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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".

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If a sample containing a particular element is irradiated (e.g. with photons or, in case of an X-ray tube, with electrons) with an energy high enough to excite inner electron orbitals, X-rays are emitted in the de-excitation. In contrast to the continuous bremsstrahlung energy spectrum, the energy liberated in the transition from the excited to the ground state takes the form of a characteristic X-ray photon whose energy is given by the energy difference between the initial and final states. If the temporary vacancy has been created in the K shell of an atom, then a characteristic K X-ray is liberated when that vacancy is subsequently filled. In particular, if the concerned electron comes from the L shell, than a Kα photon is generated whose energy is equal to the difference in binding energies between the K and L shells.

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.

Hence, in order to produce characteristic X-ray photons with a high energy, K-series X-rays of a heavy element are preferred.

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.

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  • $\begingroup$ Since I just had to look this up myself: X-ray spectroscopy uses a set of historical names for the electron shells, X-ray notation. "K" and "L" correspond to principal quantum numbers 1 and 2, respectively. The notation seems only to accommodate a subset of the orbitals, for instance there are three 2p orbitals but only two L-codes for them (this may be because some of the 2p orbitals produce indistinguishable emission lines). $\endgroup$ – zwol Aug 9 '16 at 13:15
  • $\begingroup$ So, practically is it possible to use heavy metals like Os, Ir, Pt etc. which have high melting points? $\endgroup$ – Apurvium Sep 19 '19 at 12:45
  • $\begingroup$ In places like X-ray crystallography systems and hospitals, copper (Z=29) is often used for the anode. I think this partly because of the good thermal conductivity but it may be that the lower energy K-shell X-rays are better suited for those applications. $\endgroup$ – uhoh Dec 7 '19 at 5:34

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