# Why elements with high atomic mass (like tungsten) are used in the production of X-rays, in the Cathode-Ray Tube?

I was studying about the production of X-rays, and was struck at how elements having high atomic mass (like tungsten) are used for the production of high energy X-rays. I am wondering why low atomic mass elements are not used for the production of high energy X-rays.

I have read that "tungsten has the highest melting point of any metal, and because over 99 percent of the energy put into the X-ray tube to produce the beam is turned into heat energy, the ability to dissapate the heat without loosing structural integrity is critical".

The K-series X-rays are generally of most practical importance because their energy is greatest. Their energy increases regularly with atomic number $Z$ of the element, for example, only about $1\ \mathrm{keV}$ for sodium ($Z=11$) and about $100\ \mathrm{keV}$ for uranium ($Z=92$). An empirical law concerning the characteristic X-rays that are emitted by atoms is expressed by Moseley’s law.
Since only a small fraction of about $1\ \%$ of the energy generated in an X-ray tube is actually emitted as X-rays and the rest of the energy is released as heat, the temperature of the anode target can be very high during the operation of the X-ray tube. Therefore, the anode has to be made of high-temperature materials. You already mentioned the high melting point of tungsten ($Z=74$); whereas for example the heavier elements lead and bismuth cannot be used because of their low melting points.