Why Two Geiger Muller Tubes in a Geiger Counter?

Just a guess.

When you consider what Geiger counters are designed to measure, maybe redundancy isn't such a bad idea.

Why two? Guess they couldn't fit four like this one:

Now, every Geiger tube type has an inherent "zero radiation" pulse rate, of pretty

inconsistent and random pulses, that makes a "noise floor", near which you can't

"see" much. I'm guessing that counting the sum of pulses of more than one tube,

gives more info in that, very useful low end, just above background radiation, and

instrument's useful dynamic range, increases . S.M.

I wonder what causes these random pulses? Temperature? I thought it required an ionising particle to cause the ionisation that could escalate into a count? I like the photo you posted --- thats lots of tubes.

Most of what causes the random pulses is background ionizing radiation. The background level is not zero. There is a statistical possibility that the thermal noise of a resistor or the shot noise of a bias current can be an individual pulse. Unless the electronics is a lousy design or the background radiation levels locally happen to be exceptionally tiny, this detector noise is nominally orders of magnitude smaller than nominal background levels.

where can I buy SI-29BG and SBM-20 GM Tubes at low price about $10/each? Anyone knows the bulk supplier of these? I want to thank in advance for help. I am developing electronic circuits for various types of GM Tubes. I do have LND-712 and something similar to SBM-20 with shielded top. I am also going to get LND-713. Idea is to make the circuit for about $50-$90 range easy to use high performance type for educational purpose. I can share my experimental results if people are interested.

Thanks for your really interesting answer. I have been hanging around Geiger counters a long time but there are things there I never knew. David