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Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
I was making a few adjustments to my first VTTC built around the low power Russian GI30, a double tetrode, and I am already aware of some mistakes like: poor primary to secondary insulation, feedback to secondary insulation, primary/feedback too tick wire, and a few more, well pointed out by vasil already.
While trying various caps for this oscillator and checking the RF output and power consumption, suddenly I heard a hissing sound and I thought I was also seeing some blueish light inside the tube. I turned off the light, and indeed there was some faint blue light inside the tube: see photos.
Some measurements on input potential difference showed HV getting in the driver circuit (because of the poor insulation between coils). So this might be the cause for the blue light. I also failed to get a discharge, but a fluorescent tube was lit in close proximity.
While seeing the blueish light, I took my Radex 1706 , and it quickly jumped off scale at 10cm aways from the tube - not very good since I was staring at the blue light with naked eye for minutes already. But then I used a piece of fluorescent screen, and it showed no fluorescence. My scintillation probe is not 100% completed, so besides these two tests I can't do any other (see pictures 1,5,6)
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
If the potential difference across the tube has peaks of more than 15 - 18kV, then any X-rays above that cut-off will be transmitted through the glass envelope.
The energy response of uncompensated GM tubes is such that low energy X-rays will generally produce a very much higher count rate than rays of high energy. For more information on GM tube energy compensation, see the Centronics Geiger–Müller Tube Theory Handbook
If you are concerned about EMI affecting the meter, you could put it in a small box of light plastic or paper card, and cover it with kitchen aluminium foil to make a Faraday Shield which you connect to a good low impedance Earth (i.e. a nice fat copper strap). I expect that you should still hear it beeping inside the shield.
I would have more confidence in a decent GM tube like SI-22 (which you have), used together with a very simple radiation hardened circuit. You won't be able to have a read-out in Gy, unless you have its energy response data, but you can make relative measurements of total count etc.
I am doing some experiments at the moment to determine the energy response of Gd2O2S:Tb screen, and will write them up in the forum when I have some solid results.
Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
Proud Mary wrote ...
If you are concerned about EMI affecting the meter, you could put it in a small box of light plastic or paper card, and cover it with kitchen aluminium foil to make a Faraday Shield which you connect to a good low impedance Earth (i.e. a nice fat copper strap). I expect that you should still hear it beeping inside the shield.
I did this test and it outruled the x-ray emission variant (a second indication besides the fluorescent screen). Meanwhile my VTTC is ready/up and running:
Nah wrote ...
Sometimes a hard worked tube will display a blue glow on the surface of the glass due to stray electrons. This is commen in type 45 and 47 tubes.
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VTTC emitting XRAYs or GM counter interference?
