Sergei Bezrukov writes:
The radiometer is based on СБМ-20 Geiger counter tube which is manufactured in Russia and could be found on E-Bay. The counter is in a thin metal hull, so only beta and gamma rays can snick through it. It’s working voltage is in the range 350 – 450 V, the dead time does not exceed 190 μs, and the sensitivity is about 78 pulses per micro-roentgen. Therefore, maximum frequency of pulses provided by the counter is 106 / 190 = 5263 Hz. Respectively, the maximum radiation level one can register with it is 5263 / 78 = 67.47 μR/s, which is about 243 mR/h. The embedded firmware, however, can work up to 1 R/h.
Radiation dosimeter - Geiger counter - [Link]