This is Ivan, the Aztec Research Geiger counter. It is a Ludlums model 3 survey meter fitted with a 44-7 Geiger Muller tube with a mica end window. It is sensitive to Alpha, Beta and Gamma ionizing radiation.
It's scale is calibrated in counts per second and has 4 sensitivity settings, slow/fast response and audio on/off controls.
It is a very rugged unit and is ideal for field prospecting.