The internal soundness of weldments, castings, shop fabrications, and plant piping systems can be evaluated by gamma radiography. Slag inclusions, shrinkage, lack of penetration, and porosity are among the defects that can be detected. The radiation source is placed on one side of the component being tested and a film is placed on the other. The voids or areas of lower density than the metal show up as a dark spot on the film. Iridium isotopes are most often used, although cobalt isotopes are available for heavy wall components. A hard copy of the film is provided for a permanent record of results.