I understand that in an X-ray tube electrons collide with the anode which then deflects them out of the window producing X-rays and that this anode is rotated to dissipate heat. But why is it necessary? I imagine the cathode filament will get far hotter but this is not rotated. Is the dissipation of heat connected to the release of X-rays?
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$\begingroup$ radiopaedia.org/articles/anode-x-ray-tube, radiopaedia.org/articles/focusing-cup It sounds like the reason is because the electron beam is produced by ejecting electrons from a larger cathode area and focusing them onto a smaller anode area so the hot spot is worse on the anode. $\endgroup$– DKNguyenCommented May 4, 2021 at 20:44
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2 Answers
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The electron beam power is (voltage) x (current), only a small fraction of which actually generates X-rays. Most of those watts end up dissipated as heat when they collide with the anode. This heating is manageable in small X-ray machines running at low power for short periods of time but for big industrial X-ray tubes (~1.5 million volts) the anode is hollow and is cooled by running water through it.
answered May 4, 2021 at 20:49
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$\begingroup$ Electron beam welders melt metal just fine, and are run at voltages low enough not to create many x-rays that would get out of the vacuum chamber. $\endgroup$ Commented May 4, 2021 at 20:56
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$\begingroup$ And lets not talk about melting the crucible in an e-beam deposition system. That has never happened to me. Nope, not at all... $\endgroup$ Commented May 4, 2021 at 20:59
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$\begingroup$ @nielsnielsen I wonder about your answer because a still typical, conventional X-ray tube (fixed target, e.g., Mo) for small molecule crystallography runs well with a generator set at 25 mA and 50 kV. Depending on your sample to characterize, there are of course the brighter rotating anodes, especially for proteins / biochem, or µfocus tubes with less power consumption than the other two, but where did you see "big industrial X-ray tubes (~1.5 million volts)"? $\endgroup$ Commented May 4, 2021 at 22:11
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$\begingroup$ @Buttonwood - perhaps food irradiation? On the other hand, the x-ray machine in the building I see out my office window uses a pulsed 2MV 750kA beam, which blows up the target every shot. - no cooling required! $\endgroup$ Commented May 4, 2021 at 22:30
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1$\begingroup$ @buttonwood, Raychem also used cobalt-60; the capsules were stored in deep concrete "wells" and winched up into the beam cell when they wanted gammas. the plastics we irradiated had photosensitizers mixed in them which, when hit with radiation, broke up into free radicals which did most of the crosslinking work. Different "pro-rad" cocktails were used with electron vs. gamma sources. $\endgroup$ Commented May 5, 2021 at 21:17
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There are multiple types of X-ray tubes used, e.g., in crystallography, and the one using a rotating anode is one (but not the only) type being used. The advantage compared to a "conventional" X-ray tube in a diffractometer is the higher brightness of this source of radiation, i.e. number of photons suitable for the diffraction experiment per unit of time. The temperature on the focus of the anode of the rotating anode reaches about ${2500\,^\circ{}\mathrm{C}}$. The higher brightness is useful because it allows to shorten the time to collect the data; more importantly however to record in sensible time useful data for the protein crystals which are both i) much smaller than the "small molecule crystals" routinely characterized by in-house single crystal diffractometers and ii) for most of their atoms consist of weakly diffracting matter only.
The cooling (both for the conventional, as for the rotating anode) however is only an attempt to extend the time the anode may be used. This is not made to alter e.g., the wavelength of the X-ray radiation harvested.
