X-ray vacuum tubes cannot be replaced by solid-state alternatives due to the destructive impact of ionizing radiation on semiconductor crystal structures. The design of X-ray tubes requires a vacuum chamber that allows electrons to travel freely without collisions, which is unattainable in solid or gaseous states. The Coolidge Tube, a later model of X-ray tube, utilized a hot cathode to emit electrons, demonstrating the necessity of maintaining a vacuum for optimal electron flow and radiation generation. Understanding the physics behind bremsstrahlung radiation is crucial for comprehending the limitations of solid-state replacements.
PREREQUISITESThis discussion is beneficial for physicists, engineers in medical imaging, and anyone involved in the design or maintenance of X-ray systems, particularly those interested in the challenges of replacing traditional vacuum tubes with solid-state technologies.
Ionising radiation destroys the crystal structure of semiconductors.wonderingchicken said:What are the reasons X-ray vacuum tubes could not be replaced by solid state alternatives?
Baluncore said:Ionising radiation destroys the crystal structure of semiconductors.
Do you understand how an X-Ray tube works ?wonderingchicken said:So I guess it is common sense to include some kind of chamber so the electrons can move freely instead of flowing through solids? Is that one of the reason?
davenn said:Do you understand how an X-Ray tube works ?
Baluncore said:@wonderingchicken It is time to do some reading.
https://en.wikipedia.org/wiki/X-ray_tube#Physics
Electrons from the cathode, accelerated by an electric field, will impact the anode with an energy in eV equal to the accelerating voltage. As the electron is decelerated by the heavy metal nuclei in the target, the kinetic energy is released as photons.
https://en.wikipedia.org/wiki/Bremsstrahlung
Look at the Planck–Einstein relationship between the energy and the wavelength of the photon.
https://en.wikipedia.org/wiki/Planck–Einstein_relation
The electrons need a clear run without intermediate collisions in order to deliver maximum energy to the target. The electron acceleration voltage must be insulated and not short circuited. That is simply not possible in a solid or a gas.
As a matter of interest, the early X-ray tubes contained a little air, which had the effect of increasing the beam current. From the Wiki page on X-ray tubes: "Crookes tubes generated the electrons needed to create X-rays by ionization of the residual air in the tube, instead of a heated filament, so they were partially but not completely evacuated".wonderingchicken said:So what I said earlier "So I guess it is common sense to include some kind of chamber so the electrons can move freely instead of flowing through solids? Is that one of the reason?" is basically what you said "The electrons need a clear run without intermediate collisions in order to deliver maximum energy to the target. The electron acceleration voltage must be insulated and not short circuited. That is simply not possible in a solid or a gas".
Thank you very much. I'll read these links you've shared. Much appreciated.
tech99 said:As a matter of interest, the early X-ray tubes contained a little air, which had the effect of increasing the beam current. From the Wiki page on X-ray tubes: "Crookes tubes generated the electrons needed to create X-rays by ionization of the residual air in the tube, instead of a heated filament, so they were partially but not completely evacuated".
What does it matter how many molecules are left? If the majority of accelerated electrons reach the anode without collision with a gas molecule, then the electrons that reach the anode will generate radiation as if it was a perfect vacuum.wonderingchicken said:If I'm not mistaken, it is impossible to get perfect vacuum so even for the hardest vacuum possible for vacuum tubes there are actually still gas molecules in them although the pressure is already very low. Correct?
With the early tubes the gas enabled more electrons to flow due to the ionisation resulting from collisions with gas molecules. The tubes had an arrangement to add more air as time went on as the vacuum tended to increase with time. The later hard vacuum X-ray tube was the Coolidge Tube, which had a hot cathode to emit electrons.Baluncore said:What does it matter how many molecules are left? If the majority of accelerated electrons reach the anode without collision with a gas molecule, then the electrons that reach the anode will generate radiation as if it was a perfect vacuum.