A Geiger counter, also called a Geiger-Muller (GM) counter, is a type of particle detector that measures ionizing radiation. It detects the emission of nuclear radiation: alpha particles, beta particles, and gamma rays by the ionization produced in a low-pressure gas in a Geiger-Muller (GM) tube, which gives its name to the instrument.
A High Voltage generator (U1, U2, Q1, T1, and associated components) powers a GM tube. A pulse from the GM tube is interfaced through Q2 and U1 to pulse-generator U3, which drives a speaker.
Geiger Counter – [Link]