Introduction: Homemade SBM-20 Gamma Detector Array

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.


apburner (author)2015-05-26

Kind of a misnaming of the ible. This turns the 700 into a high level
gamma detector. Stock the 700 detected both beta and gamma in the gieger
tube. Also the sorce that comes with the 700 is a small square of
yellow cake. (Uranium hexafloride). Another source we used was a
standard Coleman lantern mantle, unburnt. The element that makes it glow
so bright white is thorium which is slightly radioactive and gives off
beta particles. Back in the early eighties I fixed and calibrated these
instruments for the state in the radiological defense program. good write up by the way.

DavidN71 (author)apburner2015-05-26

The OCDM manual states that the beta window is to be opened and a beta only source used. They can be calibrated with gamma sources, but that is not how they were made to be used. Gamma sources require permits and shielding, weak beta sources don't. This array does not require a high level gamma source. I have a radium vacuum tube that will set this off pretty well. The lid comes off with a screwdriver and I can check beta and alpha source that way. The tubes are fully exposed from the top.

SoakedinVancouver (author)2015-05-24

Nice! What kind of reading do you get from the built-in check source?


The check source on CDV-700's is beta only. The OCDM manuals that came with them specify that a beta-only source had to be used for calibration. So there is little or no reaction to the CDV-700 operational check source. The plastic shell, perf board and distance block the betas. If you take the lid off and place a beta source on top of the tube array it goes crazy. The SBM-20 is thin wall for betas and gammas.

The original CDV-700 thin wall 6993 tube was energy compensated for gammas from Co60. That meant in case of nuclear was you could leave the pickle probe window closed and get a real world gamma exposure reading from a properly calibrated CDV-700.

BrettHacks (author)2015-05-24

That's cool. I don't understand the lead on the front only. If you are going for a directional gamma detector, wouldn't you enclose the whole box with with a slit on the side directed to the source?

DavidN71 (author)BrettHacks2015-05-25

You might get bremsstrahlung from the steel plate that I used for a lid, and from the steel handle. The lead sheilds the tubes from that backscatter. It also sheilds your hand. I have a homemade scintillator using plastic BC408 that is wrapped around to make it like Victoreen's old Cutie Pie detector. If I was next to a hot load in traffic, since the array is flat and broad, moving the array parallel and perpendicular to the suspected hot spot would change the reading. If I set a source off to one side it reads much less than pointing the flat surface at it. Sweeping back and forth then up and down in an arc with a speaker on the audio out will give an audible indication of the direction of the source.

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