Geiger counters

[Mark R]

Why is the anode at the centre?

What difference would it make if the polarity was reversed?

 

[brentkiosk]

Good question. It doesn't seem as if it would make any difference - you would get a discharge in the gas when it was ionized with either polarity. Maybe it has something to do with the detector connections. GM tubes have been around for many decades. It could be it was once easier (and maybe still is) to count one polarity pulses than the other.

I'd be interested if anyone knows . . .

 

[Mark R]

I've thought about this a bit. My current thought is that you might get a better avalanche from the electrons.

Electrons are attracted towards the anode - having the anode at the centre means that your concentration of electrons at the centre will be higher (because of the reduction in radius).

Additionally, the electric field is higher (again due to the reduction in radius) so the electrons accelerate more rapidly.

The heavier, positive ions may be less effective at causing avalanche than the electrons because of their lower velocity. Reversing the polarity may therefore mean that current pulses are slower and less intense.

Does this sound plausible?

 

[brentkiosk]

That does make sense. The lighter particles in the stronger field would seem to make for a sharper edge on a bigger pulse. Maybe that's it.

A long time ago, I worked with someone who was a great fan of GM detectors, and I know he explained more than just the basic operation. But, I've barely used one since and just don't remember what he said. But trying to concentrate on electron motion does sound very plausable.