So, I mentioned the dinky Geiger counter built around the stylish SI-29BG tube. Happens a friend had a stash of the ubiquitous and cheap SI-3BG, which has a reputation for low sensitivity, but why not hook one up and give it a try?
It's about the same overall dimensions:

But, wait. Let's have a closer look.

Er. Is that internal structure what I think it is?

D'OH! The actual sensitive volume is the interior of that tiny little coil of wire inside the big glass envelope.
Which might account for not-so-good sensitivity.
OK, let's have a test. With both tubes at 400V (because that's what the Probe-O-Zap is currently programmed for, and it's in the plateau range for both tubes), and a couple-years-old, uncalibrated, 1 µCi Cs-137 source at 10 cm:
SI-29BG gives around 380 counts/minute.
SI-3BG gives one count per several minutes. If I hold the source right up against the glass, I get around 50 counts/min. (Doing that with the '29 gets around 600 counts/sec.)
So, the SI-3BG's reputation for low sensitivity is well earned, and an inevitable result of its design. So, we may ask, why was it designed that way?
We may ask. I don't offhand know the answer. It's probably out there somewhere, in Russian.
But, I can speculate. Best guess is that it was designed for cheap little alarms to be scattered around nuclear installations, and if they started registering more than a couple of counts per minute it was time to get out of there right now.
It sure wasn't designed for monitoring radiation at human-safe levels.
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