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Neon lamp radiation detector

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kimbomba

Mon Jul 15 2013, 02:02AM

Registered Member #3854 Joined: Fri Apr 29 2011, 03:45AM
Location: Mexico
Posts: 95

I am trying to build this circuit that supposedly works as a simple geiger counter:

However i can't make it work. I set the voltage just below the threshold, about 71.5 V. The bulb ignites but it remains so. Ha anyone built this circuit? Thanks in advance

Dr. Slack

Mon Jul 15 2013, 07:17AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

I haven't built this circuit, but I've noticed a related phenomenom in a neon indicator light. This behaviour suggests that you may not need to be as careful about whether it ignites or not. However, remember that not all neon lamps are created equal. With a commodity lamp that has to 'just light', there will be all sorts of variation in electrode material, gas impurities etc, which I would expect would change the detailed behaviour.

Anyhows, this what I saw. I was playing with a white LED. As I intended to make it a very low drain always-on 'find me' indicator for my camping torch, I was running it at micro-amps. I therefore needed to stumble around in my darkened workshop to see it. As I swung the not very bright beam around, it passed over the neon indicator on a distribution board that had been flickering, causing it to light steadily. I was surprised, and spent the next few minutes confirming the effect, and seeing how dim it could be and still work. The answer was 'very dim'.

I figured that the blue light content of the white LED was pre-ionising the the gas in the neon bulb, so that it lit reliably every half cycle. And it didn't need much light to do it. Why it wasn't firing up every half cycle in the dark I don't know. I'm in 240v-land, so there was plenty of over-voltage available. I think I'll go back and test amber and green LEDs, just for completeness.

Maybe to use this effect, you'd need to find a soft neon that was about to die? Maybe if you've *got* a soft neon, it's breaking down prematurely? Perhaps you need to characterise your chosen neon with a variable voltage.

Steve Conner

Mon Jul 15 2013, 08:08AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Try adding a small capacitor across the neon lamp, maybe 100pF, and/or increasing the 1M resistor. Adding capacitance will make the voltage across the bulb recover more slowly after it fires, giving the gas time to deionise.

Proud Mary

Mon Jul 15 2013, 12:25PM

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

If you make a quick study of the Centronics GM tube theory manual here:

you will see that it is impossible that a pea neon bulb could operate in the Geiger region, not least because there is insufficient voltage to accelerate the ions and electrons produced by a collision, so rather than producing further collisions, they recombine. There is no Townsend avalanche and little or no gas amplification.

I would expect a very small ionisation current would flow if the bulb with voltage applied were placed in a strong X-ray beam, but this is not GM action as gas amplification is not involved.

A few years ago, Charlie Wenzel did some experiments to try and replicate the widespread neon bulb detector claims, and, like you, found he could not make it work.

kimbomba

Mon Jul 15 2013, 09:13PM

Registered Member #3854 Joined: Fri Apr 29 2011, 03:45AM
Location: Mexico
Posts: 95

Thanks you for your replies. Adding a 220 n cap in parallel with the lamp does produce an erratic behavior, as Charly Wenzel reports in his page.
I do not have a source of radiation to check however. What is interesting is that the thing can be used as a EMF detector, for example, the bulb ignites if you use a gas stove ignitor near to it, or if a source of HV is near the bulb (my 30 KV flyback driver makes the bulb blink when placed at up to 40 cm)

Adam Munich

Mon Jul 15 2013, 11:24PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Keep in mind... many of these lamps have thoriated electrodes. I can't imagine such a bulb would be useful at all, for radiation detection.

radiotech

Wed Jul 17 2013, 07:02AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Some lamps like the NE 83 have a radioactive isotope added to combat the dark effect.. If you had a more common lamp, perhaps the sensitivity of your
experiment would be more stable if the lamp, and your sample were shielded completely from light.

What the dark effect is, an unpredictable change in characteristics from
dark to light, when those lamps were used as voltage stabilizers.

Other than the voltage across the lamp, the electric field and the charge
on the glass may influence the firing (it does for fluorescent lamps )
Your experiment could revert to the tendency for radioactivity to discharge
an electroscope.

Shrad

Wed Jul 17 2013, 07:57AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

wouldn't a classic voltage stabilizer valve achieve similar behavior?

it might be interesting to see if there are avalanche processes occurring in these tubes at a certain operating region

radiotech

Wed Jul 17 2013, 08:45AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

This ia a page from the GE Glow Lamp Manual showing the regions. The regulator tubes, 0A2, etc act somewhat like a neon lamp, but at a higher
current, and are polarized, to an extent, with a positive and a negative
terminal. neon lamps mostly have similar electrodes for both poles.
The regulator tubes sometimes also have radioactive additives.
I believe the whole glow lamp manual has been scanned and is on line
somewhere.

1374050740 2463 FT155806 From Ge Glow Lamp Manual

Proud Mary

Wed Jul 17 2013, 11:50AM

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

This extract from the 1958 Philips 0A2 datasheet gives some actual light sensitivity figures - an ignition voltage difference of as much as 45V:

1374061792 543 FT155806 0a2 Datasheet Extract Philips 1958

Clear glass GM tubes will also trigger - false count - in daylight.

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