RCL Model 10305 GM Tube (late 1950s)
Unfortunately, I have no literature regarding this particular tube, a Model 10305 produced by Radiation Counnter Laboratories. However, an RCL catalog states "The RCL Series 103 Detectors are sensitive to beta and low energy gamma radioactivity. Constructed from thin-wall glass or metal tubing, they are suitable for a wide variety of general purpose radioactive detection requirements… Series 103 detectors are virtually insensitive to count rate variation due to temperature change, are non-photosensitive, and exhibit no hysteresis."
Size: 4 1/2" long and 3/4" in diameter
Operating voltage: 950-1100 volts
Cathode: Silver coating on inside of glass wall
Wall thickness: Probably 30 mg/cm²
Since this tube seems to be fairly similar to the RCL Models 10300, 10302 and 10303, it is likely that it used an organic quenched fill gas.
Donated by the University of Kansas, courtesy of Michael Lemon.
Radiation Counter Laboratories catalog, Radiation Detectors, no date.