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.

X-rays Shielding Calculation

Like gamma rays, X-rays have no definite range - the intensity of radiation transmitted through a material falls exponentially with the thickness of the material. Tungsten alloy X-rays shielding become more and more popular for radiation protection. Following is the calculation:

I = Io e-ux

where u is the linear attenuation coefficient of the material

For typical 50 KeV X-rays the half value thicknesses, H1/2 = 0.69/u, for some common materials are

         Air                     H1/2 ~ 10m

        Body tissue          H1/2 ~ 3 cm

        Lead (Pb)             H1/2 ~ 0.1 mm

        Steel                    H1/2   ~ 0.5 mm

An attenuation of I/Io = 10-10 is achieved by lead of thickness 3.3 mm or by steel of thickness 16.6 mm.

What is X-ray Spectrum

Most of the X-ray output of rotating anode sources has a continuous energy range that extends up to a value, Emax that, measured in electron volts, is equal to the acceleration voltage applied to the tube.. This Bremsstrahlung ("braking radiation") arises from the accelerations suffered by the electrons during collision with the atoms of the anode, then that is the X-ray spectrum, superimposed on the Bremsstrahlung background there are intense narrow line emissions that are characteristic of the material of the anode. These characteristic lines are important for crystallography but since they amount to only a few percent of the total X-ray output they are of little radiological importance. For the radiation during this whole process, tungsten alloy shielding will be the ideal material for radiation protection, for more details, you could visit our website: http://www.tungsten-alloy.com/X-ray-target-collimator.htm.