Principle Of X Ray Tube
The X ray tube is a vacuum diode operating at high voltage. It consists of two electrodes: a filament used to emit electrons, a cathode, and a target used to receive electron bombardment as an anode. The two stages are sealed in glass or ceramic shell with high vacuum.
Vacuum electron devices that produce X rays by high-speed electrons impinging on metal targets. According to the way of producing electrons, X ray tubes can be divided into two categories: inflatable tube and vacuum tube.
X ray tube
The inflated X ray tube is an early X ray tube. In 1895, W.C. roentgen discovered X ray when conducting the Crookes tube experiment. Crookes tube is the earliest inflated X ray tube. After the tube is switched on, the gas in the tube is ionized, and the electrons escape from the cathode under the bombardment of positive ions. After acceleration, the electrons collide with the target surface to produce X-rays. Inflated X ray tubes are seldom used because of their small power, short life and difficult control. In 1913, W.D. Coolidge invented the vacuum X ray tube. The vacuum inside the tube is not less than 10-4 PPA. The cathode is a direct heating spiral tungsten wire, and the anode is a metal target embedded on the end of the copper block. The target and electron beam energy are selected according to the usage of pipes, and tungsten is used as target. For some purposes, silver, palladium, rhodium, molybdenum, copper, nickel, cobalt, iron and chromium are also used. The working temperature of the cathode is about 2000K. The electrons emitted from the cathode are accelerated by tens of thousands to hundreds of thousands of volts and then hit the target surface. The cathode is surrounded by a metal mask with a slotted front. The potential of the metal cover is equal to or lower than that of the cathode, forcing electrons to focus in a narrow area on the target surface to form focal spots. X rays radiate from the focal spot in all directions and output through the window on the wall. The window is usually made of beryllium, aluminum or light glass which absorbs very little X ray, and beryllium is the best.
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