Neal Namowicz wrote:
Speaking of vacuum and high voltage, here's something I've done with a bi-pole coil. I mention this now, because I've been meaning to ask y'all if I might be producing a bit too much x-ray radiation in the process. I've placed a glass tube across my electrodes, running them into the tube through rubber stoppers. The length of the tube allows "normal" arcing to take place. But then as I begin to draw a vacuum in the tube, the arcs smooth out into a plasma stream. With my little manual pump, I'm only able to get down to between 20 to 25 in/hg, but is this enough to cause a dangerous amount of x-rays to form? Is there some way of easily checking for x-rays? For example, would a geiger counter work, or is there another way (short of exposing sheet film to find out)? My kids are grown, and I'm sure not anticipating more at this stage of the game, but I'd like to avoid other annoying side effects like brain tumors and such.Thanks for your input on this, Neal.
In general, if you're still seeing sparks/glowing gases, you're not making x-rays.
xrays come from fast moving electrons hitting a target and stopping quickly. If you've got a glowing plasma, you've got enough gas molecules/atoms in the path to keep the electrons from getting through.
This is where things like the "mean free path" come from.. the distance something can move before it hits something. For air, the usual approximation is 5E-3/P (cm, for pressure in Torr).. that is, at normal air pressure of 760 Torr, about 6.5E-6 cm (or, say, 65 nanometers). (or 5E-5/P for meters and torr)
To get xrays, you're really needing the MFP to be greater than the distance between the electrodes. In, for instance, a vacuum tube (like a CRT), the vacuum has to be good enough that the electrons can go from electron source (cathode or electron gun) to the target without hitting something in the way (implying a mean free path on the order of a meter and pressures less than a 1e-6 Torr)
The Crookes tubes used in early cathode ray experiments (and which produced some xrays as Roentgen discovered) were in the 1E-4 to 1E-5 torr range, as I recall (MFP on the order of a few cm)
BTW, 25 inches vacuum is about 120 Torr, which is not very much of a vacuum. Your mfp is still way smaller than a micron.
AS for detecting the xrays... A geiger tube would work, but it would depend on the energy of the xrays and the flux.
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