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What is the difference between an x-ray device and a linac (for medical purposes)? In case both have to produce photons.

Linac:

img1 source

X-ray device:

img2

source

As far as I see the major difference is, that in case of the x-ray device, the rays impinge on some medium (crystals, GM-counter(?)) to produce an image. So, neglecting such a medium, are both devices conceptually the same? Not just in terms of "Yes, they produce irradiation" but also in a technical setup and so on. Or in other words: When placing crystals below a linac, could one speak of x-rays /x-ray images then?

I also wonder why the linac navigates the electrons towards the patient. In the case of the x-ray, the electrons and photons are orientated perpendicular to each other, while in the case of the linac, they are parallel.

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  • $\begingroup$ I'm not really clear what sort of similarities and differences you are interested in. You say "are the two devices conceptually the same (...) in terms of technical setup and so on", but "technical setup and so on" is too vague. $\endgroup$
    – Martin CR
    Jan 10, 2022 at 10:28
  • $\begingroup$ Probably, sorry. I wonder whether you can turn a linac into an x-ray device or vice versa under minimum effort? Atm, for me, it seems that they are quite similar whereas the x-ray device provides some crystals or the like(?) to produce an image. But from my everyday experience, they are at least quite different due to size and weight but I don't know whether this is only due to the photon energy they provide. $\endgroup$
    – Ben
    Jan 10, 2022 at 10:31
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    $\begingroup$ While these links don't directly answer your question, you might find them interesting. Videos by Don Lincoln. 54, 56, 57, 58, and 59 talk about accelerator design. 42 talks about medical accelerators. $\endgroup$
    – mmesser314
    Jan 10, 2022 at 15:33
  • $\begingroup$ Thanks a lot!!! $\endgroup$
    – Ben
    Jan 10, 2022 at 18:20

3 Answers 3

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For the diagrams you have, the biggest difference (which is the major reason for the design difference) is the energy of the x-rays that are produced. An x-ray tube with the above design typically gives off photons up to ~160 keV energy, which is the upper range of what is useful for making a medical image (higher energies have too many photons go through the body, and you lose contrast in the resulting image). Medical linear accelerators give x-rays of uptown ~23 MeV, which are very useful for treating tumors.

To summarize, they both produce x-rays, but at different energies for different purposes

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Both of these systems are accelerating charged particles, then making them hit a target, to produce x-rays through bremstrahlung.

The linear accelerator represents a more technically sophisticated method to achieve much higher energies without the extremely high potential (voltage) differences inherent in a very high energy X-ray generator.

The x-ray generator can be considered a form of static accelerator -- it is analogous to accelerating an object by rolling it down a steep hill. This is very simple, but is limited by the height of the hill. The kinetic energy of the particle that can be achieved is equal to the potential energy difference (in voltage) between the cathode and anode.

In the image on the Wiki page, you can see a very high energy static accelerator; a great many efforts (and their attendant, unavoidable bulk) have been taken to allow a voltage drop of 5 million volts to be achieved. Insulation difficulties start setting in at voltages as low as 50,000 volts and only become more severe as the voltage increases. This type of particle accelerator became obsolete for fundamental physics research in the mid-20th-century, as they are limited to single-digit millions of electron volts.

The linear accelerator uses an oscillating (generally, radio frequency) electric field -- the same is true of cyclotrons and synchronotrons. They have in common that the repetitively oscillating electric fields let a charged particle be accelerated by the same electrode multiple times. It can be compared to rolling an object down a bumpy path where the bumps tilt back and forth at the right speed -- you are no longer limited by the maximum electrical potential -- however, the technology is rather more sophisticated.

It's not the same as a linear accelerator, but here's a video showing a model of a cyclotron for which the downhill-rolling analogy is valid.

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The difference is similar to that between a light bulb and a laser. Both produce light that you could use to (e.g.) read a book, but only one of them is useful for conducting experiments that require a highly directional and organised beam of photons that you might use for (e.g.) conducting scientific experiments on diffraction.

In the above analogy, the 'X-Ray device' is like the light bulb. It works by directing a stream of very fast electrons against a metal target which then emits a random 'hail' of photons with energies scattered all over the X-ray spectrum.

In contrast the linear accelerator uses a process much more like a laser, in order to produce a focused beam of x-ray photons that are all in phase and of identical energy.

Can you easily "convert one into the other"? NO. Could you use a linear accelerator in the role of a medical X-Ray generator? YES - and indeed this is done, for example in very high-end treatments to treat tumours, where you want to be able to focus the beam very precisely. But it's a very expensive approach.

If you want to know more about the precise internal mechanism of how a linear accelerator works, then that's a different question.

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    $\begingroup$ That is a nice analogy! Thanks a lot! So one could say, they are "conceptually" rather similar to each other but technically very different. If that makes sense :) $\endgroup$
    – Ben
    Jan 10, 2022 at 10:49
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    $\begingroup$ well ... in the same way that a stone axe is 'conceptually similar' to a chainsaw :-) $\endgroup$
    – Martin CR
    Jan 10, 2022 at 10:51
  • $\begingroup$ Sire ;) I assume a linac produces photons at higher energy then, or? $\endgroup$
    – Ben
    Jan 10, 2022 at 11:03
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    $\begingroup$ @ben not necessarily of higher energy, just a lot more organised. It's like the difference between the pure note produced by 'pinging' a wineglass, versus the sound produced by dropping a piano down a mineshaft (I believe the corresponding chord is 'a-flat minor') $\endgroup$
    – Martin CR
    Jan 10, 2022 at 11:12
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    $\begingroup$ Just wanted to add that the x-rays produced by either machine are far from monoenergetic. The average energy of the produced photons is approximately 1/3 of the maximum. $\endgroup$
    – D-MedPhys
    Jan 10, 2022 at 12:34

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