A spotter’s guide to the SBM-20 Geiger counter tube

SBM-20 and friends
click to enlarge

The picture above shows a variety of Geiger tubes that are commonly sold under the name “SBM-20”. This post is an attempt to clear up some of the confusion around these tubes and to share some information about the different tube styles I have encountered in the wild since I first started selling Geiger Counter kits in 2011.

This is my spotter’s guide to the SBM-20 Geiger counter tube.


This is the quintessential Soviet Geiger Counter tube. 400V operating voltage. 105mm overall length. Gold (brass?) color. Symmetric end caps. Positive terminal is usually marked ‘+’, but if not, it’s the end closest to the CBM-20 marking. Date codes are always four digit, but I have seen both MMYY and YYMM (assuming I am interpreting the numbers correctly). Year of manufacture varies widely but most are 1970s – 1990s.


Very common alternate version of the SBM-20. Shorter 95mm overall length. Positive terminal is exposed rod with no end cap. Soldering to this terminal can be a challenge as the plating is usually quite oxidized and the underlying metal is steel.  I’ve also seen the internal glass seals fail after soldering. These are often sold as SBM-20 tubes, and unscrupulous eBay sellers will sell the SBM-20/SBM-20U interchangeably.

SBM-20 (alternative negative terminal style)

Slightly longer positive terminal. 105mm overall length. Negative terminal is formed from the tube body (cost saving design?). Tubes I have seen are marked ’77’, presumably date code for 1977. Rare.

STS-5 / CTC-5

Older version of the SBM-20. Slightly longer overall length, approx. 110mm. Silver color. Positive terminal is marked + and closest to the CTC-5 marking. Date codes I have seen are mid-1960s through 1970s. I haven’t confirmed this, but I suspect date code is roman numeral month followed by arabic numeral year, eg. VIII 65. Aside from longer overall length, properties and performance are very similar to SBM-20. Less common than SBM-20, presumably because they are older and were not manufactured in great numbers across as many years. Sometimes sold under the name SBM-20.  They are generally interchangeable with “real” SBM-20 tubes, but given that they are significantly older, failure rate might be higher (unconfirmed).


My Geiger Counter kit will accommodate the STS-5 and all of the SBM-20 variants shown. The SBM-20U requires a short jumper wire to connect the positive terminal to the PCB. I recommend using an alligator clip or wrapping a wire around the terminal to avoid compromising the glass to metal seal with the heat of the solder.

30 thoughts on “A spotter’s guide to the SBM-20 Geiger counter tube”

  1. Which part of the SBM-20 tube is sensitive ? The ends (which ?) or the length of the body ?

    My kit gives 1-2 clicks/s, but I can’t find any substance around the house to give more; not sure where I should place something relative to the tube.

    1. The SBM-20 will only respond to Gamma rays and high energy Beta particles. Due to its metal casing all Alpha particles and low energy Beta particles will be blocked.

      The probe will pick up Am-241, Cs-137, Co-60, etc. But will not detect materials such as Po-210 which is a nearly 99% Alpha emitter.

      This is the reason that almost nothing around the house will be detected by the SBM-20. Having said that it does not mean the probe is worthless. You simply must understand the probes capabilities and employ it in that environment.

      I hope that helps.

  2. I can add from my experience that NOS CTC-5 tubes from the sixties are 50% crap-shoot. Some have low resistance and pull down the tube voltage, some are completely open circuit and don’t detect anything. But when the seals have held then they work perfectly. Storage conditions definitely play a large role.

  3. Interesting idea: some of these are MUCH more sensitive. I had a thought, put some in a pressurized helium atmosphere for months at 70 Celsius to see if this helps.
    Also for tubes where they are damaged in some way ie low threshold voltage: He should increase the mean free path length and thus affect the count rate, provided it does not get hot enough to deform or melt the sealing pip which on some tubes is a low melting point alloy of composition BixInySnz or sometimes a Cd variant.
    Source: the same effect is seen in PMTs.

  4. Also relevant: my idea inspired by dark matter experiments.
    Put a few of these tubes that have failed testing ie messed up threshold in a helium atmosphere to repair them AND increase sensitivity.
    Only works if they are just low threshold so the gas fill has the wrong pressure inside.

  5. Hi, just a thought but some tubes seems to have a bad connection to the – side.
    I found that “dead” or noisy tubes can have this symptom, the giveaway is that both end caps will be isolated from the metal case and it should read less than 10 ohms from metal case to – cap.
    Also worth mentioning that the glass to metal seal on the + side (other end) is VERY sensitive indeed and can be damaged easily.
    Hint: its somewhat transparent and you can usually see a continuous faint blue (rather than red flashes- normal discharge as seen on end window tubes) glow inside tube if its leaking.
    Might actually try and make a hybrid indicator to pick up weak discharges and do a double count indication with something like an avalanche photodiode (aka APD) with parallel 160V zener running from the existing HV supply so I can run it from 550V supplies such as the ones on cheap dosimeters.

    1. I had no idea that you could actually SEE the discharges. I’ll have to check this out. Thank you for the tip!

  6. I got from my supplier a badly dented STS-6 GM tube. As I am curious, I opened it up and found that there is no gas containing ampoule inside. Thus the sealing is demanded to a perfect tube-caps crimping.
    I have more tube of the same type. Some of them have caps loose, therefore I suspect that the inside gas has leaked out.
    However those tubes work normally.

    How is that possible?

      1. if you tell me how to post a picture, I’ll show you that there is nothing else inside the STS-6 tube.

        By analogy, there must be no gas gas containing bulb also inside the SBM-19 tube, as the weight of the two devices is exactly the same: 17.2 grams. 0.61 ounce.
        The two tubes have the same dimensions.

        Antonio Zanardo

          1. There is no sealed gas bulb inside the tube. Therefore the sole sealing is the tube well crimped to the end caps. But if crimping is faulty, the caps are loose and the inside gas leaks out. This is what happened to some of my tubes.
            However the tubes with loose caps still are working. I wonder how is that possible?

  7. Does the cover of the SBM-20 Tube, the brass colored metal conduct electricity, or should I cover it with thin layer of rubber to be safe to touch or use it on a stick? And how much would a thin layer of rubber block Beta radiation?

  8. Repeat of the question with email request.

    Do you make an x-ray detector- or have some tube that would respond to= > 10KeV (i.e no metal cylinder around tube) that would work with your electronics


    1. I haven’t built a detector specifically for xrays, but the sbm-20 tube is somewhat sensitive to xrays based on what I have heard from others who have experimented with them. A tube with an alpha window might be more sensitive.

  9. Difference between СБМ-20 & СБМ-20У is in climatic standards. 20У is for moist climate. Both for mild and cold zones (-60,+40) Since Soviet Union was vast, there were many different regions from far north to east. Requirements is in ГОСТ 15150-69.
    http://www.istok2.com/data/2398/ СБМ-20
    http://www.istok2.com/data/3003/ СБМ-20У
    Also you should not solder those terminals, there are connectors for them, like for tube fuses.

    There is more sensitive СБМ-19

    1. Hi Senz,

      There were two type of Sensitivity
      Ra is 29
      Co is 22

      if I measure CPM, which one should I use for public?


      1. A Geiger tube just tells you an X-ray or gamma ray has been detected. It does not tell you the energy of the radiation, which is what determines the type and depth of damage done to your body. Really, all a Geiger counter will do is tell you if there is radiation in the area and the relative amount. To get true dose information, you need more sophisticated equipment that measures the energy as well. In general don’t attempt to use the Geiger counter for dose measurement.

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