Re: X-rays;
There will always be electron bombardment of the glass envelope to some degree, and as such there are always some xrays produced from Bremsstrahlung, but the key issue is; can those very low energy photons exit the glass envelope. Generally speaking, with even 1.5mm wall borosilicate glass, you need to exceed about 25kV and have a metallic anode to get any detectable xray emission. When there is no electrode in the tube, you need upwards of 50kV. It wasn’t too long ago that someone on 4HV did a study of ultra low energy bremsstrahlung emission vs glass thickness. Using the most sensitive detectors available at quite a cost to the hobbyist, it was found that even very thin wall glass (0.5mm) blocked all the emission below about 7keV.
Re: blue glow;
Blue glow is normal and not to be worried about. It is when the glass glows GREEN that it is a surefire warning of xray production. I don’t mean the seal-in point where the metal pinch seal is either, because traditionally Vaseline (Uranium) glass is used for those areas and it fluoresces under UV to be a brilliant green.
Re: your tube;
Additionally; from the description of your tube, it most certainly does not have a hard vacuum present. This is key; the mean free path of an accelerated electron in even a very low pressure gas (~1Torr; diffuse glow in just about any gas, easy ignition) is quite short. This means that electrons are far more likely to collide with the gas atoms/molecules and not the glass envelope. This is why a hard vacuum is used for thermionic devices; if there were gas atoms in the way it would effectively shield the anode from the emitted electrons and you would get negligible current amplification. Electrification of a Hard vacuum produces no glow, as there are no gas atoms to ionize. Without thermionic emission from a heated cathode, there will be no current flow either, and the vacuum behaves as an insulator.
The higher the pressure inside a tube the higher the voltage needed to strike/ignite/ionize the gas inside is. Different fill gasses have different ignition voltage coefficients, with Ne and Hg being some of the easiest to strike out of the common lot of gasses used. Nitrogen, the main component of air, is VERY hard to strike. Above about 90Torr it is nearly impossible to strike an arc in rarified air using capacitive coupling. Direct electrode or direct-streamer-to-glass coupling will still strike the arc up until about 220Torr. From this point up until atmospheric pressure (760Torr) the gas will simply not ionize without direct electrode coupling.
Given that you stated your tube ignites with as low as 10kV and glows diffuse blue, it is a very strong indication that Hg vapor is present. Now, a warning; Neon Sign shops almost always use Hg (mercury) vapor in their penning mixture as this is what actually produces the light emission from the plasma. Hg emits mostly UV (254, 365nm), NUV (404nm), blue (435nm), and a bit of green (546nm) light. To our eyes this appears light blue, approximately 488nm in color (google Argon Ion laser to see this wavelength). This is how all fluorescent lights work; the UV and most of the 435nm line are used to cause a phosphor coating on the tube to fluoresce red; completing the color trio needed to excite the cones in our eyes and cause us to perceive the light as white. In “Neon” sign making the phosphors are tailored to emit certain bands of light to create the desired color. Only in true Neon Gas (orange/red) signs is there negligible Hg or Ar content as the blue emissions from these gasses would spoil the intended color. As there is no phosphor coating present there is nothing to absorb the UV emissions, which means that your tube is an eye and skin hazard. You can get simple Plastic Cling Wrap which is opaque to UVB and lower wavelengths and wrap the tube with several layers, this will block most of the UV emission and nearly eliminate the eye/skin hazard. Just watch out for dielectric heating and breakdown when holding the tube and not using the electrodes for electrical contact.
Atmospheric air, rarified to certain medium pressure ranges (~65-80Torr) can glow diffuse blue/white as well, especially with elevated traces of CO2, but it is notoriously difficult to ignite/ionize, even with HF HV AC.
Here you can see one of the tubes I made recently: https://www.youtube.com/watch?v=iea-mGFeeKY
It is simply rarified air, pumped down to about 60Torr. At this pressure air appears as a pinkish color, but due to the strength of the blue/violet lines camera perceive it as violet. As these are electrodeless tubes you need HF AC to ignite them (plasma ball drivers, AC flybacks, or TC’s). I have a second, shorter tube made that has a more intense violet color that does not look pink, as it is pumped down further to approximately 52Torr. This is the limit of my vacuum equipment. I will be making these large tubes available for sale at very reasonable prices in the near future.
Further more, I have an array of radiation detection and monitoring equipment here in my lab, some of which is sensitive down to 8keV, and no detectable increase in background radiation has been observed during any of the dozens of hours of run time on any of my electrodeless discharge tubes (and I have an extensive collection now). All my detectors are Faraday shielded from the TC interference as well.
Sincerely,
Matt Sig Giordano
Sigurthr Enterprises
www.SigurthrEnterprises.com
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