In 1933, the rotating anode X-ray tube was invented, in which the anode is made to rotate before the electron is emitted. It was developed to increase the heat loading with higher X-ray output. In these tubes, the electrons transfer their energy over a large area of a rotating target. Rotating anode tubes are larger in size, but the principle and function are similar to that of stationary X-ray tube.
Principle
Consider a rotating anode of radius R and circumference L. The electrons bombard a region of height ab and
width cd. The length may range up to the circumference (L 2pR) depending on the exposure time. But, the X-rays always appear to come from a focal spot of area cd × cd.
Let us consider a stationary anode of actual focal area 7.3 mm × 2 mm, and rotating anode of R = 30 mm and length 7.3 mm, then
Actual focal area of rotating anode
Loading gain = ————————————————————
Actual focal area of stationary anode
2 p × 30 × 7.3
= ——————————— = 94.2
7.3 × 2
Thus, the rotating anode arrangement helps to increase the loading to a greater extent of the order 100. The construction of such a rotating anode is a remarkable technological development. The diameter of the tungsten disk determines the total length of the target track, and obviously affects the maximum permissible loading of the anode.
Cathode
Rotating anode X-ray tube consists of a cathode and an anode which are kept in a glass bulb . The cathode is a tungsten filament which is offset from the long axis of the X-ray tube to face the target near the periphery of the anode disk. Usually, rotating anode tubes are fitted with two filaments , one larger than the other set side by side in the cathode assembly. One filament is designed to focus the electrons on a larger area of the anode, which require heavytube loading. The other filament is used to focus the electrons on a smaller area of the target. This type is used when high resolution is required. Both filaments should focus the electron on the same part of the anode, so that focal spot is at the same point for both modes of operation. Some tubes provide two target angles for two filaments, so that each filament will have a separate focal spot. Smaller angle is used with the smaller focal spot.
Focusing Cup
The focusing cup (cathode block) surrounds the filament, shapes the electron beam width. It is used to focus the electrons on a small area (focal spot) in the anode. There are two ways by which the focusing cup is energized, namely, unbiased and biased . In unbiased setup, same voltage is applied to both focusing cup and filament. In this type, the electron spread is wider and the focal spot width is larger. In biased X-ray tubes, insulated focusing cups are used and it is given more negative supply (–100 V) than the filament. This creates a tighter electric field around the electron, reduces the electron spread and gives smaller focal spot width. Thus, the focusing cup width determines focal spot width and the filament length determines the focal spot length
.Anode
The anode is made in the form of large disk of tungsten, or an alloy of tungsten, which is saucer shaped. The target track is near the periphery of the disk, to maximize the length. The track is a mixture of 90% tungsten and 10% rhenium (Z = 75), which reduces the crazing effect caused by thermal stresses. Modern rotating anodes are made from solid molybdenum onto which a thin tungsten-rhenium focal track is coated. The specific heat capacity of molybdenum is higher than that of tungsten (250 vs 130 J Kg–1K–1). The mass of molybdenum anode is also lower due its lower density. Some high output X-ray tubes have radial slots cut into the anode disc to reduce thermal stresses caused by repeated heating and cooling. Heavy duty X-ray tubes have graphite layer (Carbon) in the back of the anode disk. The anode disk has a beveled edge and the angle of the bevel may vary from 6 to 20o. The bevel is used to achieve the line focus principle.
Anode Stem
The anode disk is mounted on a stem, which is attached to the rotor. The anode assembly rotates with the help of bearings. The stem is made of molybdenum, which is having high melting point (2620°C) and poor heat conduction. The molybdenum stem prevents the flow of heat from the tungsten to the bearings of anode assembly, due its small cross section. Thus, the bearings are protected from heat, which may cause them to expand and bind. Higher the length of the stem, higher the inertia of the tungsten disk, more will be the load on the bearings. Hence, it is desirable to keep the stem as short as possible.
Rotor
The anode disk is connected to a rotor, which is made up of copper bars arranged around a cylindrical iron core. There are electromagnets surrounding the rotor, outside the glass envelope is called stator . Both the stator and rotor is called as an induction motor.
When the stator coils are energized, a rotating magnetic field is produced, that induces current in the copper bars of the rotor. This induced current produces an opposing magnetic field that causes the rotor to spin. The rotor rotates at a speed of about 3000–9000 revolutions per minute (rpm). This will facilitate the electrons to bombard a constantly changing area of the target. Since the electrical conductivity of copperis higher, it will facilitate induction of strong currents from the induction coil supply. The surface of the rotor is blackened to enhance heat dissipation by radiation process. The rotor support is made of steel and the positive high tension supply is made at the end of the rotor support outside the glass envelope. Low speed rotor operates from 60 Hz power (single phase) and gives rotation of about 3000 rpm. This speed is too slow for short exposure of the order of mill-seconds. High speed rotor operates from 180 Hz power (3 phase) and gives a rotation speed of 9000 rpm. If the speed of rotation is increased, the heat generated at the focus will be spread over a larger area. Modern X-ray tubes employs higher rotor speeds by increasing the frequency of the stator supply with frequency multiplying circuits. The X-ray machines are designed so that the tube cannot be energized until the anode attains its full speed. This delay time (1–2 s) is incorporated in the exposure buttons. Thepower supplied to the induction coils, produce eddy current which causes heating of the rotor.
Bearings
The anode assembly is rotated with the help of bearings, which are made of steel ball races. Bearings are in the high vacuum environment and require special heat insensitive, nonvolatile lubricants. The bearings are coated with lead or silver to act as (metallic lubricants) lubricant. Commonly available lubricants such as oil and grease cannot be used,
since it will vaporize while heating and destroy the vacuum. Even dry lubricant (graphite) would wear off as a powder and destroy the vacuum. When the X-ray unit is turned on, the motor alone is energized first for a few seconds until the rotor reaches its operating speed. Then, a high voltage is applied to the tube for the required exposure. After the exposure, the rotor is slowed down quickly by dynamic braking to avoid the wear in the bearings. Since the electrons are striking the whole circumference of the anode, no part of the anode attains very high temperature. The heat from the tungsten disk is dissipated by radiating through the vacuum to the wall of the tube, and then into the surrounding oil and tube housing. The life of a rotating anode X-ray tube is limited, because of pitting due to continuous electron bombardment on the anode surface.These changes are due to thermal stress. This pitting reduces the X-ray yield and changes the spectral distribution. The decreased output, results in excessive scattering of X-rays and increased absorption of X-rays by the target itself. A pitted anode will affect the electric field between cathode and anode, so alter the size of the focal spot.
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