Geiger counters are devices that detect radioactivity. The four major types of radioactivity include Gamma, X-Rays, Beta, and Alpha radiation. All geiger counters detect Gamma and X-Rays. We use this in the paranormal field because of a theory that relates ghosts to radiation. Ghosts are rumored to emit low-levels of radiation. What kind of radiation is yet to be determined.
Geiger counters do not detect neutron radiation, microwave radiation, radon gas, and laser energy.
Items that emit radiation: Smoke detectors, lantern mantels, watches, camera lenses, jewelry, rocks, pottery, furniture.