Cheney C8600 Geiger Counter With CI-3BG GM Tube

I haven’t built a kit in a long time, so when the Cheney C8600 Geiger Counter kit and a CI-3BG GM tube went on sale I had a pleasant interlude:

Chaney C8600 Geiger Counter Kit - CI-3BG GM tube
Chaney C8600 Geiger Counter Kit – CI-3BG GM tube

It’s a good thing I have a pretty deep parts stock, as one of the caps didn’t fit into its holes at all.

The Russian CI-3BG glass tube, according to the datasheet and discussion on MightyOhm, is sensitive to gamma and beta radiation, so it should serve as a simple cross-check on my ionization chamber results. It’s not clear the C8600 is applying the correct voltage to the CI-3BG tube, but it probably doesn’t make much difference; the supply is so feeble that there’s no way to actually measure the results.

A closer look at the CI-3BG suggests the active volume lies inside that spiral-wrapped section between the white insulators:

Russian CI-3BG Glass Geiger Tube - detail
Russian CI-3BG Glass Geiger Tube – detail

In round numbers, that section is 6 mm long and 3 mm OD. Figuring the ID at 2.5 mm, that’s a volume of 30 mm3 = 0.030 cm3. That’s maybe 1/7300 of the ionization chamber volume, so, (handwaving) assuming roughly equal sensitivity, the chamber should report three orders of magnitude more pulses than this little thing.

It’s mildly sensitive to a radium-dial watch and perks up when a watch hand lines up along the spiral-wrapped volume. Given that the radium decay sequence spits out betas and no gammas, the (scaled) count may be a bit higher than the ionization chamber produces, but there are so many other imponderables that it might not matter in the least.

Obviously, it needs a case of some sort…