Why are more x-rays made from metal than glass

  • #1
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i know that x-rays are produced when a beam of high energy electrons hits a metal anode but why does that work better than when that same beam hits glass (like in the Crookes tube)?

Thanks
 

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  • #2
mjc123
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When an energetic electron hits an atom it can knock out a core electron. Then one of two things can happen:
(i) An electron from a higher shell can fall into the vacancy in the lower shell, accompanied by the emission of an X-ray photon.
(ii) An electron from a higher shell can fall into the vacancy in the lower shell, while another outer electron is expelled from the atom (Auger electron).
The quantum yield of X rays increases with atomic number; light elements give mostly Auger electrons and heavy elements mostly X rays. The crossover is somewhere around zinc (Z = 30). Thus glass, containing mainly Si and O, will give a lower yield of X rays than a heavier metal, such as iron or tungsten.

Thus much I know from my acquaintance with SEM. Why things are that way round, I don't know.
 
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  • #3
ZapperZ
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When an energetic electron hits an atom it can knock out a core electron. Then one of two things can happen:
(i) An electron from a higher shell can fall into the vacancy in the lower shell, accompanied by the emission of an X-ray photon.
(ii) An electron from a higher shell can fall into the vacancy in the lower shell, while another outer electron is expelled from the atom (Auger electron).
The quantum yield of X rays increases with atomic number; light elements give mostly Auger electrons and heavy elements mostly X rays. The crossover is somewhere around zinc (Z = 30). Thus glass, containing mainly Si and O, will give a lower yield of X rays than a heavier metal, such as iron or tungsten.

Thus much I know from my acquaintance with SEM. Why things are that way round, I don't know.
But this is not the only way to generate x-rays. The other is via bremsstrahlung process where the electrons generate the EM radiation because they are being slowed down. This is where it is relevant to the OP's question, i.e. why is it more efficient to generate x-rays with a metal anode.

First of all, obviously, it is because one can efficiently set a higher potential for a metal than for an insulator like glass. So electrons can easily be collected and diverted to collide with the metal anode.

But secondly, metals tend to be more dense than glass (at least, regular glass) and are able to more efficiently stop the penetrating electrons. The quicker they stop, the higher the energy of the bremsstrahlung radiation, which is why we tend to get x-rays if the incoming electrons have sufficiently high energy. This x-ray generated via bremsstrahlung is a very common radiation in particle accelerators, especially when there are stray high energy electrons hitting the stainless steel beam pipe.

Zz.
 
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  • #4
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Thank you so much. I was actually interested because of the history of the x-ray and the x-ray machine. I have two follow up questions:

1. Does the bremsstahlung have to do with the density *or* the charge of the nucleus? Or, is that basically the same thing?

2. Do you know why lighter elements create mostly Auger electrons and the heavier elements create mostly x-ray fluorescence?

Thanks again
 
  • #5
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Thank you so much for replying. I really appreciate it.
 
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  • #6
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<Moderator's note: Merged from another thread.>

It seems pretty clear that when a core electron is ejected and another electron falls into that hole, it can either release that energy in a photon or in ejecting another electron. That makes plenty of sense to me. What I can't understand is why the percent chance of getting an x-ray vs. a Auger electron is dependent on the atomic number.

Any of you smart lovely people know the answer?

Danka,

Kathy
 

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