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Conundrum

Tue Jan 07 2014, 09:13AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

Watch out guys, seems that some of the GM tubes from less reputable vendors may not be fully working, which is why it is important to test them against a known working calibrated coiunter.

I only found this out by accident when talking to someone online about their counters, they had to buy 6 tubes to find two that worked properly. It seems that they may be taken from old submarines etc and "worn out" by exposure over years so the presumably organic fill is depleted. Symptoms are that the bad tubes can be unstable and/or have a very narrow count/breakdown threshold whereas a new tube has a near linear increase for about 100V.

I've had this happen here, at least one of the tubes on my Bull Electrical counter was bad but it only became obvious when it was replaced as it was just very insensitive.
It might have been one of the old style organic tubes, however both were made during the same era so its possible that it was simply old stock where they can and do wear out.
Buying new tubes like PM said is best, ideally with datasheet even though it might cost a bit more.

10 tubes for Â£120 may "seem" good value but invariably there will be a few duds mixed in.
I had to buy three in the end because one of mine simply wouldn't count at all even when the pins on the counter were checked etc.
Bought from a reputable seller and all was well, had to change them in a pair because the voltages were slightly different and one went "unstable" ie random bursts of clicks.

As an aside, if you do get one of these a simple 0.01uf 6KV capacitor and 2*10M in series will stop it in its tracks without affecting the sensitivity.

edit: updated some of the 3rd hand reports with useful information.
There are a few sellers of these who sell "dud" tubes for collectors, but these normally mention that the tubes don't work.
Its possible that the kits online which use end window tubes may be using old stock which has corroded, because even EPE mentioned problems with theirs.
A 5 second dab of conducting paint is the most effective fix, and avoids damaging the barrel which might explain a lot of the reports of DOA's.

No fake tubes have been seen yet, however there really is a lot of junk available if you look hard enough.
Mostly dead/broken tubes sold as working, or the problems mentioned above, occasionally sellers will put a dud tube in a counter and sell it as "Needs calibrating".

Proud Mary

Tue Jan 07 2014, 11:55AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

It would be helpful if you could give even a single example now on ebay.

Mica window tubes sometimes leak over time as seals can perish with age - even ones that come in unopened boxes.

Miniature dosimeter GM tubes will only give one or two cpm at normal background, and will disappointment enthusiasts hoping for something sensitive.

Valves including GM tubes sold as NOS should only be purchased if they come in unopened boxes, with the original paperwork still inside.

Ex-Soviet GM tubes are so cheap it's difficult to understand how anyone could 'counterfeit' one and make a profit without the massive and non-profit economy of scale that the Soviet state enjoyed.

The best thorough test for a GM tube - in my opinion - is to plot its plateau slope gradient and compare with the datasheet figure.

Halogen quench gas is not depleted in the same way as the organic quench agents like ethanol used in tubes manufactured before about 1955.

Typical life expectancy of halogen GM tubes is of the order of 5 x 10^10 counts at 25Â°C. This equals millennia of background counting.

What I have noticed on ebay over the last year or two are some really well-made glass beta-gamma GM tubes manufactured in the former East Germany @ Â£20 or so - remarkably good value.

A quick glance on ebay just now showed some very nice SI-22G and STS-5 for sale in Ukraine, as well as the usual small dosimetry tubes from the usual East European and Russian suppliers.

The things I wouldn't buy that I've just seen are old end-window tubes, ex-equipment probes with cut off wires etc, stuff with assembled PC boards, and so on, which might work, or might not. Go for the stuff with the datasheets still wrapped around the tube inside its original cardboard box, avoid all mica window tubes to be on the safe side, and the chances of getting a dud are very low in my experience.

I think it a shame to spread unnecessary fears about ebay sellers of GM tubes, when a few precautions of the type I've suggested above, together with a look at the seller's feedback rating, will make disappointment unlikely.

Conundrum

Sun Jan 12 2014, 02:09PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

Just found this, looks good. It appears to have four separate tubes in one housing, any ideas why they did this?

Also

Suitable for high radiation use, ie balloon monitors.
Also not too expensive so ideal for large area sensing applications, and possible to run them with a weak UV source to boost sense efficiency at the cost of accuracy.

Proud Mary

Sun Jan 12 2014, 03:36PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Conundrum wrote ...

Just found this, looks good. It appears to have four separate tubes in one housing, any ideas why they did this?

To quadruple the effective aperture of the detector. I'd expect to see a detector like this in a 'frisker' used to detect contamination spots on workers' clothing as they exit the hot zone in a nuclear installation etc.

Conundrum wrote ...

Also

I've noticed those HV modules before, and thought they would be useful if they work well and are reliable.

Conundrum wrote ...

When glass GM tubes are not already painted black, they must be operated in a light-tight shield or case so they don't false count on UV photons.

The counting efficiency with which a GM tube can detect Î³-rays depends on the probability that an incident Î³-ray interacts in the wall and produces a secondary electron, and the further probability that this secondary electron enters the fill gas before the end of its track. Where the tube itself is glass, with a transparent conductive cathode layer formed on the inside of the glass tube, then UV photons that pass through the glass can knock secondary electrons out of the cathode layer and into the counting gas, where they initiate a Townsend avalanche that registers as a 'click.'

Adding what would amount to a UV noise signal would not improve the sensitivity or efficiency of a glass GM tube.

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