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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
After reading reports of X-ray emission by American 2X2A high voltage rectifier valves, a Russian 2X2 was obtained.
This valve has an octal base, and an anode top cap, and was set up for the experiment inside a 3mm lead shielding 'well.'
In order to increase the impedance of the valve when used in field emission mode, the anode was connected to earth, while all the pins at the plug end - heater and kathode, were connected to HT+. (i.e. the valve is connected in reverse to its normal polarity and the heaters are not used at all, but shorted to the kathode pin)
A 50uA moving coil microammeter was inserted between the valve top cap (the new 'kathode') and earth.
20kV DC was applied, and a current of 30uA was immediately shewn on the meter - presumably the combined total of the field emission current, and leakage current across the surface of the glass bulb.
At this point, it was observed that X-ray fluence was about 100uSv/hr at 30 cm, and in excess of 5mSv/hr close to the tube, the majority of the radiation emerging at rightangles and downward from the open end of the anode bell.
Molybdenum is used extensively in thermionic valve technology, and it is a fair guess that the unexpectedly large X-ray output of 2X2 is due to excitation of the strong 17.3keV Mo line, for which 20kVp would be an optimum voltage.
So the 2X2 does produce X-rays with even as little as 20kVp, albeit very inefficiently compared with a professional tube using the same voltage and current. Its principal problem as a source is that radiation is emitted round 360 deg, and that the target has no focal point. Images would therefore be slow to form on film, and poorly defined because of their lack of a point source origin.
Even though the X-rays emitted by 2X2 under the conditions described are relatively weak, they are still easily capable of causing permanent harm.. Soft X-rays are almost entirely absorbed by the body, and so are much the most hazardous.
No one should attempt to repeat this experiment who has not already set themselves up with suitable lead shielding, remote operation capability, time delay switch, and a method of measuring soft X-rays i.e. prop tube, ionization chamber, etc, but explicitly not a GM counter.
Addendum: Replication with PD500
In the last hour I have repeated the experiment, identically in every respect, except that the TV shunt triode PD500 was substituted for the 2X2.
Kathode current settled at 18uA. X-ray output was diffuse and hard to measure, but was about 30-40 uSv/hr at 30cm, and at least 1mSv/hr close to the tube. Small movements of the detector to right or left could cause a large change in received dose, presumably due to the presence of 'windows' in the valve's internal structure.
Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699
Sorry for re-opening an old thread, but I've just purchased some 2x2's from ebay, knowing about their regular glass and x-ray emission properties, and now I just stumbled upon this report, so thanks for the excellent details, Proud Mary.
What I'm trying to do is to use my CW multiplier to drive a rectifier tube to a level where it can impress my intensifier screens, enough to be captured by a digital camera.
I also have the 6LJ6A, 3CZ3, 3BW2, 3A3C.
EDIT: you might have more details on your experiment, that would be useful here, for eg. can you draw an approximate distribution of the x-ray emission as compared to the tube's perimeter?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
This Russian 2C2S is the 'equivalent' of 2X2A - which does not mean it is identical. It means that the pin outs are the same, and the principal parameters are similar, so the rectifier can be expected to perform well in circuits designed for 2X2A. The state-owned manufacturer may or may not have used the same alloys in the electrode assemblies, since 'equivalence' is an empirical concept based on actual working performance, and not on metallurgy and manufacturing processes.
An X-ray source emits two types of radiation, as we know - continuous radiation, the so-called bremsstrahlung or 'braking radiation,' and the characteristic spectra of the chemical elements being excited by the electron beam. It is here that we may discover big differences between 2C2S and 2X2A, and perhaps even between batches of the same valve, perhaps reflecting changing costs in the metal market
I have not personally examined the American 2X2A, so what I say here is my opinion based around the Russian 2C2S.
The sharp and jagged edges seen in these pictures all become significant sources of field emission at high voltage. (This is just a valve I've taken at random out of a box in my stack)
While the anode has been carefully shaped to avoid sharp edges, the ends of the terminals spot welded to the top are sharp wedges presumably made by hand-held wire cutters.
Now here below is what I call a real girl power rectifier - the legendary Russian V1-0,15/55 * - which is what it says it is - a 55kV 150mA rectifier, which emits abundant radiation whether you like it or not.
Tired of those fussy little HV silicon diodes that go phut at the smallest sign of a flashover? The V1-0.15/55 is the answer to your prayers! Worried your X-ray tube isn't emitting all you hoped it would? Use the V1-0.15/55 and let it add its own soothing rays to the radiation emitted by your tube.
Yes, girls, it really is true! Tests prove that nine out of ten women who expressed a preference for Russian high voltage rectifiers chose V1-0,15/55!
Warning: Not suitable for pets or children under six.
Warning: May cause permanent chromosomal change
* I have written this in Roman script. In Cyrillic it looks like B1-0,15/55
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
radhoo wrote ...
nice to see you're in a good mood, it is almost contagious.
so what do you think, will this diy rectifier-tube approach work with an intensifier screen?
First, do not apply heater current, as this will cause the voltage drop across the rectifier to fall to 200V or so. You must use the valve in field emission - cold cathode - mode.
If you put 30kV and more across the 2C2S, you'll have enough X-rays to make a Gd2O2S screen fluoresce, and form an image.
Now, if you reverse the normal polarity of the valve, connecting EHT+ to the cathode on pin 4, and the anode top cap to Earth, you will get a sharper image by reducing the target size. This will also increase the impedance of the valve, keeping the voltage drop across it to a higher level than the conventional polarity and so help to produce plentiful field emission.
If you hold the valve with the top cap uppermost, then most - but not all - of the radiation will come from the open end of the anode 'bell' - from almost at right angles to the vertical axis of the valve, and to about 45° down from this all round. The emission will occur unevenly round 360° with the valve at the centre of the circle.
If you had 50kV/50µA across 2C2S, the dose rate distributed around a circle of radius 200mm would be somewhere between 2 and 4 Sv/hr - a non-trivial quantity.
To reduce the risk of flashover, and glass 'puncture' failure, I'd suggest putting the valve in a bath of vegetable oil - sunflower seed oil, and so on. You should not expect the tube to last long, even with this precaution, but it will last longer!
Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699
I couldn't dare to waste your time, but just to make sure I got it right, would you draw a simple sketch of the tube's optimal position? You know what it is said about pictures as compared to words. In geometry this comparison goes even further.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
radhoo wrote ...
I couldn't dare to waste your time, but just to make sure I got it right, would you draw a simple sketch of the tube's optimal position? You know what it is said about pictures as compared to words. In geometry this comparison goes even further.
I think the field emission is coming from the sharp edge of the anode 'bell' or 'thimble' and impacting on the rods going up to the heater and cathode.
Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699
I was just imagining some x-ray sensitive (photosensitive maybe) non-conductive foam, put in a recipient around the tube, powering the tube, then carefully dismantling the foam to see the beams pattern.
Besides this, is there a realistic approach that I could use?
What about shielding the tube in lead, except a tiny hole, checking the output, then moving the tube inside the shield, checking again and so on, for optimal output?
What about the measurements? I've got the following: All geigers.
I also have the CDV 717 (ionisation chamber: bottom ) however there's that joke that the CDV 717 measures how dead you already are.
Then there's the CI-12B and a custom made circuit: Unfortunately the CI-14B was over-volted before I got it so it was useless.
Any suggestions on how to get an idea of the actual X-rays level? The Radex 1706 is very sensitive, it goes screaming near an over-volted DY86 , but I'm not sure how reliable it is.
I know it is said that geigers are not good for X-rays, but why?
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