Abstract: The expansion cloud chamber，invented by C. T. R. Wilson，is the earliest charged particle detectorin the world. It is effective，and yet the conditions for operation are not continuously maintained. Hence the diffusion cloud chamber is developed. The diffusion cloud chamber is continuously sensitized to radiation. A diffusion cloud chamber is designed and made according to its principle. With an Am-241 α radiation source ( from a smoke detector) put inside the cloud chamber，the tracks of α-particles are observed. In addition，the tracks of protons，β particles and cosmic rays are observed as well. The radioactive activity of the source is estimated based on the video footage and the geometry data acquired.

Key words: diffusion cloud chamber, charged particle track, activity of an α radiation source