons, and enough voltage (kilovolt level) is applied between the anode and the cathode, the cloud is pulled towards the anode. At this time, the electrons hit the tungsten target at high energy and high speed. When the high-speed electrons reach the target surface, their motion is suddenly stopped. A small part of their kinetic energy will be converted into radiation energy and released in the form of X rays. Changing the current of the filament can change the temperature of the filament and the amount of electrons emitted, thus changing the tube current and the size of the X-ray intensity. Changing the excitation potential of the X-ray tube or choosing a different target can change the energy of the incoming X-ray rays or the intensity at different energy points.
Due to high-energy electron bombardment, the temperature of the X-ray tube is very high, so it is necessary to carry out forced cooling on the anode target. Although the energy efficiency of X-ray tube generating X-ray is very low, at present, X-ray tube is still the most practical X-ray generating device and has been widely used in X-ray instruments. At present, the medical use is mainly divided into diagnostic X-ray tube and therapeutic X-ray tube.
The X-ray tube is required to have a small focus and a high intensity to form a large power density. Therefore, a relatively large power must be supplied on the anode, but the efficiency of the X-ray tube is very low. More than 99% of the electron beam power becomes the anode heat consumption, and causes the focal spot to overheat. The method to avoid anode overheating is to cool the anode or tube in different ways to reduce the temperature at the focal spot, or tilt the target surface at a certain Angle to provide a larger heat dissipation area.
Then came the rotating anode X-ray tube, which allowed a high power density and a small focus due to the high speed of the target surface rotation (up to 10,000 RPM).In modern times, an X-ray tube is equipped with a control gate between the anode target surface and the cathode, and pulse modulation is applied on the control gate to control the output of X-rays. The timing and repetition exposure can be adjusted by changing the pulse width and repetition frequency.
X-ray tube structure composition
The fixed anode X-ray tube is one of the simplest X-ray tubes in common use. Its structure consists of three parts: anode, cathode and glass shell to fix the poles and keep high vacuum in the glass tube. The anode is composed of an anode head, an anode cap, a glass ring and an anode handle. The main role of the anode by the anode head target surface (generally choose tungsten target) to block the high-speed movement of the electron flow and produce X-rays, and the resulting heat radiation or through the anode handle conduction out, but also absorb secondary electrons and scattered rays. X-rays produced in tungsten alloy X-ray tubes use less than 1% of the energy of the fast-moving electron stream, so heat dissipation is a very important problem in X-ray tubes.
The cathode is mainly composed of filament, focusing cover (or cathode head), cathode sleeve and glass core column, etc. The electron beam bombarding the anode target is emitted by the filament of the hot cathode (usually tungsten filament), which is focused by the focusing hood (cathode head) under the high voltage acceleration of the tungsten alloy X-ray tube. The high speed moving electron beam impinges on the anode target and is suddenly blocked, which produces X-rays with continuous energy distribution (including characteristic X-rays reflecting the metal of the anode target).
