This simple circuit is for testing a Geiger-Muller tube such as the popular SBM-20 / STS-5, other tubes will likely work as well, input voltage should be at least 360V, a boost converter is an ideal source, ignore the transistor in the middle as this is just a crude way to simulate a GM tube.
Replace the 1Meg output resistor with a piezoelectric speaker, keep in mind it will be rather quiet due to the very short pulse length, an average background radiation count is 16-24 cpm (counts per minute).
