SBM20 The quintessential russian geiger tube.

The SBM20 is the most produced of the russian geiger tubes. Probably it’s also the most produced tube in all world. It’s characteristics and cheapness makes it as a first choice of every DIY geiger project.

Two SBM20 tubes.

There are different versions of this tube:

  • SBM20 This is the standard one. 400V operation 105mm lenght clip-style connectors
  • SBM20U This is shorter 95mm and positive pole have no end cap. You can solder a wire to it but pay attention that the glass seal around the rod could crack and your tube became unusable. The better way to use it is wrapping around various turns of a thin wire.

How to wrap a wire at the positive rod of the SBM20-U

  • CTC5 This is the older version of the SBM20. They usually work well and with the same characteristics of new SBM20 but pay attention! This tube stocks are older than SBM20 and tubes could do not last as long as new’s in service.

A comparison chart between this tubes. Thanks to for the image.

Mightyohm comparison chart

Original datasheet


Gas Filling: Ne + Br2 + Ar
Cathode Material: Stainless Steel, 50 mkm
Maximum Length (mm): 108 / 101
Effective Length (mm): 91.0 / 83.5
Maximum Diameter (mm): 11
Effective Diameter (mm): 10
Connector: Pin
Operating Temperature Range: -60 to +70 C
Areal Density (mg/cm2): 40
Thickness (mm): 0.05

Minimum Anode Resistor (meg ohm): 1.0
Recommended Anode Resistor (meg ohm): 5.1
Recommended Operating Voltage (volts): 400
Operating Voltage Range (volts): 350 - 475
Initial voltage (volts): 260 - 320
Plateau length (volts): at least 100
Maximum Plateau Slope (%/100 volts): 10
Minimum Dead Time (at U=400V, micro sec): 190
Working range (mkR/s): 0.004 - 40
Working range (mR/h): 0.014 - 144
Gamma Sensitivity Ra226 (cps/mR/hr): 29
Gamma Sensitivity Co60 (cps/mR/hr): 22
Inherent counter background (cps): 1
Tube Capacitance (pf): 4.2
Life (pulses): at least 2*1010
Weight (grams): 10 / 9