This is a very popular surplus Russian military Geiger Tube. It was used mainly with other tubes such as the popular SBM-20 model in Geiger counters meant to measure both high and low levels of mostly Gamma radiation. The SI-3BG was engineered to have relative insensitivity to low level Gamma radiation; its main function was to act with other tubes in areas of high levels of contamination such as would occur in a Fukushima-level disaster zone. However, in close proximity to common radiation sources, such as those found in smoke detectors, uranium-bearing ore samples, medically-used radionuclides or radioactive isotopes, etc., it will easily detect radiation. It is a well-made, low-cost alternative to the more common GM tubes, and for this reason, is an ideal starter tube to the novice hobbyist, inventor,  or experimenter.  https://www.youtube.com/watch?v=SwIHr2Qd42o