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External quenching mechanism for Geiger tubes

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radhoo

Mon Jan 02 2012, 07:41PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

Hi!

I was wondering if anyone here has any experience with non-quenching geiger tubes. I would need some advise for the Russian SI14b / 3 electrodes.

Any docs/links/pointers would be appreciated. Schematics (preferable using vacuum tubes) would be perfect.

Conundrum

Tue Jan 03 2012, 08:54PM

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

Seem to recall briefly interrupting the Vcc being the preferred quench method.
Think "coherer tapper" here, a simple feedback circuit using a HV transistor such as the MPSA42 ought to work.
You don't need to take it down to 0V, below the "on" voltage will do the trick.

-A

Proud Mary

Wed Jan 04 2012, 07:38PM

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

Happy New Year, Radu!

As you know, quenching in GM tubes is most commonly performed by the voltage drop across the anode resistor during tube conduction bringing the voltage across the tube down to a level below the threshold of ionisation. Common anode resistor values are between 2.2MÎ© and 10MÎ©, but resistances as high as 100MÎ© are sometimes seen. Clearly, the RC time constant of the circuit will rise with added anode resistance, increasing the dead time before the circuit is able to detect another event. (This is why it is important to keep total capacitance in the anode circuit as low as possible, by, for example, taking the output signal from a measuring resistor in the cathode circuit).

Where higher count rates must be measured, a system similar to the impulse noise blanker found in good quality communications receivers must be used.

See: United States Patent US4453076 Method and apparatus for reducing the dead-time of a Geiger-Muller, gamma-ray detector tube

The grid potential in three-electrode gaseous detector devices is used to modify sensitivity to Î² particles.

A final tip! When you have your tube set up and working, you will be able to see - in total darkness - tiny flashes of light with each Townsend avalanche due to Ne ionisation in the fill gas. You can detect these light flashes with a PIN photodiode etc, and use it as an isolated output signal source of high counting rate. Your particular tube does contain Ne - with Br as quench agent - but clearly this technique will not work with all blends of fill gas

radhoo

Sun Jan 08 2012, 06:49PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

Thank you for your replies,

I am looking for an external quenching for my russian SI14 because this tube is damaged, and enters a strange "growing" Townsend avalanche, which makes it unusable. It is clear the chemicals inside do not work as expected , probably the tube is either too old, or too used. On the other hand it still has a nice large Mica window, and I need to put that to some good use.

So what I need is a fast switching mechanism, that would reverse the polarity of the circuit connected to the tube, for an extremely short period of time, after a pulse is registered. I'm not sure how well would this work, especially without further increasing the tube's dead time.

I spent some time reading about the Neher-Pickering mechanism. Is this a good way to go?

Proud Mary

Sun Jan 08 2012, 08:32PM

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

Neher-Pickering Circuit using Thermionic Technology (1948)

There is nothing difficult or complex about this circuit, but clearly component values in the timing circuits must be followed, and I'd guess it would take a bit of setting up with a calibrated pulse generator.

It actually specifies our old friend 2X2 as HV rectifier, and it does look good fun, I must say, though quite expensive

radhoo

Mon Jan 09 2012, 06:26PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

nice find

Proud Mary

Mon Jan 09 2012, 11:02PM

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

If we just consider the Neher-Pickering sub-circuit at the top, we can see from the circuit configuration - cathode follower - and the very high resistance values - a total of 6MÎ© in the cathode line - that this is a very high impedance circuit of the type that may need a number of rebuilds before the right layout is found for stable operation.

You'll see that the designer specifies a ceramic valve socket mounted in a "Lucite"* plate, a sure sign that the greatest care is needed to stop leakage currents which can easily exceed the desired signal in a circuit like this.

Because the entire circuit is at high potential relative to Earth, a separate isolated heater supply is needed to prevent heater-cathode breakdown.

It certainly is a most interesting circuit, and I might even find time to have a go myself. Thank you for drawing it to our attention.

* i.e. Poly(methyl methacrylate), "Perspex" etc

Proud Mary

Wed Jan 11 2012, 05:45PM

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

I've stripped this circuit down to the essentials.

In the original circuit the screen grid bypass capacitor is specified as "0.5Î¼Î¼F 600V Paper" which is clearly wrong - Î¼Î¼F is the old way of writing pF.
I have therefore increased this value to 500nF, much more in keeping with both screen bypassing and paper capacitors.

The HV supply is adjusted until the cathode voltage is the same as the operating voltage required by the GM tube.

There is nothing really critical about the resistor and capacitance values in this circuit, so I would change them to the nearest modern
preferred value.

I can see no reason to stick with the antique octal-base 6J7 pentode, when a much smaller B7G-base sharp cutoff RF signal pentode like EF91 would make the apparatus more compact.

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