also: ガイガーカウンター
noun
Geiger counter
A device that detects and measures ionizing radiation. The full term ガイガー計数管 is technical; ガイガーカウンター is more common in general use.
放射線量を測るためにガイガー計数管を使った。
We used a Geiger counter to measure the radiation level.
A broader term for any radiation measuring instrument, while ガイガー計数管 specifically refers to a Geiger–Müller counter.
From German 'Geiger' (after Hans Geiger) + 計数管 (けいすうかん, 'counter tube'). The alternative form ガイガーカウンター is directly from English 'Geiger counter'.