Abstract: A method and apparatus designed for the purpose of determining quickly and with high resolution the spatial distribution of radioactivity within an object emitting radioactive particles, especially those of low energy. A gas-filled position sensitive detector is used having an elongated electrode wire enclosed inside of and running the length of the detector. The object is placed inside the detector in close proximity to the wire. An electric circuit means is operably connected to the wire to determine the point along the wire at which an emitted particle is sensed. A two-or three-dimensional spatial distribution can be obtained by using a plurality of such wires.