What is the optimum voltage for an X-ray tube and why is it important?

[JordanU94]

Hi all,

I hope this is the right place for this question.

What effect does increasing voltage have in an X-ray generator? What's the optimum voltage for the most efficient production of X-rays and why?

Thanks

 

[SteamKing]

Is this a homework problem?

 

[JordanU94]

Is this a homework problem?

No just reading around and my subject and trying to understand voltage and x rays

 

[mfb]

Depends on the material, and the way you define "most efficient".

 

[UltrafastPED]

The usual x-ray generation process is to produce electrons, accelerate them through a voltage - say 50,000 volts.
Then each electron will have 50 keV of energy when it hits the target where the x-rays are generated.

The target is usually a metal, but need not be. The high energy electrons knock out inner-orbital electrons from the material ... these will strike other atoms, etc ... with an inner orbital electron missing, there will be a series of electron transitions from the outer orbitals, each falling to a vacant inner orbital - and for each transition there is a "characteristic x-ray" generated which has the energy corresponding to the difference of the two levels.

See http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/xrayc.html

So for a given target there will be a small set of characteristic x-rays of the highest energy, and that energy will _always_ be less than the energy of the electrons striking the target. There will also be a bunch of "braking radiation" x-rays, but the lower energy ones can be filtered out.

So the most efficient voltage for x-ray generation depends upon the target material. A silver (Ag) target can generate 22 keV x-rays if the incoming electrons are it bit more energetic than that ... say 25 keV. If the energy is much higher you will still get braking radiation, but you won't get any more of the 22 keV x-rays.

If you want to filter out the lower energy x-rays you can put in an extra layer of a lower-energy material. If you filter with palladium (Pd) you will intercept most of the x-rays less than 21 keV.

The details now depend upon the electron fluence (number of electrons per unit area per second) and the thickness of the target material. This is where your optimization will occur.

 

[Menaus]

I have a question also related to X-rays, although I don't mean to intrude.

If the electrons have a high enough energy level, and they happen to hit an atom, is it possible that the electron would accelerate the atom to such a velocity that it would pass through the tube? If this is true, then would it be correct to say the in addition to X-rays, these tubes also eject streams of particles?

 

[UltrafastPED]

If the electrons have a high enough energy level, and they happen to hit an atom, is it possible that the electron would accelerate the atom to such a velocity that it would pass through the tube?

The x-ray can be scattered by an atom; it interacts with the charges of the electron "cloud". Thus it is the orbital electrons which see the momentum transfer ... some are driven out, others are rearranged.

But if you increase the energy enough you arrive in the "gamma ray" energy range. Here you will have enough energy for the "x-ray" to interact with the nucleus. If the energy is high enough you can have "photo disintegration", where the nucleus breaks up. This can generate a stream of particles. But you won't be able to "push" the atom this way. It is way too massive.

So in the x-ray range you won't see any "atomic beams".

PS: The easy way to make an atomic or molecular beam is to ionize the atoms/molecules, then accelerate them with a voltage, and steer them with magnetic or electric fields.

 

[sophiecentaur]

There is an arbitrary divide between X Rays and γ Rays and I have concluded that the best way to categorise the two (if you really insist that the name is that important) in terms of how they are generated. Earlier in this thread, there has been mention of X Rays of 'a mere 50kV' energy. Radiotherapy is carried out with X Rays with energy of 10MV. A linear accelerator is used to produce this energy and the photons are produced in exactly the same way as in more familiar, diagnostic X Ray machines. I don't think is is common, these days, to use radioactive sources for radiotherapy because you can't turn them OFF as easily.
The 10MV energy is certainly very "optimal" for deep radiation of tissue.

 

[mfb]

If the electrons have a high enough energy level, and they happen to hit an atom, is it possible that the electron would accelerate the atom to such a velocity that it would pass through the tube?

If an electron with 50 keV makes a 180°-turn at an atom (the maximal momentum exchange), a single proton just gets 27 eV of kinetic energy - but a single proton cannot do this. If you use a material with A=50, for example (like iron), this maximal energy drops to 1/2 eV. That is not enough to dislocate it in a solid material, so this process does not happen at all.

With higher electron energies - in the GeV range - this process is possible, but extremely rare. And then you still get an atom that moves a few atom diameters in the material, it won't fly around freely.

 

[Menaus]

If an electron with 50 keV makes a 180°-turn at an atom (the maximal momentum exchange), a single proton just gets 27 eV of kinetic energy - but a single proton cannot do this. If you use a material with A=50, for example (like iron), this maximal energy drops to 1/2 eV. That is not enough to dislocate it in a solid material, so this process does not happen at all.

With higher electron energies - in the GeV range - this process is possible, but extremely rare. And then you still get an atom that moves a few atom diameters in the material, it won't fly around freely.

In a Crookes tube an electron need not make a 180° turn. Also, somewhere I've heard of the use of single-electrode vacuum tubes for x-ray experiments, although I forgot where.

In any case, do you think it is plausible in this case?

 

[sophiecentaur]

So what sort of target and electron transitions are involved with producing 10MV XRays?

 

[mfb]

In a Crookes tube an electron need not make a 180° turn. Also, somewhere I've heard of the use of single-electrode vacuum tubes for x-ray experiments, although I forgot where.

In any case, do you think it is plausible in this case?

To get atoms to move through any macroscopic material, they need an energy of the order of MeV. You won't get this with an electron collision, independent of the energy - if the energy is really high (GeV++), you can split atoms, but that is a different process.

 

[UltrafastPED]

There is an arbitrary divide between X Rays and γ Rays and I have concluded that the best way to categorise the two (if you really insist that the name is that important) in terms of how they are generated. Earlier in this thread, there has been mention of X Rays of 'a mere 50kV' energy. Radiotherapy is carried out with X Rays with energy of 10MV. A linear accelerator is used to produce this energy and the photons are produced in exactly the same way as in more familiar, diagnostic X Ray machines. I don't think is is common, these days, to use radioactive sources for radiotherapy because you can't turn them OFF as easily.
The 10MV energy is certainly very "optimal" for deep radiation of tissue.

This is inconsistent with the terminology that I learned in nuclear physics and plasma courses.

As for synchrotron radiation, let them speak for themselves:
http://www.esrf.eu/about/synchrotron-science/synchrotron-light

I agree that the division point is arbitrary, but I don't read papers where they call a 10 MeV photon "an x-ray". At higher energies they tend to simply mention the energy.

 

[sophiecentaur]

This is inconsistent with the terminology that I learned in nuclear physics and plasma courses.

As for synchrotron radiation, let them speak for themselves:
http://www.esrf.eu/about/synchrotron-science/synchrotron-light

I agree that the division point is arbitrary, but I don't read papers where they call a 10 MeV photon "an x-ray". At higher energies they tend to simply mention the energy.

That's useful information. Thanks. I'm not sure where synchrotron radiation comes in - at such high energies.
I have a feeling that the medical application of 10MV photons is often referred to as X Ray therapy because of 1. The scary nature of Gamma Rays and 2. The Star Trek associations of Photons. The therapy has to be 'acceptable'.
I would still like to find out how the 10MV photons are actually generated. I know the equipment has a linear accelerator and that there is an alternative application of the equipment which uses the Electron Beam directly. In the only picture I could find of a typical Radiotherapy unit they do, in fact, refer to a Photon Beam and not X ray and http://www.varian.com/euen/oncology/radiation_oncology/clinac/clinac_dhx_high_performance.html shows the basic layout. The treatment head is described in this link. It involves an electron beam striking a target to produce the photons.