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Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
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Posts: 843
Well it looked good in the pictures, it was described as being in "near new condition", the seller had 100% positive feedback, and the price seemed right, so I bought it.
It's a combination "Alcatel Adixen Pascal" 2005SD mechanical pump, with an attached VBC Microtorr 11 diffusion pump.
But after already having two bad experiences with ebay vacuum pumps, I'm a little nervous. I suppose I won't be at ease until I receive it and test it.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
It looks like it's in good clean condition and well put together.
If it's in good condition and hasn't been abused I'd say it probably is worth $500, but perhaps not a lot more.
Art's point about what to do with it is a good one, as you can do a great deal of interesting things with the medium vacuum from a single stage rotary air-con type pump. For example, you can make GM tubes, glow discharge devices like Grimm sources, and you can make cold cathode x-ray sources.
You can increase the electron mean free path in your device by first filling it with helium balloon gas, and then pumping it out, and you can put a getter in there like metallic calcium and melt it from the outside with a brazing torch, and so on.
If you still feel that you would feel frustrated if you couldn't produce a hard vacuum then prepare yourself for the costs of the vacuum oil and all the flanges and seals that will see your dollars vapourize.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
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Posts: 843
@ Art: I've always wanted to make a simple x-ray tube, something like the following:
@ Woodchuck: Thanks! I hope to have it and test it (at least the mechanical pump part of it) by the end of the week.
@ Proud Mary: That's exactly what I had in mind. And that's an interesting idea you mentioned regarding helium. The following graph (output from the tube shown above) seems to support that idea:
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
All x-radiography from 1895 - 1913 was done with Crookes' (cold cathode) tubes which depended on the ionisation of low pressure gas. As the gas was slowly absorbed by the glass walls of the tube, the vacuum became higher and higher, until the glow discharge ceased altogether and the (expensive) tube stopped working. Various gimmicks were used to 're-soften' the fill gas to extend the life of the tube.
All this changed in 1913, when Coolidge invented the heated cathode tube, which needed a high vacuum for electrons to motor across from cathode to anode.
My take on this: if you want a Coolidge set-up, you might as well save yourself a lot of time and treasure and buy an old dental tube, as many experimenters do, and take a few radiographs of your mobile phone, and a silicon chip or two, before you get bored.
In contrast to all that high-vacuum Coolidge stuff, soft x-rays illuminate the path followed by the blessed saints and martyrs on their way to enlightenment, and you don't have to horse around with sheets of toxic lead to maintain a good safety margin - though soft x-rays can cause serious burns at close quarters. If you Google 'glow discharge x-rays' you'll find lots of interesting papers that don't need high vacuum to get into.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
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Posts: 843
You've got me thinking about the cathode in that tube (depicted above). In the text of the paper the authors refer to it as a "cold cathode", but I'm wondering what exactly they mean by that, because they say almost nothing else about its operation/performance. If I understand correctly (based on the little information they give), it seems the cathode must be supplying 400 to 500 amps in 20 nS pulses, so maybe in the case of this particular device, cathode operation depends on "explosive" emission from the high E-field at the sharp edge?
Anyway what I'm interested in doing is to make is a "demountable" tube that can have either a tungsten target or a thin foil window. I've seen things like this for sale, and some of them seem rather crude and simple to make; e.g., like this one:
For field emission cathode material I was able to get some reticulated vitreous carbon "foam", similar to what's described here:
Proud Mary wrote ...
All x-radiography from 1895 - 1913 was done with Crookes' (cold cathode) tubes which depended on the ionisation of low pressure gas. As the gas was slowly absorbed by the glass walls of the tube, the vacuum became higher and higher, until the glow discharge ceased altogether and the (expensive) tube stopped working. Various gimmicks were used to 're-soften' the fill gas to extend the life of the tube.
All this changed in 1913, when Coolidge invented the heated cathode tube, which needed a high vacuum for electrons to motor across from cathode to anode.
My take on this: if you want a Coolidge set-up, you might as well save yourself a lot of time and treasure and buy an old dental tube, as many experimenters do, and take a few radiographs of your mobile phone, and a silicon chip or two, before you get bored.
In contrast to all that high-vacuum Coolidge stuff, soft x-rays illuminate the path followed by the blessed saints and martyrs on their way to enlightenment, and you don't have to horse around with sheets of toxic lead to maintain a good safety margin - though soft x-rays can cause serious burns at close quarters. If you Google 'glow discharge x-rays' you'll find lots of interesting papers that don't need high vacuum to get into.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
There's been a lot of research on cathodes employing nanotubes and nanotips to create miniature field-emission X-ray sources, though I hadn't heard of Reticulated Vitreous Carbon before.
XinNano is one of the brand leaders in CNT field-emission devices and details of some of their cathodes can be found here:
A cathode made of shungite might make an interesting experiment.
Flash X-ray - high vacuum breakdown - devices are outside my experience.
I think demountable tubes are excellent for the experimenter, as they are easy to modify and improve, and you don't have to disconnect the pump, so there is not the same concern about slow leaks.
I have a Svetlana BS-7 transmission target grounded anode microfocus device which I run at 4.99kV so as to comply with EC regulations. There are no regulations or legal requirements about ionising radiation sources in the EC where the PD across the device is less than 5kV. Hence, Hamamatsu make x-ray sources optimised to work at 4.9kV for use in their 'Electrostatic Removers' such as their ' PhotoIonBar' technology for use in semiconductor assembly areas. The soft rays emitted by the Hamamatsu sources are between 3 and 4.9 keV.
Of especial interest is Hamamatsu's empirical ultra-soft dosimetry, where a charged plate 150mm square and 20pf is charged to 1kV, and the time taken for that voltage to fall to 100V is measured.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
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Posts: 843
I never heard of "shungite" until you mentioned it. Carbon is just such a fascinating element, isn't it? It never seems to run out of surprises.
And speaking of that, I've been looking into graphene lately, and that stuff, with its unique properties, seems to have the potential to "change the world".
I was wondering how to make an electron beam window for my demountable tube (for anyone interested in the subject I came across this and this and when I thought about the characteristics that a good window material would have, I thought about graphene as a possibility.
As it turned out, the key words that I was using to research the idea of a graphene foil electron beam window also led me to some papers which suggest the use of graphene as an excellent field emission cathode material - even better than nanotubes. See e.g., this paper:
As that paper implies, graphene seems to be easily applied to conducting substrates by electrophoretic deposition, and that might be all that's necessary to make a robust FE cathode.
I also found this place which sells something they call graphene "electroplating" solution, but I think it's just an aqueous dispersion for electrophoresis:
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Another Vacuum Pump Gamble (from ebay)
