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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Radiation detection and measurement instruments: their construction and calibration
CI-22 is a robust Russian gamma-only GM tube with a working voltage of 390V. Its plateau extends over 100V, with a plateau gradient of 0.125%/V
It is 215mm long and nominally 18mm in diameter, but its diameter increases to 18.45mm at its widest points, which are the crimps at either end where the metal skin and the dielectric are bonded.
The 18.45mm maximum diameter lends itself perfectly to housing inside a 25mm acrylic tube with 3mm walls, leaving 0.275mm clearance to accommodate thermal expansion.
This external diameter of 25mm opens up a range of useful standard parts. The hand-grip of the GM sensor is a 25mm Lambretta handlebar grip replacement part. The anodized aluminium end stop was a control knob taken from consumer electronics scrap.
There was a very small hole at the end centre of the hand-grip, which I enlarged with a Dremel cutter to take the cable gland and strain reliever.
In order to have the shortest possible distance between the anode resistor and the anode electrode, I had to place the cathode at the far end. There being so little clearance between the GM tube and the inner acrylic wall, I decided to make the connection by means of adhesive copper tape:
The alloy wall of the GM tube is also connected to the cathode . It screens the central anode wire which is at high impedance and prone to EMI. I ran the tape down the body of the tube, making a tag at the end by folding the tape back on itself.
An 8M2 resistor was soldered directly to the anode pin, and protected by adhesive heatshrink. The screen on the coiled coax (a cheap guitar lead) was soldered onto the copper tag running down the side from the cathode pin.
It proved impossible to slide the hand-grip onto the acrylic tube without lubrication, so I applied a very thin coat of silicone grease to make the job easier.
Now I can see that the end stop made out of an aluminium tuning knob continues to look like a tuning knob, so I shall replace it with a simple metal disc for the sake of aesthetics.
Registered Member #2261
Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581
Nice! It looks a bit like an old metal workers 100W soldering iron I've got.
The shot of the fragile tube in a vice is amusing after reading that some metal Geiger tubes collapse from as little as being in a car boot when it's slammed closed!
A few thoughts:
How much effect does 3mm of Acrylic have on Gamma detection?
Why not cap the tube with an insulator?
Could you make the cathode connection to the metal tube itself and dispense with the copper tape?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
IntraWinding wrote ...
The shot of the fragile tube in a vice is amusing after reading that some metal Geiger tubes collapse from as little as being in a car boot when it's slammed closed!
It's a Russian tube, much more robust than those frightful Victoreen 1B85s formed from the type of metal that was once used for tubes of toothpaste and oil paint. CI-22 makes no pretence of detecting betas, and so is formed from sturdier stuff. The famous collapsing Victoreen 1B85 tried to do two jobs, and so did neither properly. Another difference is that the CI-22 has dielectric support at both ends, while the Victoreen 1B85 thyrode (a name which didn't catch on) only has internal support at one end. The Russian tube was designed to be pickled in vodka, and piled on the backs of grumpy mules in long damp marches across the taiga.
IntraWinding wrote ...
How much effect does 3mm of Acrylic have on Gamma detection?
Negligible so far as the rays emanating from uranium minerals are concerned. But clearly it wouldn't be able to detect the feeble gamma emission of Americium smoke detectors etc, but one has other means for detecting those
IntraWinding wrote ...
Why not cap the tube with an insulator?
After waking up in a cold sweat thinking how dreadful that aluminium tuning knob looked on the end of the tube, I racked my brains to think of a more aesthetic alternative that was 25mm across. I came up with the 2p coin. I'll criminally deface it, erasing the Royal image with corundum, and then paint it with black Hammerite before epoxying it onto the end of the tube.
IntraWinding wrote ...
Could you make the cathode connection to the metal tube itself and dispense with the copper tape?
The tube body metal certainly isn't solderable. I was concerned that the adhesive on the tape might form a dielectric barrier, but found the resistance between the metal shell and the copper tape was less than 0R05, so it must be conducting directly in places at least. The tape is also soldered directly to the copper cathode pin at one end, and to the coaxial screen at the other.
Ever since I heard about the uranium-vanadium nodules in the red mudstone exposures in Devon, my determination to add it to my collection of uranium and thorium minerals from the UK grows by the day. It's my idea of good fun!
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Now I am applying the same techniques to the famous G26 GM tube, a Holy Relic from the Golden Age of Atmospheric Atomic Testing.
It needs 1150V on the anode before it will perform.
Here's the glass GM tube, a length of adhesive heat-shrink, and a heat gun to do the shrinking:
I set up the tube with heat-shrink on a retort stand, so I would be able to play the hot air all around it from every angle.
This is the obsolete B4 (British 4-pin) socket. I found it in a sadly neglected state, so first pickled it in phosphoric acid, and then applied Brasso by way of rejuvenation.
Here is the tube now shrink wrapped, and hopefully more shock resistant and opaque to UV photons which cause spurious counts in glass GM tubes. By its side is the 50mm OD acrylic tube that will house it once I have finalised details of the shock absorbing medium inside the housing.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Assembly, Probe, GM Mk II
Having learned a few lessons, and got a few ideas, from my Mk I effort, I decided to rebuild it to IP66H - and do everything properly this time!
The 10M 5% VR37 resistor assembly at the GM tube anode inside the handle is sealed with adhesive heatshrink, and a silica gel pack is introduced into the anode end of the acrylic tube.
The Vishay VR37 series was selected for compliance with UL (510K to 11M), IEC, VDE, BSI, standards and RoHS compliance.
The end plug has two 'O' rings in fitted grooves, and is sealed with RTV. The assembly is held vertical while the RTV cures to prevent it running back down the tube.
The neoprene compression ring in the cable gland in the base of the handle is coated with RS silicone grease.
Below - A demountable clamp - adapted from a 25mm handlebar camera mount - is fitted as an accessory. The clamp is of use both in general monitoring, and for use in the many experiments where repeatable geometry is necessary.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
IntraWinding wrote ...
Nice work. I'm not sure why, but the style is definitely a step up from the MkI.
What did you use for the end cap?
The end cap is a standard 25mm end cap - but I had to bevel down the the three gripper rings which had been made to fit a wall thickness of 2mm, rather than the 3mm used in my acrylic tube. The beveled down grippers formed two perfect slots into which I was able to seat 'O' rings.
The IP66H test was performed by:
A. Measuring the resistance across the coax ouput connector with a Takeda-Riken TR8601 High Megohm Meter, which was found to be 12G. I did this test on the 25V range so as to keep ionisation of the GM gas to a minimum - it will otherwise act as a variable shunt across the dielectric resistance.
B. Immersing the detector head in 30cm of cold water (in the bath) for 60 minutes and measuring again. The resistance held at 12G.
The "H" suffix test was performed by shorting across the coaxial termination, and letting this form one node of a measuring circuit, while a 60cm length of Cu pipe sunk below the waves in the bath formed the other. After the one hour immersion as above, the resistance between the two nodes was then measured with the Takeda-Riken on the 1000V range, and determined to be >100G.
I realise that my water test was somewhere between IP66 and IP67, but I did not have one metre deep water readily to hand! So we'll call it IP66-and a-half, or >IP66 which sounds more scientific.
As the only physical difference between your SI-21 and the SI-22 is length, you're welcome to copy any part of my idea that you might find helpful.
It has certainly paid off using a 25mm OD acrylic tube, since there are lots of fittings, end plugs, and so on for it.
Not photographed are heavy duty plastic 25mm snap in pipe clips, which are sold for use in koi carp-keeping paraphernalia, and provide another form of solid and secure but demountable anchoring device.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Here, at last, is the finished probe housing for the G26 glass GM tube.
The handle assembly is made from a metal desk lamp shade that fortuitously had an internal diameter of 51mm, an excellent fit for the 50mm OD acrylic tube. A mounting hole in the metal lamp shade was made to disappear with self-adhesive stainless steel tape, and a coating of black Hammerite. The G26 glass GM tube, previously encased in adhesive heatshrink as a first protective layer, (see above) is held snugly inside the acrylic tube by a second layer of 1mm-thick closed-cell neoprene. The clear acrylic end stop is a piano leg coaster - one of those plastic dishes that you put under the brass wheels to stop them gouging the carpet. It is an exact fit, and is glued to the tube end with clear epoxy. The two hefty shock absorber rings are car exhaust pipe hangers.
The anode resistor is 12M 5% VR37, (the anode voltage for G26 is 1150V) and secured to the anode pin inside stout adhesive heatshrink. The cable gland on the handle base was assembled with RTV, except the compression ring, which was coated with silicone grease.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
IntraWinding wrote ...
Nicely done. What do you have in mind for the electronics?
I have several GM counters which can be adjusted to provide the 1150V for the G26 anode, and others which can be set to 390V for the SI-22. All are Mini Instruments Mini-Monitors. (Mini Instruments was taken over a few years ago, and is now called Thermo Scientific, but they have retained the name Mini-Monitor and its back-compatible inventory.)
But I'll have to build myself the coincidence/anti-coincidence detector counter for using the matched pair (ha ha! A brace! ) of SI-22s - I'm still awaiting the delivery of the second tube from Alex in Odessa, who is generally cheaper than Denys. It certainly won't be anything very sophisticated - parallel lines of pulse shaper/conditioners, variable pulse stretchers (LMC555 CMOS timers, I suppose) all coming together in a CMOS quad 2-input NAND gate gizmo to compare the pulses and output LOW when the inputs coincide in time. This signal will go to a 50R line driver, and thence into my Thurlby Thandar counter-timer. I'll just fix ordinary linear rate-meters onto Left and Right for analogue display.
I've been lucky enough to buy a small stock of that blue double-sided high-density Veroboard with DIL pads and whatnot on it, (which perhaps isn't made anymore) so I'll design around the use of that.
I'll also have to make a weatherproof, ruggedized, minimalist prospecting unit for real-time measurement of radon in streams - like the one that snakes past the flooded South Terras mine and the ruins of the U/Ra concentration plant.
IntraWinding wrote ...
I suspect that my design with the SI-21 will depend a lot on what I have lying around but I'll be informed by your work with the SI-22. The scope should arrive tomorrow, so I'll be back to work on the project as soon as I've finished playing with that First job is to find the Geiger Plateau and see if I can use that HT supply you offered
I'm not at all clear why a 'scope is needed for determining the Geiger plateau, though I s'pose you could find a way of pressing it into service.
This is how I do it:
Set up the tube horizontally in a clamp.
Place beneath it a lively piece of pitchblende.
Adjust the position of the tube and the source such that the source is at the (downward) apex of an equilateral triangle, the opposite side of which is the effective length of the tube (Take this to be the incised crimp grooves near either end of your SI-21.) Clearly, pitchblende isn't the point source of perfect geometry, but this is the best we can do!
At a guess, your SI-21 will be going 20-30 cps at this distance and with the best pitchblende.
Determine the correct polarity of the tube (the kathode end is connected internally to the outer metal tube in your case). Solder a 5M6 resistor with a very short lead to the anode, and a 125K to the kathode. Connect your variable HT supply to the tube via these resistors, and take your output pulses from the kathode pin.
Connect your counter and switch on HT, starting at 250V. Crank this up quite quickly until you see the pulse count start to rise, then back off to a point where you have only a few cps.
Now crank up the voltage in increments of 5V and plot your graph for your first approximation.
To determine the plateau gradient, go to the starting voltage (an important GM tube parameter) as determined in the 5V series, but use 1V increments, or even less if you have nothing else to do.
If the plateau hasn't begun by 550V or so, I suggest increasing the anode resistor to 10M, and the kathode resistor to 220K, (so as not to shorten tube life with excessive current pulses) before raising the voltage higher.
Having plotted your graph, (the which, in my case, involves sharp pencils, rulers, flexible curves, and graph paper) you should designate the plateau centre as the Working Voltage. This means that the HT voltage can wander significantly with very little change in count rate for a given gamma flux, a good feature for the simpler battery power supplies without power management/voltage maintenance etc.
If you haven't got a pulse counter, you can still do a good job by taking your pulses to a source follower, so that they are now at low impedance, and then integrate them on a capacitor/s to feed a voltmeter.
Anyway, this is my method of determining the plateau geometry, but there's more than one way of skinning a cat, so I shan't be surprised to hear of great improvements from you using your new oscilloscope. (I have an elderly Gould OS300 2-channel 20MHz scope - 20MHz for 20 squid - but have in practice found that only very occasionally do I do anything which would make me wish it were faster)
If you don't want to go through all this palaver, you could try my first guess of 390V as the working voltage, and your tube won't come to any harm if it isn't.
IntraWinding wrote ...
Update: The protective tube I have turns out to be too short so it looks like I'll be copying that from your design straight away, thanks
I strongly advise you to use the 25mm tube with 3mm walls, a very close fit to the SI-21/2 maximum diameter, which I found to be about 18.45mm at the crimp points. The firm I buy acrylic tubing from makes the first four cuts in your 1000mm length for free, and much more accurately than I could manage with a mitre saw. I give them lengths to 1mm, and have found them to have been cut faithfully to my instructions on both occasions I've ordered from them.
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Geigermania
