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Registered Member #2261
Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581
I've been needing a new scope for a while after my OS300 developed a fault I couldn't fix. The unexplained behaviour of my recent Geiger test circuit, which I expected to give me a nice graph showing the Geiger Plateau, just put more pressure on me to make a spending decision. I'll be delighted if I get everything working - please don't expect it to result in a ground breaking design! On the other hand, there is some interesting circuitry at the end of that Centronics Geiger Tube bible intended to deal with problems caused by closely spaced pulses, but I'll leave that for another day...
I've ordered some 25mm/3mm tube. I was tempted by the 25mm/2mm tube which would allow a bit of clearance around the Geiger tube for a resilient mount, but in the end I figured that extra acrylic was probably more valuable than a tiny layer of silicone or other rubber. I'll have to cut and finish the tube myself but at least I won't have to decide what length I want until closer to the build. I plan to solvent weld a 5mm acrylic end cap, which should form an invisible bond after a bit of sanding and polishing.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
IntraWinding wrote ...
I've ordered some 25mm/3mm tube. I was tempted by the 25mm/2mm tube which would allow a bit of clearance around the Geiger tube for a resilient mount, but in the end I figured that extra acrylic was probably more valuable than a tiny layer of silicone or other rubber. I'll have to cut and finish the tube myself but at least I won't have to decide what length I want until closer to the build. I plan to solvent weld a 5mm acrylic end cap, which should form an invisible bond after a bit of sanding and polishing.
The advantage in using a standard plastic end plug, is that you can locate and centre the kathode terminal inside it by using a suitable rubber grommet as a bushing. This holds the GM tube about 0,25mm away from the acrylic wall all round, which is more than enough clearance to allow for the expansion of the metal on hot days, while the rubber bushing inside the end cap allows linear expansion of the stainless steel tube, which could otherwise - possibly - crack the seals if the metal had nowhere else to go but buckle.
Registered Member #2261
Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581
Proud Mary wrote ...
The advantage in using a standard plastic end plug, is that you can locate and centre the kathode terminal inside it by using a suitable rubber grommet as a bushing. This holds the GM tube about 0,25mm away from the acrylic wall all round, which is more than enough clearance to allow for the expansion of the metal on hot days, while the rubber bushing inside the end cap allows linear expansion of the stainless steel tube, which could otherwise - possibly - crack the seals if the metal had nowhere else to go but buckle.
Good point. I'll need to allow the Geiger tube to move freely inside the acrylic tube, axially & radially, but without rattling. I think I'll put a small blob of silicone (non acetyl RTV ) on the anode pin and fit the the Geiger tube into the acrylic tube so it makes contact with and adheres to the previously welded in place end cap. A cylinder of paper wrapped around the Geiger tube will hold it on axis as the silicone sets (hopefully it will set). I'll deal with the cathode end when I get to it using similar techniques. I wonder if I can get transparent non-acetyl RTV silicone? The stuff I have is white. What's yours like?
An appealing alternative would be to fill the space between the Geiger tube and the acrylic tube with silicone oil and just let it float there. Sealing the oil in whilst allowing it to expand and contract and making electrical connections looks a bit daunting though, unfortunately.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
IntraWinding wrote ...
I'll need to allow the Geiger tube to move freely inside the acrylic tube, axially & radially, but without rattling. I think I'll put a small blob of silicone (non acetyl RTV ) on the anode pin and fit the the Geiger tube into the acrylic tube so it makes contact with and adheres to the previously welded in place end cap. A cylinder of paper wrapped around the Geiger tube will hold it on axis as the silicone sets (hopefully it will set). I'll deal with the cathode end when I get to it using similar techniques. I wonder if I can get transparent non-acetyl RTV silicone? The stuff I have is white. What's yours like?
It is 'clear' but has a petrol blue caste. But it's not aesthetically disfiguring, like horrid white smears of putty and painty thumb-prints round the edge of a badly-fitted window pane.
IntraWinding wrote ...
An appealing alternative would be to fill the space between the Geiger tube and the acrylic tube with silicone oil and just let it float there. Sealing the oil in whilst allowing it to expand and contract and making electrical connections looks a bit daunting though, unfortunately.
I have about ten of those Romanian blue glass GM tubes (Va = 1.1kV) with the clunky B4 base. The glass is thin enough to admit even some of the lazier betas - in a word, the glass is so fragile that it's a miracle that so many have survived since 1960. With very great care, it's possible to cut off the base with a Dremel wheel and connect directly to the flying leads, one of which, the kathode, emerges at an angle from the side of the tube. The metal is so brittle that this wire will snap off with the slightest excuse, and it cannot be wetted with ordinary lead solder to reconnect the stump, which is dead flush with the glass.
I wondered if it might be possible to embed these fragile tubes in a clear resin polymer, to render them shatter-proof and immobilise the leads in a very permanent way.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Coincident GM pair with Gamma-Ray Photometer on Variable Geometry Alignment Frame
Making the second GM tube probe housing only took a morning, as I had nothing to do but copy the first one.
A sample or source to be investigated is mounted at any desired position on the centre rail. The frame geometry allows the positions of the scintillation head and the GM pair to be varied in relation to the source or sample (in a single plane.) in a completely repeatable way.
Registered Member #2261
Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581
Another problem with my silicone oil idea is that it'd transfer shocks to the Geiger tube very efficiently.
Proud Mary wrote ...
I wondered if it might be possible to embed these fragile tubes in a clear resin polymer, to render them shatter-proof and immobilise the leads in a very permanent way.
I think setting shrinkage might crush or pre-stress the tube, and if not, the same shock transfer mechanism I fear with silicone oil might limit improvements. I'd have thought the best thing would be to mount them in soft supports, like your rubber end caps or my RTV idea. A perforated tube cover would still let beta through.
As for the connections, perhaps it's another job for conductive epoxy or paint? I plan to use some on my project to make a connection directly to the tube wall at the anode end of the tube.
Shame my silver conductive paint project wasn't more successful - I'll have to get back to that one day as silver conductives are hideously expensive!
What's that device in the middle of your alignment frame?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
IntraWinding wrote ...
What's that device in the middle of your alignment frame?
That's a PMT scintillation head - as mentioned in the text - configured as a soft gamma-ray photometer - as mentioned in the title!
The à la carte scintillator block de jour is St Gobain BC-452 5% organo-lead-loaded anthracene plastic which really rocks below 200keV, the warm 'n fluffy end of the spectrum where everyone's going this year.
Registered Member #2261
Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581
Proud Mary wrote ...
IntraWinding wrote ...
What's that device in the middle of your alignment frame?
That's a PMT scintillation head - as mentioned in the text - configured as a soft gamma-ray photometer - as mentioned in the title!
Ahh! Sorry, I was in a rush when I read that...
Proud Mary wrote ...
The à la carte scintillator block de jour is St Gobain BC-452 5% organo-lead-loaded anthracene plastic which really rocks below 200keV, the warm 'n fluffy end of the spectrum where everyone's going this year.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Al/Pb Shielding Jacket For Detector Calibration
An uncalibrated detector is like a ruler without any gradations on it. Such a ruler can establish the fact of extension in space, but not how much of it there is for purposes of analysis and comparison.
Commercial calibration sources using such horrors as Sr-90, Co-60 and Cs-137 have no place in my scheme of things, even where they might be subject to a UK radioactive substances license exemption order under RSA93. So what's a girl to do?
Natural potassium contains 0,012% of K-40, a beta gamma emitter of half-life 1.248 x 10-9 yr.
11.2% of the time, K-40 decays to Ar-40 by electron capture or positron emission, but for the remaining 88.8% of the time it gives up the ghost by beta decay to become stable Ca-40.
All the fancy beta emissions of K-40 we can filter out with aluminium, leaving us with juicy 1.46MeV gamma rays produced during the 11.2% of the time when K-40 atoms turn themselves into argon gas.
So, if we take a weighed amount of common or garden anhydrous potassium hydroxide (KOH), we can calculate how many atoms of K-40 there are in it, how many of these unstable atoms will disintegrate in one minute, and how many of those disintegrations will be of the K-40 to Ar-40 kind that eject a 1.46MeV gamma ray during elemental reincarnation.
As we know by theory the number of gamma-producing distintegrations per minute that are happening inside our weighed KOH sample, we only need compare this figure with the number of gamma photons actually detected (total count minus external background, minus the detector's own intrinsic radioactivity due to lead solder etc) and we can calculate the detector's efficiency - and more....
And so we get round to the need for the clunky DIY Al/Pb shielding jacket of our title.
The amount of gamma rays coming off a spoonful of KOH isn't going to take your breath away, telomeres shortening by the minute - - in fact, it's not going to be dramatically above the background, so we need a shield to keep as much unwanted background out of the calibration set as is reasonably possible on a shoe-string budget.
There's no keeping cosmic rays out of anything, but a worthwhile amount of terrestrial background can be filtered out by taking measurements inside a lead pipe.
Unfortunately, natural lead contains Pb-210 and Po-210 in sufficient amounts to interfere with low-level detection*, so we have to filter out alpha and beta interference arising within the lead by means of an additional internal layer of aluminium.
And so our journey takes us to an aluminium tube inside a lead tube - a 60mm OD aluminium tube with 3mm walls, wrapped around with lead roofing sheet, both ordered via eBay.
*The impact of natural radioactivity in solder on low background experiments Brodzinski, R. L.; Reeves, J. H.; Avignone, F. T.; Miley, H. S. Nuclear Instruments and Methods in Physics Research Section A, Volume 254, Issue 2, p. 472-473.
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Geigermania
