Those damn 'X' Rays, from Tubes etc.. - Page 1
Forum section: electronics / general

author: GlennSprigg
author_rank: Super Contributor
timestamp: April 10, 2020, 01:08:36 pm
content: The 'X' (for originally unknown/non-understood) Rays, are still 'foreign' to a lot of people!!Many decades ago, I used to wonder why/how such 'Tubes' were special, and how they worked? Turns out that there is NOTHING special about the mechanical construction, (ok, I get it!), as opposed to a valve rectifier tube. The DIFFERENCE, is the High Voltage across the Tube!! While trying to find a simple explanation for someone, I recently came across this youtube video from a "Doctor Klioze". He is not talking 'medicine' as such, but the full history of X-Rays, from accidental discovery, design & implementation....BASICALLY, it is simply the result of a very high voltage across the plates! What I propose is THIS!!!In SPACE, there is no better vacuum, so would very high voltage 'plates' (with a radiant/direct heater to heat the 'Cathode'), generate 'X' rays outside of a spaceship etc, as SPACE itself becomes the 'Tube' ??

author: jogri
author_rank: Frequent Contributor
timestamp: April 10, 2020, 02:49:39 pm
content: You could, but why would you want to do that? The process of creating Bremsstrahlung is spectacularly crappy when it comes to energy efficienc as 99-99.8% of the energy you put it just gets converted to heat. And you don't even get a good signal as the energy of the emitted xrays isn't one exact energy (okay, for the most part) but is a wild bunch of different wavelengths, ranging from 0.2 to over 1 Angstroem (for a Wolfram anode@50kV).How and why that's the case: Bremsstrahlung is the result of an electron missing an atom (the nuclei) and releasing its energy in the form of photons as it changes its path (it gets attracted to the nuclei as the nuclei has a positive charge and the electron a negative). That's why you get a energy spread that accords to Kramer's law as the energy of the Bremsstrahlung is related to the distance between the nuclei and the electron and its speed->there is an infinite amount of possible distance/speed combinations.An electron can also just happen to bump into an electron that is just sitting inside an atom orbital of an atom: If that happens, the electron that was bound to the atom gets released and both electrons fly away (the energy of the first electron is reduced by the amount of energy it took to free the second electron). If the atom has another electron that has a higher energy as the one that was just released this third e- will take the place of the second, releasing the difference in their energy as one photon.For example: The first e- has the energy 1 and it takes 1 energy to release the second e-. They collide-> second e- is released. The third electron has an energy of 2 as it has to occupy a atom orbital with a higher energy (the lower energy orbital was occupied by e- two). That electron jumps from its orbital to the one where e- two just was, releasing 1 energy in the process.Now this energy only depends on the atom and the orbitals of the two electrons involved: If we have a copper atom and the electrons goes from the orbital with the second lowest energy (L) to the orbital with the lowest energy (K), we call it the K-alpha radiation of copper. K because thats the orbital were electron three goes, alpha because the electron came from the orbital with the next higher energy (beta for the orb after that etc).If we want to use xrays as a form of communication we should go with that approach as we can just pick an element that doesn't occur naturally as our cathode and every signal of that specific wavelength can only come from our space probe. There is just one tiny problem: generating Bremsstrahlung is extremely inefficient (and the amount of the K alpha radiation we want is even lower), you'd need a power supply in the kW range (solar can do that, but not for deep space probes). The second, more important problem is that there is no way to cool the cathode as the only way to cool an object in space is to just let it radiate the heat as black body radiation and that is painfully inefficient compared to air/water cooling on earth. Just look at the ISS, those massive white panels are there to keep it cool and they need roughly 6m^2 of panels for every kW of heat dissipation.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: April 10, 2020, 03:45:55 pm
content: If one wanted to make x-rays in space, it might be more feasible to use secondary x-rays emmited by hitting a target with a strong gamma source.If one wanted to modulate the x-rays in the frequency domain, a space synchrotron would be a cool option. Very intense x-ray beams are generated that way on earth. And you're likely right, you could do away with all the vacuum confinement structures. You'd still need magnets, though.

author: jogri
author_rank: Frequent Contributor
timestamp: April 10, 2020, 05:22:01 pm
content: Quote from: ChristofferB on April 10, 2020, 03:45:55 pmYou would need to launch an extremely powerfull gamma emitter into orbit and people tend to freak out if you launch a few kg of enriched plutonium into space without proper shielding. Yes, we launched a bunch of RTGs but those were designed to remain sealed even if the rocket undergoes a rapid unscheduled disassembly and you obviously can't use a sealed container if you want to access your gamma emitter once it is in space. And we would also encounter the problem of cooling...

author: TimFox
author_rank: Super Contributor
timestamp: April 10, 2020, 05:43:17 pm
content: Quote from: ChristofferB on April 10, 2020, 03:45:55 pmWhen the ill-fated SSC synchrotron was being planned for Texas, I thought that it would be interesting to plan a huge installation on the moon, where the radius of the ring could be very large and no vacuum chamber would be necessary between the bending magnet/accelerator stages.

author: jogri
author_rank: Frequent Contributor
timestamp: April 10, 2020, 06:07:04 pm
content: The moon wouldn't be a good place for a open synchrotron as the moon is notorious for producing large dust clouds due to cosmic x-rays charging the dust particles positive. I don't think that shooting electrons through a cloud of positively charged dust would work...

author: TimFox
author_rank: Super Contributor
timestamp: April 10, 2020, 08:48:00 pm
content: The SSC and similar synchrotrons accelerate protons, not electrons.If it be necessary to keep the beam away from dust, it would not require the <10-10torr stainless steel vacuum chambers used on terrestrial accelerators: aluminum foil on cheap wire frames would suffice.

author: GlennSprigg
author_rank: Super Contributor
timestamp: April 12, 2020, 11:30:34 am
content: Ok... To be honest, I was not initially thinking about any real practical uses for this!I was simply wondering if say a 11/50kv differential across a pair of 'plates' outside of some SpaceCraft would start radiating uncontrolled X-Rays?? With or without a directly/indirectly radiant heated 'cathode'. I didn't think about Modulating anything for Communications etc!!Mind you, since reading the interesting comments here, it opens a whole new ballpark. Thank you all !Now I will have to read about the "ill-fated SSC synchrotron" etc...

author: pwlps
author_rank: Frequent Contributor
timestamp: April 12, 2020, 11:53:51 am
content: Quote from: ChristofferB on April 10, 2020, 03:45:55 pmAn XFEL would be even better: no bending magnets needed (except magnets in undulators) and millions times more brilliance for the same beam current (all electrons radiate in phase).

author: coppercone2
author_rank: Super Contributor
timestamp: April 12, 2020, 03:13:43 pm
content: would you say a tube being used this way for Xray generation is akin to a random noise source?what does the profile look like (spectral density?)

author: pwlps
author_rank: Frequent Contributor
timestamp: April 12, 2020, 03:44:58 pm
content: Quote from: GlennSprigg on April 12, 2020, 11:30:34 amWithout a heated cathode the current would be much smaller. Yet, as far as I understand X-rays were discovered by Roentgen without a heated cathode ? I wonder how big was the intensity by today's standards.Quote from: GlennSprigg on April 12, 2020, 11:30:34 amAnd here some refs on XFEL:https://www.xfel.eu/facility/overview/facts_amp_figures/index_eng.htmlhttps://photon-science.desy.de/sites/site_photonscience/content/e62/e189219/e187240/e208351/e187350/felbasics_eng.pdf

author: jmelson
author_rank: Super Contributor
timestamp: April 12, 2020, 04:16:54 pm
content: Quote from: pwlps on April 12, 2020, 03:44:58 pmRoght, but I guess Roentgen's tubes had a little bit of gas in them, which ionized and provided electrons to get a discharge started.Jon

author: jogri
author_rank: Frequent Contributor
timestamp: April 12, 2020, 04:18:17 pm
content: Quote from: coppercone2 on April 12, 2020, 03:13:43 pmWhat do you mean with "random noise source"? An emitter that has the same intensity over a wide frequency/energy range? That's definitely not the case.Here is the x-ray spectrum of a tungsten anode @110kV, that picture is from a PTB (national metrology institue of germany) news report (link:https://bit.ly/3c5Tykq). As you can see it really isn't linear as the spectrum accords to Plank's law. The higher the voltage the sharper the peak of the curve (the one that has its maxima at roughly 30kV, not the three smaller peaks) and vice versa. The peaks on the far right come from K-alpha/beta radiation, the ones in the middle from the L-radiation and the ones on the left from M-radiation.

author: pwlps
author_rank: Frequent Contributor
timestamp: April 12, 2020, 04:37:14 pm
content: Quote from: jogri on April 12, 2020, 04:18:17 pmActually it has nothing to do with the Planck law, for bremsstrahlung radiation profile see e.g.https://en.wikipedia.org/wiki/Kramers%27_law

author: jogri
author_rank: Frequent Contributor
timestamp: April 12, 2020, 04:56:47 pm
content: Yeah, that sentence was rather misleading. What i meant was that the curve for the intensity of the x-ray emissions looks like a spectral radiance curve from a black body radiator (because most people have seen those and kinda know what they look like) but the way i said it makes it look like Planck's law applies to this situation, and that is most definitely not the case.Props for pointing it out.

author: TimFox
author_rank: Super Contributor
timestamp: April 12, 2020, 05:11:08 pm
content: Quote from: jmelson on April 12, 2020, 04:16:54 pmThe original x-ray generating tubes used by Roentgen were actually glow-discharge tubes filled with low-pressure gas ("Crookes Tubes", q.v.), and were common until roughly 1920 (see Wikipedia). As in VR tubes, the anode-cathode voltage in such a tube is roughly constant, determined by the anode-cathode distance and gas pressure. Electrons hitting the anode produce x-rays by Bremsstrahlung, as in a hot-cathode high-vacuum device. I believe that this is where the term "hardness" for x-ray spectra originates, since a "harder vacuum" operated at higher voltage.