A radioactive source is just like a light bulb. One gives off radioactive particles and the other just photons . The power of the lamp is like the activity of the source. The activity captured in an area of 2 cm2 at a distance of 10 cm away from the source is (Activity of source/area of sphere of radius 10cm ) x 2 cm2. Just like power = intensity x area. In fact count rate is activity captured per unit area at a distance away from source .
