Anode

Two basic construction principles exist: stationary anodes and rotating anodes. The stationary anode is a much simpler, more reliable and cheaper construction than the rotating type. However, it cannot be exposed to the very high tube loads that are demanded for most modern X-ray equipment. Stationary anodes are therefore found only in dental equipment, mobile C-arm units for fluoroscopy and low-load radiography. Stationary anodes are also found in X-ray units for orthovoltage radiotherapy.

In the stationary anode (Fig.1), the electron target is made of tungsten or an alloy of tungsten and up to 15% rhenium. Tungsten has a very high melting point, 3370 C. The target is commonly mounted into a large copper block, shaped as a stem, due to the heat conducting capacity of copper. The copper stem carries the heat away from the target and is sometimes cooled by circulating water or oil. The load limit of a stationary anode is given by the temperature that is reached in the copper at the boundary between the tungsten target and the copper stem. With a stationary anode, the maximum tube load for radiography is around 100 W/mm² and for continuous fluoroscopy around 30 W/mm².

The rotating anode as we know it today, came into use in 1929. However, the first construction of a rotating anode tube was done by Wood in 1897. In his construction, the cathode filament was placed inside a rotating glass envelope, which also functioned as the anode. The modern rotating anode tube uses a disk-shaped anode, normally made of molybdenum, with a 1-2 mm thick surface layer consisting of an alloy of tungsten and rhenium (5-15%) (Fig.2). The rhenium is added because it makes the alloy more elastic, which prevents cracking of the surface and extends the life of the X-ray tube. Molybdenum is used as the anode base because it has twice the heat capacity of a pure tungsten disk with the same mass. For X-ray tubes where extremely high heat capacity is needed, a graphite backing is often used behind the molybdenum disk because graphite has a very high heat capacity per unit mass - twice that of molybdenum. However, the heat is dissipated more slowly. Anodes with large graphite backings are therefore found in equipment where momentary high tube loads are used, for instance in CT scanners. One drawback with graphite anodes is the risk that the graphite can come loose, which happens if the temperature exceeds 1 2001 300 C. Compared to stationary anodes, tube loads for rotating anodes are much higher and can exceed 10 000 W/mm² on the anode surface.

The technique of making the anode disks has changed during the last decade. The disks are now forged instead of sintered. New anode surface materials are being introduced, such as alloys of tungsten, rhenium, titanium and zirkonium, which will also contribute to anodes with longer life time.

A special problem is the removal of heat from the anode and anode stem. When the temperature difference of the anode and the cooling medium (oil) is large, most of the energy is given away as radiated heat, the efficiency of which is proportional to (T_anode - T_oil)^4. However, when the oil is heated, most of the anode heat is transported away through the bearings of the anode stem. For common ball bearings, the contact surface between balls and stem is very small and the heat conduction is inefficient, giving a long cooling time. Some newer constructions therefore make use of another bearing technique for the stem, which is inserted into a cylindrical casing with some degree of spacing between stem and casing. In this space, an alloy with a very low melting point is introduced. When the anode rotates, this alloy melts and functions both as lubricant and heat conductor with a very large contact area. This arrangement makes the tube cool off much quicker.

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