Linked from
The 11 pages that link to Geiger counter, each with the reason it gives.
Ionizing radiationRelated: Its pulses arise when radiation ionizes gas inside the detector.
Beta particleRelated: It can detect beta particles entering its sensitive volume.
RadiotracerRelated: It can detect radiation emitted by a radiotracer near the instrument.
UraniniteRelated: Its radiation can be detected in the field with a Geiger counter.