2009-05-24
Soft X-ray Output from a Vacuum Rectifier
This old 2X2 rectifier has a bit of a reputation of generating significant x-ray radiation when operated at high voltages. As I found out, this reputation is quite well deserved. I ordered some Russian clones of the original 2X2/879 (labelled 2U2C) from eBay seller kwtubes. Unlike the newer production 2X2 (2X2A, et al) they do not appear to have lead-oxide glass to attenuate softer x-ray emissions. At only 20-30 kV and a few hundred uA in cold-cathode mode the x-ray radiation pours out, making the end-window Geiger counter scream from more than a metre away.
I put a few gig-ohm resistors in series with the coronatron PSU to reduce the anode current (and hence brightness of the source). This not only reduces the dose rate of being in proximity to the unshielded device, but also prevents the GM tube from saturating so one can map the radiation output.
Although the maximum energy of the x-ray quanta is only about 25 kV in this situation (quite soft), it is still fairly penetrating. A piece of metal or your hand absorbs almost all the radiation, but less dense matter is more radiolucent.
The radiation is too soft to penetrate the Geiger tube walls. If you rotate the tube such that the window is not pointed at the source the count rate drops towards that of the natural background. Similarly the radiation can not penetrate the tin-plate ion chamber walls, but with the lid open the radiation will trigger the detector from a foot or so with a Saran film or a sheet of paper over the end as a window.
Although you might call this radiation "soft", experimental radiology has show it actually has a larger biological effect than harder radiation. Some studies suggest a RE coefficient of about 1.5 - 2 for 10-30 keV x-rays compared to 1 for harder x-ray and gamma radiation. This is probably because softer radiation is more completely absorbed in tissue. X-ray imaging machines generally have an Aluminium hardening filter to attenuate softer radiation from their output beam.
Just How Crazy Was I?
What was my worst-case dose from this experiment? Well the HV PSU has an input power of about 10 watts. Typical supply efficiency is around 50% based on previous measurement, so say 5 watts into the vacuum tube. A spectacularly good x-ray tube might be 1% efficient, so 50 mW of x-ray power (probably much worse and lots wasted absorbed in the envelope, plus most of the time I wasn't running anything like 200 uA). That's 20 seconds per Joule and I operated the tube for less than 1 minute total, so say 3 Joules of x-ray energy emitted by the tube. Assuming the radiation is isotropic (it isn't, I measured much stronger count rates broad-side than from the tube ends) and I was about 1 metre away the energy flux is about 230 mJ per square metre. The unshielded cross-section of my body was less than 0.1 square metres, so I intercepted less than 23 mJ. Assuming it was delivered to about 10 kg of tissue (debatable) then worst absorbed dose in Gray is about 2.3 mGy. Factoring in that most exposure was shallow (skin, fat and muscle) then the N factor is about 0.04, which means about 92 uSv affective dose. Even using a Q factor of 2 for the softness of the x-rays that is less than 200 uSv.
In more useful terms that is about a week's worth of background radiation around here - not a significant risk compared to a dental Orthopantomogram I had earlier this year. The Orthopantomograph had an anode voltage of 140 kV, ran about 40 mA for more than 20 seconds and passed the radiation beam right through my rather radiopaque head.
Future Direction
Clearly shielding and collimation are required to make practical and safe extended use of this x-ray source. The photon energy is sufficient for diffraction and fluorescence experiments and could be improved somewhat with a better PSU. At some point the tube will flash-over and limit the maximum supply voltage. I think the source is bright enough for my purposes even at the fairly low currents the coronatron supply offers, but we'll have to wait until after collimation to tell for sure.
The GM tube is a fine detector, but an proportional detector would be handy. I'll likely experiment with ion chamber detectors, semiconductor and phosphor scintillators. I'm trying to source some Sodium Tungstate and Vanadium so I can prepare an x-ray phosphor screen. Calcium Silicate with a suitable activator might work, and Zinc Sulfide with a Copper Activator is worth a try but afterglow will probably be a problem. I *may* be able to find an old x-ray cassette on eBay. One also wonders how sensitive Cyanotype chemistry is to x-rays? Exposure times might be impractical though.
Notes
- If viewed in the dark the glass envelope of the vacuum tube fluoresces bluish-white in operation.
- The tube emits much more radiation when the current is applied "backwards" (i.e. the anode cap running as the cathode).
- The cathode and heater also glow quite brightly in operation. The heater in particular emits a fairly white light.
- The bulk of the radiation escapes from where there is a gap between the anode cup and the shield between the getter ring and the other tube elements. The getter mirror doesn't seem to have much effect on the radiation emission.
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