Geiger-Müller Counter
A gas-filled radiation detection instrument that measures ionizing radiation by detecting current pulses produced by Townsend avalanches in a high-voltage tube.
Governing formula
Townsend Avalanche ionization in high electric fields
SI unit
Counts per Minute (CPM) or μSv/h
In depth
An incident ionizing particle ionizes inert gas molecules inside the GM tube. High voltage accelerates liberated electrons, triggering an avalanche ion cascade that produces a measurable electrical pulse.
Examples in the real world
Radiation safety officers using portable Geiger counters to scan laboratory surfaces and nuclear facilities for radioactive contamination.