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Vacuum / Electron Tubes and X-rays

  1. Jul 13, 2017 #1
    I've read that if you apply enough voltage to the plate of certain types of tubes, they will generate a fairly good amount of X-rays (even more so if the cathode is hot) -- I even read about a guy who managed to take relatively detailed radiographs of random objects using a standard, run-of-the-mill radio tube.

    This makes sense, since that's basically how an actual modern x-ray tube works.
    But will ANY vacuum tube generate x-rays? For example, I have a pentode, model 802, used for RF transmitting/amplifying applications...
    Data on it can be found here: http://www.radiomuseum.org/tubes/tube_802.html

    The reason I'm asking is because I do have a high voltage source which is the plasma globe that sits on the computer desk.
    Will the vacuum generate x-rays if it's brought close the globe, due to the high alternating voltage being produced?
  2. jcsd
  3. Jul 13, 2017 #2


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    Staff: Mentor

    How much do you know about x-ray safety?
  4. Jul 17, 2017 #3


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    Science Advisor

    When an electron is accelerated by a voltage of 1 kV it gains a kinetic energy of 1 keV. When it hits a heavy target, that energy is released as a photon. The photon energy of soft x-rays is centred on about 1 keV. Vacuum tubes that operate on voltages above about 200V can be expected to generate soft x-rays.

    The photon energy of the hard x-rays that are used for imaging is between 10 keV and 100 keV. The cathode ray tubes once used in TV screens needed lead glass shielding because the accelerator voltages were in the 10's of kV range.
    You need to avoid exposure to x-rays that are generated by voltages above 500 volts.

    The resolution of an imaging system is about the same as the size of the anode target. The size of the anode is very large in the 802, so it would generate a very diffuse source of x-ray radiation. Any x-rays generated by the 802 will need to pass through the metal anode material, and then the glass, to escape the tube.

    An old 802 tube will now be gassy and so ionise at a low voltage. That will limit the voltage available to accelerate electrons. When a high current flows through the 802 tube, the anode will glow red hot and so melt the glass and suck in the envelope.

    In short, for many reasons, an 802 will not work for x-ray imaging.
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