The Soviet-made SBM-20 tube is sensitive, large, plentiful and affordable in bulk. I got 15 from eBay for about $12 each. I wanted to make a cheap array that would detect a gamma source from a distance. I have CDV-700's with me at all times (yeah, I'm a "prepper"). I tried an array of 12 and found that it works properly connected to a CDV-700, even a Lionel 6B with their weak power supply. I used 500 volts of zener diodes turned backwards to drop the 900v CDV-700 output to the 400 volts the SBM-20 works best at. The lid has lead sheet under it held in place with aluminum tape. The array will peg a CDV-700 on the 100x scale if you open the box and set a piece of Fiestaware on the tubes. The plastic box bottom, perf board and metal SBM-20 tube block alphas and most betas from the bottom, so you can point the array at your suspected hot spot.
Step 1: The Parts Needed
A US seller of SBM-20's. Most other sellers are from Russia, Ukraine or Bulgaria as this was a Soviet Bloc geiger tube. They aren't hard to test, so I would refuse to buy untested tubes. They have a thin wall and should be handled gently.
I bought fuse holders and cut them in half. These clips have a solder ring. The SBM-20 is polarized. The positive end is marked +. A plain copper wire through the solder ring holes was used to bridge together all the positive ends and another wire used to bridge all the negative ends. They are Gorilla glued to the perf board
Since Radio Shack has gone away, this is the closest project box you will find to replace the black plastic ABS enclosures they used to sell. The posts are used to mount the perf board off the bottom.
I used a 300 watt zener and 2 100's because I had the 300v zener. Those are hard to find. Five 100v zeners with the + end facing the bnc connector will drop the CDV-700 voltage to 400v for the SBM-20. There is a 10M ohm resistor in line with the anode (negative) end of the BNC to limit the inrush of current when the geiger counter is turned on. Soviet tubes are a bit twitchy without some anode resistance, but the SBM-20 doesn't need much. It's hard to make one avalanche without going way over voltage.
The lead sheet under the lid is from a roof vent. Home Depot sells the same item. The lead is so soft you can cut it with common scissors. I cut the tube open and used that to make my shield. The lead sheet is held in place with aluminum duct tape.
This is the perf board that I used. Radio Shack used to sell it a sheet at a time.
This is a BNC panel mount. Most of my CDV-700's have been modded with BNC connectors. Other makes of geiger counters might need an different connector. Ludlum used a propietary "C" connector
and Eberline and others sometimes used a medium high voltage connector
that is not exactly compatible with a regular BNC, but the trend is toward using common BNC connectors for interchangable probes.
This plain drawer pull makes a nice handle because it's wide enough to put all 4 of my fat fingers under.