Search for Superheavy Elements and Actinides Produced by Secondary Reactions in a Tungsten Target

WE have presented evidence 1 for the possible existence of a superheavy element with atomic number 112. This evidence came principally from the observation of spontaneous fission activity in a mercury source separated from a tungsten target which had been irradiated by 24 GeV protons. It was subsequently shown 2 that at a later stage 70% of the fission activity was due to 252 Cf . The origin of the remaining 30% of the fission activity was unexplained, however, although measurements of the activity made when the source was first produced indicated that it could not then have been due to 252 Cf.