b. One such requirement is a high melting point. The production of x-rays is a
very inefficient process as less than 1% of the energy supplied to the x-ray tube is
converted into x-rays. The remaining 99% is converted into thermal energy (heat).
Consequently, the target temperature frequently reaches high proportions. Tungsten's
3,370C melting point makes it suitable to withstand high temperatures. The target
must also be able to conduct the heat away as it is generated in order to keep
temperatures below the melting point. Tungsten's heat conductivity, while not extremely
high, is considered acceptable in view of its other excellent characteristics.
c. Some x-ray targets are constructed with a rhenium-tungsten alloy. The
addition of rhenium makes the target surface more resistant to surface etching at high
d. Molybdenum is used for the target material in some x-ray tubes made
especially for mammography. The characteristic radiation produced in molybdenum is
somewhat lower than that produced in tungsten, which makes the x-ray beam more
suitable for x-rays of thick, soft tissue.
e. Even with tungsten1s high melting point, overheating of the target to the point
of melting, cracking, pitting, etc. is a continuing problem. One way to provide better
heat dissipation is to embed the tungsten in copper, as illustrated in the stationary
anode in figure 3-8. Copper has higher heat conductivity than tungsten and, therefore,
carries the heat away more quickly. Another way to reduce point heat buildup is to
rotate the anode, as seen in figure 3-9. By continuous spinning of the target, the focal
spot presented to the electron stream is always changing. This spreads the electrons
and, consequently, the heat over a larger area. Most modern x-ray tubes have a
rotating anode. In some special-purpose tubes, the anode can be rotated at speeds up
to 10,000 rpm.
Figure 3-8. Copper anode used to help dissipate heat.