X-ray Output & Tungsten Alloy Anode

The area over which the electrons from the cathode strike tungsten alloy anode is referred to as the focal spot. The cooler the anode can be kept, the smaller the focal spot can be and the greater the image detail that is possible. If a high X-ray output is required, a larger focal spot would be needed to mitigate the temperature increase.

In the early tubes the angle of the target was usually 45 degrees (see figure below left). Later tubes often employed the so-called line-focus principle in which the target angle was closer to 20 degrees (see figure below right). This reduced the effective area of the focal spot (as viewed from the perspective of the object being x-rayed permitted) without significantly affecting the area of the anode bombarded by the electron beam from the cathode. In other words, it permitted high loading (x-ray intensity) without having to sacrifice image details.

Tungsten Alloy Anode for X-ray Radiation

Tungsten alloy anode is the most commonly used target material because it has a high atomic number which increases the intensity of the x-rays, and because it has a sufficiently high melting point that it can be allowed to become white hot.

During operation, the tungsten alloy anode can get as high as 2,700 degrees centigrade. In many cases, it is surrounded by copper - the high heat capacity of copper improves the dissipation of heat. It is very suitable for X-ray radiation.