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Registered Member #2261
Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581
That's interesting.
I have a memory of reading about something with the same design as THGEMS (double sided pcb with lots of tiny holes) some years ago except the interest in that case was because the small holes could support a stable discharge at atmospheric pressure without the discharge turning into a spark, the point being that normally this requires operation at reduced pressure. Unfortunately I can't find anything about it despite lots of Googling.
I've tried reading a few papers but they're aimed at people who've been working on GEMS/THGEMS for ages. I need to look back for the earliest papers to get a grasp of this, but meanwhile I found a couple of links that you might like if you haven't seen them already:
This one because it includes pictures of related technologies, one of which happens to look rather like Veroboard. Micro-Pattern Gas Detectors Technologies (CERN)
The other seems to be a projection display to support a presentation and includes dimension ranges for THGEMS devices that includes the dimensions of Veroboard, I think. It gives Diameters: 0.3 - 1mm, and thickness 0.4 - 3mm on page 9
Possible problems: Where you thinking of using two pieces of Veroboard sandwiched to get the second side electrodes? It seems the rim or ring of removed electrode around the holes is important, something Veroboard doesn't have. The phenolic dielectric might interfere with things.
If I understand correctly and THGEMS operate at atmospheric pressure, testing Veroboard might be very simple if it can work with air as the gas. If an inert gas is essential I think argon is readily available in small cans for welding.
If I was serious about this I think I'd have a thin PCB made with the smallest available drilled holes, but a two layer Vero sandwich might work.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
IntraWinding wrote ...
Where you thinking of using two pieces of Veroboard sandwiched to get the second side electrodes? It seems the rim or ring of removed electrode around the holes is important, something Veroboard doesn't have. The phenolic dielectric might interfere with things.
Strange you should mention that, Alan. I was thinking of using my Dremel drill press and something like a countersunk bit with a depth stop to make hour-glass or venturi shaped holes, which seem to give the best results if I've understood the papers correctly. This would be a bit tedious for large sheets, but a matrix of ten by ten holes for a proof-of-concept detector shouldn't be too much of a sweat. In any case, the papers are targeted at imaging systems, and particle altitude/azimuth analysers, whereas I'm not looking for anything but detection, at least not to start with.
I've got some double-sided FR4 proto boards with various pad and DIL patterns rather than the Vero strips (which I also have) and am looking to make perforated dielectric sandwiches out of them. The bottom sheet would be a collector of unetched PC board.
I've got a Bertan 313A variable PSU 0-3kV @ 10mA, and thought of having three or four boards fed by a tapped potential divider like the dynode resistor chain for a PMT, with the full 3kV on the collector plate at bottom. . I got the impression that the thick GEM-like detectors could be made to work at STP, but if not, it will take me a month for two to have a vacuum system up and running, using backfilled Ar as you suggest, and butane too. (A pallet of Harwell vacuum cast-offs is coming my way)
It might be interesting to see what happens in a GEMS detector when the pressure is reduced into the Geiger region, which suggests a postage stamp sized detector might be possible.
I wonder how an electric sewing machine would be able to handle punching rows of holes in a Al-Kapton-Al sandwich.
Anyway, it's lunchtime, so I'm off out for a bite to eat on my way to vote Green Party at the local polling centre.
Added after lunch:
Here's my invoice for the Harwell auction lot which arrived while I was out adding inches to my waistline:
I'm hoping to get my hands on a bit more of this kit next week, so once I've got a Javac A/C pump and all the hoses, greases, and oils, I should be well set up in the vacuum biz, with some left over, I'd say.
While I was having lunch, I drew some sketches on the paper table cloth, and came to the conclusion that the GEM detector could be considered as an assemblage of holes joined together by dielectric, while a metal gauze could be described as a collection of air dielectric holes joined together by a conductor. I'm not quite certain how, but I think it may be possible to carry the GEM detector concept a bit further by thinking in these terms.
Registered Member #2261
Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581
I might easily be wrong here, but I got the impression those tapered holes where just a consequence of the etching process that has come to be 'the standard'.
I don't know what they etch Kapton with that wouldn't also take off any etch resist over the copper, but I can see how it would result in those tapered holes when etching simultaneously from both sides. Anyway, I think the tapered holes became 'standard' simply because that's how they came out and they want to keep conditions identical from one experiment to the next. I think there's plenty of room for other styles of hole to turn out just as good or better, so don't make work before the simple approach has failed is my advice. On the other hand, you could take a little copper away from around each hole using a sharp countersink or a conical abrasive tool twiddled by hand perhaps in a pin vice (I'm really glad I did Metalwork at school ). A countersink would be better if it's sharp at the point where it would cut in a hole of the tiny diameter we are discussing. Some will, some won't, I think you'd find. By hand it would take a couple of fast twists on each hole to strip away a little copper. With the Dremel you'd get the same problem as when polishing soft material, that it will cut too quickly and cause damage in a moment. One concern about this technique is the sharp copper edge it will leave nearest the holes. It might encourage sparking, I don't know.
I like the sewing machine idea. It will distort the plastic, but perhaps that funnel shaped hole will be the next big thing!
Yes, I'd spotted that THGEMS could be used as an alternative to a Geiger counter or for imaging. If you get it going and can make a fine enough array it might make a nice, direct X-Rays to pixels imager.
Is your Harwell pallet all vacuum gear? I have a quite large Javac two stage rotary vacuum pump I got cheap and managed to repair . I also have an old turbomolecular pump, but no driver for the motor. That's another project I might get back to one day...
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
IntraWinding wrote ...
I might easily be wrong here, but I got the impression those tapered holes where just a consequence of the etching process that has come to be 'the standard'.
I don't know what they etch Kapton with that wouldn't also take off any etch resist over the copper, but I can see how it would result in those tapered holes when etching simultaneously from both sides. Anyway, I think the tapered holes became 'standard' simply because that's how they came out and they want to keep conditions identical from one experiment to the next. I think there's plenty of room for other styles of hole to turn out just as good or better, so don't make work before the simple approach has failed is my advice. On the other hand, you could take a little copper away from around each hole using a sharp countersink or a conical abrasive tool twiddled by hand perhaps in a pin vice (I'm really glad I did Metalwork at school ). A countersink would be better if it's sharp at the point where it would cut in a hole of the tiny diameter we are discussing. Some will, some won't, I think you'd find. By hand it would take a couple of fast twists on each hole to strip away a little copper. With the Dremel you'd get the same problem as when polishing soft material, that it will cut too quickly and cause damage in a moment. One concern about this technique is the sharp copper edge it will leave nearest the holes. It might encourage sparking, I don't know.
I like the sewing machine idea. It will distort the plastic, but perhaps that funnel shaped hole will be the next big thing!
Yes, I'd spotted that THGEMS could be used as an alternative to a Geiger counter or for imaging. If you get it going and can make a fine enough array it might make a nice, direct X-Rays to pixels imager.
Is your Harwell pallet all vacuum gear? I have a quite large Javac two stage rotary vacuum pump I got cheap and managed to repair . I also have an old turbomolecular pump, but no driver for the motor. That's another project I might get back to one day...
If you go to Thick GEM-like hole multipliers: properties and possible applications and look at Figure 3. Equipotential- and electric-field lines calculated for TGEMs of 1.6mm thick and 1mm holes you'll see why having a venturi or hour-glass shape would fit in better with the distribution of the electric field.
1.6mm thick and 1mm holes sounds mighty like yer actual perfboard to me!
The guff about the double conical hole geometry is mentioned in this paper here:
but it's quite clear that the ThickGEM folk who don't have CERN's resources and workshops are just using off-the-shelf stripboard and getting results that I would be pleased with.
All the latest lot of Harwell hand-me-downs are high vacuum stuff - 5 oil diff pumps, together with a number of Pirani-type and Penning-type torr meters and their heads. Next week I'm hoping to get more in the way of baffle valves, vacuum stopcocks, and whatever flanges I can get together, but don't expect to get the same give-away prices a second time around as a rival has put his oar in.
PS There were a couple of turbomol pumps in the sale last week, but I didn't go for them because there's too many moving parts to wear out, misalign, and otherwise go wonky compared with the diffusion pump, which has no moving parts to give up the ghost. You'd have to get a second mortgage just to buy spare parts for them!
Registered Member #2814
Joined: Wed Apr 21 2010, 12:28PM
Location: Scotland, UK
Posts: 24
Apologies for butting in but this looks very interesting. It certainly appears they're using standard PCB laminates to construct the sensors, 1.6mm is a bit of a giveaway and Kapton is used for flexible PCBs. As for gas mix, Ar/CO2 is available in small disposable cylinders for small welders and on a quick search appears to be 80/20% but I couldn't find much info to confirm it. I'm building a small CNC drill/milling machine at the moment and producing a board with hundreds of identical holes would be no problem once (if) it's ever finished. Hopefully this will give me a little more incentive to do something about finishing it .
Proud Mary wrote ...
All the latest lot of Harwell hand-me-downs are high vacuum stuff - 5 oil diff pumps, together with a number of Pirani-type and Penning-type torr meters and their heads. Next week I'm hoping to get more in the way of baffle valves, vacuum stopcocks, and whatever flanges I can get together, but don't expect to get the same give-away prices a second time around as a rival has put his oar in.
Were you by any chance responsible for pushing the price of that spool of 'Tungston' wire up to over £11?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Kilham wrote ...
Were you by any chance responsible for pushing the price of that spool of 'Tungston' wire up to over £11?
So many items came up at the same time that I had to let the Tongsten go. In any case, it's too thick for good GM and proportional tubes.
Now that x***b has showed up, I don't expect to do half so well in the week coming, though as he buys to re-sell, he cannot go too high. I bought a Philips X-ray photometer off him a while back, with him starting off at £399.99 for the whole giant aluminium transit case full of kit, and finally letting it go for £16 after three months of haggling.
I should have taken that Leybold Torr Meter for 99p when I could last week, but decided against it because the bottom view showed the pipe is heavily corroded, so I thought there was good reason for chucking it out. But now I see that the rag-and-bone man is bidding on it, I stuck some dosh on it out of sheer perversity, just to hear the pips squeak! I've got about half-a-dozen speculative bids on there at the moment, but don't think I'll pursue them to any great heights, because they pretty much duplicate kit I've already got more than one of.
Lastly, here's another CERN GEM detector paper, with details of the hole morphology:
Registered Member #2261
Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581
You're far more familiar with this than I am, but here are some thoughts that might include something useful:
Proud Mary wrote ...
If you go to Thick GEM-like hole multipliers: properties and possible applications and look at Figure 3. Equipotential- and electric-field lines calculated for TGEMs of 1.6mm thick and 1mm holes you'll see why having a venturi or hour-glass shape would fit in better with the distribution of the electric field. 1.6mm thick and 1mm holes sounds mighty like yer actual perfboard to me!
The dimensions sound good, but perfboard only has copper on one side. If you sandwich two pieces together there are two ways of doing it. If you have both copper layers uppermost you get the gap between the electrodes about right, but the buried copper layer may not perform in the same way as an exposed layer. If you have the two copper layers outermost you have a double thickness board, which may be too thick.
Proud Mary wrote ...
The guff about the double conical hole geometry is mentioned in this paper here...
OK, up to 30% improved gain due to 'Charging up'. Another, possibly manufacturable, 'Hour Glass' hole profile could be made if the inside of each hole was coated with epoxy resin. Capillary action will pull it into a shape similar to the hole in a torus, but with a cross section more like a parabola than a circle. It might be possible to coat an array of holes evenly as follows: Apply degassed epoxy to one face of the perfboard. Force it through the perforations using a soft scraper blade. Use the blade to remove excess. All the holes should now be full of epoxy and the board faces only covered in a film. Next, two methods of evenly removing epoxy from the over full perforations come to mind. First, place the board down on a piece of paper, perhaps resting on a warmed flat glass plate. Allow a moment for the heat to reduce epoxy viscosity and then quickly pull the board away. Allow a moment for the 'torus holes' to form, cool the board and clean the board faces by pressing against clean paper repeatedly. The other method that might work is to apply a pulse of pressure to one side of the board to blow out excess epoxy. The 'torus hole's will form will very smooth highly polished surfaces which might be good or might be rubbish, but might be worth investigating. The precise technique for making the torus holes would have to be developed by experiment.
Harwell: OK, I think I've worked out where the Harwell stuff came from thanks to the toongstone wire. I tried my luck with a couple of those pumps, but I was torn between a preference for water and air cooling. I only saw one Turbo pump though, and I didn't think Harwell was in Wiltshire ???? And I don't want to get in an unnecessary bidding war etc. etc. on the remaining water cooled unit.
The turbo pumps I've pulled to bits had just one ball race to wear out (magnetic bearing at the top to avoid grease in the high vacuum end). It was an impressive ceramic bearing, but they're getting easier to come by and if that was a problem I'd try a steel bearing. I know the rpm's are extreme, but that might just mean a shorter bearing life, not a problem for amateur occasional use. To me the big problem is that you only ever get Turbo pumps cheap if they have no PSU, and the PSU is complex. I think the pump I have needs a three phase clean sine wave PSU with slowly increasing frequency to ramp it up to full speed. The motor is run near it's temperature limits I think, so it needs to be driven carefully. I like the idea of using a turbo pump with a multistage diaphragm pump so you get (virtually) oil free operation.
If you need to get the CO2 out of the welders Argon mix, I think Sodium Hydroxide will do it. I think it will need to be wet, but if the solid is in excess I think the resulting gas will be dry. If not, some drying agent like a tube of silica gel might be all that's needed.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
I was very pleased we now have Caroline Lucas as Britain's first Green Party MP here in Brighton.
I have two sorts of double-sided prototyping perf board. One sort is RS Components No. 434-267, and the other is Vero High Density board No. 10-0581 B. Both types are Eurocard sizes with a blue ground, and both have different copper layouts on obverse and reverse.
The vendor of the Harwell cast offs must have offered the cheapest tender, or has sub-contracted the disposal from a larger contractor dealing with government disposals. There is a similar contractor dealing in disposal from Winfrith, from whom I've also bought interesting bits and pieces.
more later -
Later...
Argon: You can buy 'pure argon' in disposable cylinders too. I'd guess that the technical grade of the 'pure' gas means better than 97% or so.
As for the bits and pieces of vacuum gear that Simon has left, if you want to bid on any of them, I won't increase my bids further if you tell me what they are.
Thinking more on the nature and shape of these GEM holes, I vaguely remember something about dielectric surface discharges which none of the GEM papers I've browsed seem to take into account. When the holes are chamfered or beveled (or whatever the word is) at both ends, the discharge track along the dielectric surface would be made longer, would it not, and the surface discharge path and the gas ion path might be considered as two interacting resistors in parallel shunted initially by C as a provisional very crude model to start thinking about.
Thank you for your suggestions about possible ways of producing the venturi shaped holes en masse which I took to mean that they could be formed as some kind of surface tension meniscus effect, perhaps.
Anyway, I have to start sorting out my vacuum bonanza, and chucking out the heaps of packing material that is blocking up my bathroom and hall.
Registered Member #2261
Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581
Yes, warm epoxy forms very handy meniscus shapes, particularly if bubble free and warm. Gravity distortion of the shapes can be used to advantage or to some extent overcome by rotation until the resin sets.
Hopefully surface leakage will be small through the holes. I don't know about dielectric surface discharges, but I suspect the environment in those holes will be hostile to surface contamination as I think it may be constantly 'Plasma Cleaned' , although that might lead to the gas fill becoming contaminated as it slowly strips material from the holes' interior surfaces. I have my eye on this
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
IntraWinding wrote ...
I have my eye on this
I can't tell from the picture whether this is the 11cc 230V 175W model, or the 30cc 230V 350W model, which look very similar. I have both types.
There's not much that can go wrong with either type, except heater failure, jets blocked with crud, and varnish-like deposits of ancient pump fluid, all of which can be fixed.
You can see Les Wright's ("Plazmatron") ingenious set up using an over-clocker water cooling system here:
704 pump fluid isn't cheap, but 100cc will last a long time with these small pump capacities.
You need circlip pliers to take the heater jacket off.
I won't get in your way, and hopefully the Rag n' Bone man won't go too high on it. Good Luck!
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Radioactive Mineral Sample Prospecting Equipment - Advice Sought
). A countersink would be better if it's sharp at the point where it would cut in a hole of the tiny diameter we are discussing. Some will, some won't, I think you'd find. By hand it would take a couple of fast twists on each hole to strip away a little copper. With the Dremel you'd get the same problem as when polishing soft material, that it will cut too quickly and cause damage in a moment.
.