How do electrons in tungsten atoms produce X-rays?

[tschneider11]

Hi Everyone,

I'm writing a research paper on the role electron transitions play in producing X-rays and there are some points I am having a tough time clearing up.

When the the cathode fires electrons at high velocity towards a tungsten anode, I understand that a series of events may take place. Bremsstrahlung radiation can be given off or electrons can collide with tungsten atoms. Assuming the energy of the electrons is great enough, this can knock loose electrons from lower orbitals of the tungsten atoms leaving a vacancy. Spontaneously, electrons occupying higher energy levels drop down to fill the vacancy and restore order within the atom. The energy the electron loses when it drops energy levels can either result in the emission of a characteristic X-ray photon or with the emission of an auger electron. I was under the impression that the lower orbital electron is ejected into space and leaves the atom altogether and a separate electron from a higher energy level replaces it. But when I read other material about tungsten atoms absorbing the energy and becoming excited I sometimes get lost. Is this electron just promoted to a higher energy level and not really completely ejected from the atom? And does this mean that the same electron falls back to its ground state and fills its own vacancy releasing the energy that it once absorbed in the form of an X-ray?

I hope this makes sense and thanks in advanced.

 

[Bob S]

Is [the] electron just promoted to a higher energy level and not really completely ejected from the atom? And does this mean that the same electron falls back to its ground state and fills its own vacancy releasing the energy that it once absorbed in the form of an X-ray?.

It would be extremely rare for the ejected electron to fall back to the ground state and fill its own vacancy. It predominantly gets completely ejected from the atom and becomes a free electron.

Bob S

 

[tschneider11]

So if an electron moves from a higher energy level to a lower energy level, it too must leave behind a vacancy. Do transitions continue to occur to fill the new vacancies? And if so, does a free electron fill the last vacancy? If that were to be the case I would imagine the atom remains ionized for an extremely short period of time before the process starts over again.

 

[Bob S]

Atoms in tungsten anodes have lots of electrons, and a vacancy (e.g., in the K-shell) will be filled in a few picoseconds by a bound electron, e.g., from L-shell. The probability that the ejected electron has exactly the right energy to fill any vacant atomic state, and there aren't any before the ejection. The ejected electron will lose any excess kinetic energy in the tungsten by the Bethe-Bloch ionization dE/dx process, stop in the conduction band, flow into the positive terminal of the power supply, and out the negative terminal to the cathode of the x-ray tube.

Bob S