When nuclear radiation enters the tube of a geiger counter, it the atoms of the gas contained in the tube.

The Geiger counter contains a metal tube containing a wire (electrode) and gas. When nuclear radiation (X-ray or Gamma) enters the tube it ionizes the gas (strips electrons from gas atoms) allowing the gas to conduct electricity when the metal tube and electrode are maintained at a voltage differential. A circuit counts the frequency of ionizations. Alpha particles (helium nuclei) can be stopped by a sheet of paper and Beta particles (electrons) are often not energetic enough to penetrate metal. See source

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