NEW (NOS) SBM-20 Geiger-Muller counter tube.
Tested and fully working.
One counter in one lot.
The counters may have different production years and slightly different appearance. 

The Geiger-Muller counter SBM-20 is designed to measure hard beta and
gamma radiation for wide application in the field of radiation monitoring.
Technical characteristics of Geiger-Muller counters SBM-20:
Radiation source - Cs;
    Sensitivity to gamma radiation:
    EDR - 4.0 µR∙s-1;
    67.5 µR-1±7.5 µR-1;
    The rated operating voltage of the device Geiger-Muller counter SBM-20
is 400V;
    Counting voltage - 260V-320V;
    The length of the counting characteristic is not less than 100V;
    The slope of the counting characteristic is no more than 0.1%/V;
    The intrinsic background of the device Geiger-Muller counter SBM-20 is
no more than 1.0 s-1;
    Maximum operating DER - 1400 s-1, 40 µR∙s-1, k.n. ±20%;
    The maximum permissible DER is not less than 360R∙h-1;
    Dimensions - 11x11x108mm;
    The weight of the Geiger Muller counter SBM-20 is 10g.