Which instrument is used to detect radioactivity?

The Geiger counter is the instrument specifically designed to detect and measure ionizing radiation, including alpha particles, beta particles, and gamma rays. It operates by using a Geiger-Muller tube, which generates a pulse of electrical current when it encounters radiation. This pulse is then counted and displayed, allowing for the quantification of radiation levels in a given area.

In contrast, a barometer is used to measure atmospheric pressure, a caliper is a tool for measuring the dimensions of objects, and a seismometer detects and records the motion of the ground during seismic events. Each of these instruments serves a distinct purpose and is not capable of measuring radioactivity, which is why the Geiger counter stands out as the appropriate choice for this specific function.