A while back I posted an excerpt from a blogpost from a person with an SBM-20 tube who made polarity reversal tests with a Cs-137 test source and found that the response at the "wrong" polarity was lower, similarly to what I observed, even if in theory it should haven't. That's what motivated my hypothesis. However, that is no Am source.
Relevant quote below:
http://uvicrec.blogspot.com/20…0-geiger-muller-tube.html
QuoteDisplay MoreThe tube is clearly marked with a "+" on one of the terminals. But theoretically it should collect charge on either terminal. So what happens if you reverse it?
At +400 V I recorded 3068 CPM. At -400V I got 1381, about 45% of the + reading. When I removed the source altogether I got 9 CPM. So clearly its working as a detector, albeit less efficiently.
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I tried to take current measurements and by 700 V spikes were already occurring
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By 900 V the signal was unstable as they no longer settled to a steady state. Maybe oscillating with the capacitor?
Overall appears to be less efficient and works under a much narrower range. Presumably GM tubes are optimized for positive voltage though so maybe it could be improved a lot if you intentionally tried to design a negative center GM tube. In any case, the biggest takeaway is that under standard 400 V operation the tube works. This is actually unfortunate as you might not notice you installed it backwards (mechanically possible).
At the moment I don't have any known/certified radioactive source to check for myself.