This is a custom built meter that I configured for myself, after looking at some of the choices commercially available at the time that I found lacking in some way or other. I’ve occasionally built small batches for others.
You may contact me if you would like me to make you one, at mike@badgertechnical.com My asking price is 250 USD. There is also sufficient information on this page to make your own as a project item, if you like.
The base design uses a cold war era soviet SBM-20 Geiger Mueller detector tube. The manufacturing quality is robust. I test and benchmark each tube against a calibration standard.
Other tubes, such as the CT11 and SI39 may be easily used, to obtain different ranges.
Tubes may be obtained from several places, such as Cheney Electronics (https://theelectronicgoldmine.com/collections/geiger-tubes)
The electronics package is called GK-mini, from DIYGeiger (https://sites.google.com/site/diygeigercounter/home/gk-mini).
I chose this off-the-shelf solution versus designing from scratch, because the gentleman at DIY has already done an outstanding job (hat tip), and I’d be reinventing the wheel.
The electronics package limits maximum counts to a Co-60 + Ra-226 equivalent dose rate of about 144mR/hr, which is fine for the scope of this project, although it does not utilize the full available range of the SBM-20 tube. The counter has a removable beta shield window. Range scalars are switch-selectable between beta+gamma, and gamma only. I have alternatively configured it with other tubes, to obtain ranges up to about 8.5R/hr (i.e., Fukushima-meter)
I perform a calibration with a microcurie range Cs-137 source obtained through Spectrum Techniques (https://www.spectrumtechniques.com/product/beta-gamma-disk-sources/). Calibration factors for each detector are inscribed on the casing. Calibrations are not certified in any way for industrial or professional usage; the intention is to provide a useful approximation only. With that caveat, however, the machine is suitable for hobby and contingency use.
The meter is dimensioned to use readily obtainable 4 AA sized batteries, and will work with rechargeable types such as Enloop or generic NiCad cells as well. Battery life is approximately 90 hours, with background levels of radiation, when using good quality alkaline cells.
