The SBM-20M Geiger tube is a shorter version of the famous SBM-20 Russian tube. This is a modern tube, currently made in Russia. It has very similar specifications to the SBM-20. This is tube is more advantageous for use in smaller handheld (portable) Geiger counters, where size is definitely an issue (reduced from a 4” length in the SBM-20 to less than 2” in overall length for the SBM-20M.) Background CPM is less than that of its big brother due to the smaller cathode area; however, the tube was engineered like this so it could be used in smaller geiger counters. This is a great GM tube for use in a wide variety Geiger Counters that is comparable to GM tubes selling for many times the price we are offering. The production, however, was quite limited, and the tube is difficult to find elsewhere in the market.