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| Since heat is unable to flow out of the
focal spot as fast as it is pumped in, the maximum energy must be
limited, in order to prevent the surface of the target melting. The
maximum rating is further limited by the need to concentrate the
electron beam onto a small area, in order to reduce the inevitable
unsharpness. An X-ray image is essentially a shadowgraph. X-rays cannot
be focused, and so the bigger the point of origin of the radiation, the
more blur there is in the image. The Dutch company, Philips,
introduced a tube in about 1938 that overcame the problem, by making the
anode in the form of a Tungsten disk that was rotated by means of an
induction motor, in which the Stator windings were outside of the glass
envelope, and the copper rotor on the inside. In this way the focal spot
was extended into a ring. The area of tungsten that was heated by the
electron beam was rapidly moved out of the way to be replaced a cooler
area of tungsten. As the heated spot on the anode moved around, the heat
was able to diffuse into the body of the tungsten disk, until it reached
equilibrium with rest of the anode. Early tubes running from a 50Hz
mains supply rotated at 3,000 rpm, but modern high speed tubes may run
at up to 12,000 rpm. In order to extend the life of the bearing, which have to run in a vacuum, without lubrication, the anode disk is mounted on a Molybdenum stem. Molybdenum is an even worse conductor of heat than tungsten, and so limits the amount flowing down the shaft and into the bearings. Heat is therefore lost from the anode disk by radiation, rather than conduction. |