A small, radioactive plastic sphere that is insoluble and used to make a tracer-loss measurement. The bead is designed to have the same density as the injection fluid so that it travels with the fluid when it is placed in the flow stream of an injection well. However, the bead does not enter the formation. It remains on the rock face in openhole, or within the perforation channel in cased hole, where it can be detected by a gamma ray log. A high radioactivity opposite a perforation indicates a large number of beads and hence a high injectivity. The technique was used mainly in the 1960s and 1970s.